Running a Tube-Type Radio on "Car Batteries" - PART 1-

This was starting to get a bit looong, so I'm splitting it into two parts... 

I also made a few changes so "Part 2" shows up after "Part 1".

I was having a discussion with Harry over on his blog on the subject of shortwave listening, types of receivers, and other things.

I made my standard "If you're worried about EMP, take your little solid-state (transistorized, but more likely uProcessor controlled DSP radio these days) radio with the batteries out (because if you don't, you will forget they're in there, they will leak, and likely ruin the radio. Trust me, it WILL happen!), and seal it up in a steel ammo can, using adhesive-backed aluminum tape on all the seams. NOT aluminum colored duct tape, but real metal aluminum tape. You can get it at big box home improvement stores. This gives you a homemade Faraday Cage, and should protect the radio".

Have I tried this?

No, mostly due to the lack of a suitable EMP generator, but I consider it sound advice, knowing what I do about EMP.

I'd tell you more about why I know this stuff, but uh...well...you how that goes...

Yes, you could make a special box from "Mu Metal", but Mu metal is expensive, hard to work with, and generally difficult to buy in small quantities.

Then we started talking about types of receivers, and the subject turned to tube-type radios.

Tubes are inherently "hardened" to EMP because of their construction. They have large metal elements, separated by substantial spacing (compared to solid-state devices), and all the metal elements are encased in a vacuum envelope. Granted, the vacuum isn't "perfect", but the breakdown voltage (aka "Dielectric Strength") between two metal elements in a vacuum is is substantially higher than the breakdown voltage in air.

Wikipedia lists the breakdown voltage for air as 3e6 (3,000,000 Volts) per meter of spacing, and vacuum as 10e12 Volts per meter.

That's TEN TERRA Volts per meter, a staggering amount of voltage!

That's a thousand billion Volts.....

Speaking in term of "Volts/mil" (voltage per .001"), air breaks down and conducts at around 800 Volts per .001" of spacing (I always remember "1000Volts/mil") and breakdown in a vacuum is something like a MILLION times higher.

ANYWAY......before I ramble too far off the original topic, tubes should survive an EMP (especially if powered off), while conventional "wisdom" claims solid-state devices will be destroyed.

SO......In all of our favorite TEOTWAWKI/TSHTF novels, there always seems to be some "Old Guy" who just happens to be a Ham operator, always "lives on a hill top", has "big antennas", and he just happens to have some old tube-type radios laying around. Of course, they get pressed in to service powered up by "car batteries" scavenged from all the abandoned/broken/non-functional cars which just happen to be strewn around everywhere.

It makes for a great read, and shows how ingenuity can overcome adversity, but would it work?

Well, that's what I'll try and analyze here (whew! finally got back on topic), and give some recommendations for trying to do this.

The closest radio manual I had at hand is for my Heathkit SB-310, so that's what I'll go with. This radio is not "100% Hollow State", as it has three rectifier diodes in the power supply (B+ and bias rectifiers), two "small signal" diodes for the Automatic Noise Limiter ("ANL"), and two more small-signal diodes in the Automatic Gain Control ("AGC") circuits. Since this is an AC to DC conversion, the rectifier diodes will be bypassed, and you can use the receiver with the ANL and AGC diodes clipped out of their circuits if they get blown from an EMP-type event. It wouldn't be as "pleasant" to listen to with those two circuits disabled, but it will work just fine without them.

The principles involved with doing this to most any tube-type receiver would be the same, but some of the voltages will probably be different.

YMMV, don't try this at home, don't blame me if you get zapped or blow up your receiver, and yes, I am a professional at this stuff!

First, let's look at how the receiver is powered during "normal" operation.

Tube-type receivers use a power supply (transformer, rectifiers, filter capacitors, dropping resistors, and possibly a filter choke) to produce the required voltages from the 120 VAC power line.

There are (usually) three voltages required to make the receiver operate:

1. An "A" voltage, which is the filament voltage, either 6 Volts or 12 Volts. Some receivers may have a mix of both 6 and 12 Volt filament tubes in them. The filaments "don't  know/don't care" if they're being fed AC or DC. Some people claim that running the filaments from DC will give a "quieter" receiver, while others claim that running them from DC will shorten the filament life due to metal transfer off the filament to the other tube elements, similar to what happens during electroplating.
Way back when I was repairing/modifying electronic stuff for all my musician friends, I did a full make-over on a buddy's Fender Twin Reverb amp. I went completely gonzo on it, beefing up the power supply, regulating all the voltages, converting the filaments to a regulated DC supply, redoing all the grounds inside, and shielding the daylights out of it. *I* couldn't tell much difference, but *he* could, and I wound up doing a half-dozen or so for other people. AFAIK, there was NO difference in tube life.

2. A "B" voltage, commonly called "B+", which is the plate/Anode supply voltage, and generally the highest voltage in the radio. Sometimes there are multiple B+ voltages used at different points in the circuit, and these are generally derived from the highest voltage using dropping resistors. This voltage is always well filtered DC, and in some cases is regulated over a narrow range. This voltage is positive with respect to ground.

3. A "C" voltage, which is the bias voltage applied to the tube grids, and used to set the "operating point" of the tube. This voltage is negative with respect to ground.

In "Ye Dayes Of Olde", long before residential homes were wired for that new-fangled "AC" stuff, these voltages were all supplied by batteries, so running tube radios on batteries is nothing new.

An "A" battery powered the filaments, a "B" battery powered the plates, and a "C" battery provided the bias.

First, we have to deal with how to get the voltages we need from "car batteries", and discuss some things about "car batteries" that need discussing. Then we'll move on to modifying the wiring in the radio to use our new external voltage sources.

At this point some of you are probably scratching your head and wondering why all this talk about using ONLY "car batteries" to power the radio, and you'd be correct. The simplest thing to do is get a 12 Volt DC to 120 Volt AC inverter, and just run the radio that way. Most receivers don't draw a whole lot of power, so you don't need a big inverter. The SB-310, for example, only draws 50 Watts AC power, less than 1 Amp, so even a small El Cheep-O inverter like this $20 one from Harbor Freight would work. My Drake R-4B, a superb ham band only receiver, draws 60 Watts, still well under 1 amp of AC line current.

BUT, as all the stories go, we're stuck using "car batteries", so that's why I'm writing this (very long...) post.

You'll notice I keep putting "car batteries" in quotes. There's a reason for that.

"Car Batteries" fall into a couple of broad classifications. First, and most common, are what's called "SLI" rated batteries, for "Starting, Lighting, and Ignition" batteries. This type is designed to put out a huge blast of current (several hundred amps) for a short time to get the vehicle started, and then to be recharged by the alternator immediately. They do NOT like to have a constant, low drain applied to them, like the drain that a radio would cause. If used in this service, they will not produce their rated "Amp Hour" capacity, will go "dead" quickly, and will get to the point that they will no longer hold a charge, or even accept a charge. I've personally ruined several SLI batteries by using them as a back-up power source for my "12 Volt" Ham radios. Even though I kept them charged with a properly deigned "battery tender", and watched them carefully, they never lasted more than 12 months.

Expensive lesson, but well-learned.

The next common type is the "Deep Cycle" type. These batteries are designed uses that require a lower current draw for extended periods. Sometimes they're called "motive" batteries, and are used for wheel chairs, golf carts, trolling motors, and solar power storage. Every time I've replaced my car battery I've always replaced it with a deep cycle battery, as there's times I'll run my radio for extended periods without running the engine, and I want to make sure I have enough power left to start the car.

And within the battery types are some sub-classifications depending on the type of construction used.

"Flooded" types have the liquid electrolyte (aka Battery Acid) and removable caps to check the levels. Most of us grew up with this type, and are somewhat familiar with it. I always used distilled water to top off the level, although many sources say that any potable water is OK. Personally, with the cost of distilled water being so low, I could never see using tap water, especially in areas with hard water.

"Valve Regulated Lead Acid" (VRLA) batteries were first seen in the late 1960's, and were marketed as "Maintenance Free" batteries. They used a slightly different chemistry and construction/

"AGM" or Absorbed Glass Mat batteries are newer still, and have a different construction that keeps all the electrolyte in a fiberglass mesh.

And as of 2016 some cars are using Nickle Metal Hydride and Lithium Ion batteries for their power source.

So, with that said, be aware that using old fashioned, flooded construction SLI batteries would work, but the batteries probably won't last as long as you'd like.

Now as to "How Many Batteries Will We Need", we need to look at voltages, and we'll use my trusty SB-310 as our example.

The SB-310 "requires" 185 Volts for the B+, -85 Volts for the Bias, and 6 Volts for the filaments.

Nominal, fully charged voltage for a "12 Volt" automotive battery varies some with the type. We'll go with the current VRLA batteries as they're most common in newer cars.

Fully Charged = 12.66 Volts

50% Charged = 12.10 Volts

25% Charged = 11.95 Volts

  0% Charged = 11.70 Volts

We'll pick 12.5 Volts just to make the math a bit easier.

185 Volts/12.5volts-per-battery= 14.8 batteries.

Kinda of hard to have 8/10ths of a battery, so we'll say 15 batteries.

Fully charged, 15 batteries gives 189.9 Volts, a few volts higher than the nominal 185 Volts for the B+, but nothing to worry about. These radios were designed with 10% tolerance resistors, and most of the capacitors (except in critical tuned circuits) were about as "loose", and the AC line voltage was never exactly "117VAC", so plus or minus a few Volts on the B+ isn't going to matter.

Fully discharged, we'd have 175.5 Volts, enough to keep the receiver running, but pretty hard on the batteries.

For the -85 Volt Bias Supply, we have it easier. Since the Bias Supply is feeding the grids of the tubes (a very high impedance), the resulting current draw is extremely low, on the order of microamps. Rather than lugging another 7 car batteries to make the bias supply, it's much easier to use ten 9 Volt "Transistor Radio" batteries, or any other combination of standard dry cells that gives about 90 Volts.
Then we'd make a simple resistive voltage divider, and adjust the voltage for the 85 Volts (or other required voltage) we need. The bias voltage is a little more critical than the B+ voltage, because if the tubes aren't biased close to where they should be, the radio won't operate properly.

This radio has  6 Volt filament tubes exclusively, with a total draw of 3.3 Amps.

While there are ways to drill and tap into a 12 Volt battery at the 3rd cell and get 6 Volts from it, I've never done it, and only seen pictures of it. I'm going to assume that whoever is attempting to do this (the old "Ham on the Hill"!) has a pretty extensive "Junque Box", and would just make up a 2:1 resistive voltage divider, and use a single 12 Volt battery for the filaments.

So that gets us our required operating voltages. 15 car batteries in series for the B+, one more for the filaments, and a bunch of dry cells for the bias voltage.


In "Part 2" I'll get into modifying an actual radio

Running a Tube-Type Radio on "Car Batteries" - PART 2 -

OK....on to the radio....finally!

There's nothing magic or wizardly going on here. The circuitry doesn't care if the voltages come from AC, DC, Solar, Nuclear, coal, or filtered unicorn farts. If you supply the radio with the correct voltages, it will operate.

In order to take an AC powered radio and run it from batteries, we'll need to make some changes to the wiring. Specifically, we'll need to connect the correct voltages to the correct circuit points for both the B+ and Bias supplies. To do that we'll add some wires to those points and bring them out of the radio for the external power to be applied, along with a ground connection to a new plug.

And we'll need to separate the filament wiring from the secondary of the power transformer, and bring those two (or three, in the case of a radio having both 6V and 12V filaments) leads out, too.

So, we'd need to use a 6-conductor plug and socket, rated to withstand, say 300 Volts.

But in a real SHTF/TEOTWAWKI situation, we'd probably just run wires out of any convenient opening in the radio case.

Here's the schematic for the power supply section of an SB-310 General Coverage receiver:

Just about smack dab in the middle, you'll see two symbols that look like right-pointing arrows, labeled "D6" and "D7". The right side of this symbol, the direction the arrow is pointing, are the positive (Cathode) ends of the rectifiers. Solder a red wire to this point, and bring it out for later connection. The wire has to be rated for at least 300 Volts to be safe, but wouldn't have to be much more than 20 gauge as the current draw is pretty low. Since we'll be feeding in a positive voltage, and that voltage will be blocked by D6 and D7, no other surgery is required. *IF* D6 and D7 are shorted (blown by that commie EMP blast!), just clip them out of the circuit.

A little below that point, you'll find D8, which is the rectifier for the bias supply. This time you'll want to solder an orange wire to the junction of D8, the capacitor C233, and the resistor R211. This will be where we apply the -85 Volt bias supply. If D8 is shorted, just clip it out like you did to the rectifiers.

Damn commies.....

Underneath the chassis should be numerous terminal strips. Look for one with a "foot" that's grounded, and solder a black wire to it.

On the terminal strip pictured below, the "#2" position is the "foot" I was talking about, which bolts it to the chassis, and provides a ground point.

That takes care of the "high" voltages. Just make sure the red, yellow, and black wires you added are long enough to come out of the radio a foot or two, so you'll have some length to work with when you connect your battery bank.

For the filaments, we'll have to be a bit more careful. Since we'll be using DC to power them, we must disconnect the secondary winding of the transformer or otherwise it will short out the new DC supply to the filaments.

Look back at the schematic for "T1", which is the power transformer. You'll notice (sorry, but I'm assuming you can "read" a schematic...) that leads "1" and "3" are the yellow filament power leads. One side is grounded, which means it's common to the B+ and Bias supplies. Pick the yellow lead that's NOT grounded, and cut it loose from the terminal strip. Add a new yellow wire to this point, and bring it out like you did the others.

That should complete the modifications to the radio. Simple, and pretty easy to reverse if the AC power ever comes back!

The battery connections will simply be 15 car batteries connected in series, with the positive lead going to the red lead coming from the radio, and the negative lead going to the black lead coming from the radio, and that takes care of the B+ supply.

For the bias supply, you'll have ten 9 Volt transistor radio batteries in series, with the NEGATIVE side of your battery stack going to the orange wire coming from the radio, and the POSITIVE side of the battery stack going to the black lead coming from the radio.

For the filament supply, you'll have a single battery in series with a 2 Ohm, 25 Watt resistor (drops the 12 Volts down to 6) on the positive side connected to the yellow lead coming from the radio, and the negative side of the battery going to the black wire from the radio.

Will this work, and power the radio as if it were plugged in?

I have no doubt it will. As I said at the beginning, the radio doesn't know/doesn't care where its power comes from. Apply the correct voltages to the correct points in the circuit, and it will function, assuming it was an operating radio to begin with.

If anybody wants to loan me 16 car batteries, I'd be willing to modify the receiver just to prove this will work!

Personally, I'd rather just buy a few inverters and some extra ammo cans to keep them in. Saves a lot of work lugging batteries up the hill the old Ham lives on.......

Now....a few words of caution here.....

CAUTION! 

You've just built a 185 Volt DC power supply capable of SEVERAL HUNDRED Amps output.

This is a LETHAL voltage source! You will NOT get a second chance if you accidentally contact the positive lead while grounded.

Use EXTREME care when operating this power supply!

You have been warned!

It would definitely be a good idea to fuse the positive lead of the battery string, and properly insulate ALL exposed metal connections, but make sure you use a fuse rated for 250 VDC, and with a very high "interrupting rating" so that the fuse doesn't turn into a bomb. Fuses for this service usually are packed with sand, and use a ceramic body instead of glass, so that when the fuse link inside opens, the grains of sand fall into the gap, and quench the arc. Otherwise, with a supply this "stiff", there will be enough energy available to keep the arc "lit" when the fuse link opens, and a plain glass body fuse will violently disintegrate, possibly causing somebody to get hurt.

And a 1 or 2 Ohm, high wattage current limiting resistor in series with the positive lead wouldn't hurt anything, either.

Friday Already?

Still waiting for the "4-pin" cable from the Maestro RR people so I can finish installing the new Kenwood radio into my Jeep. They're in Canada, so it might be here today, but I'm betting it won't be here until next week.

And the two switches on the steering wheel that control the radio channel selection (left switch) and the volume (right) switch haven't shown up yet, either.

The USPS "tracking number" claims they were delivered last Saturday, but my buddy where my mail drop is hasn't seen them.

I dropped the vendor a note, and he sent another set out on Wednesday, so they also might be here today since he's in AZ.

And of course, it's supposed to rain some this weekend, starting on Saturday night. *IF* the parts get here today I should be able to swap out the radios tomorrow before it starts raining.

And on the Iowa, we were (FINALLY!) able to get audio routed from one of the "Red Phones", through the "Coke Machine", and the transmitter audio switchboard, down to transmitter #3, and were able to get 500 Watts output from the transmitter into the dummy load. We then noticed that the Power Output knob was backed off a bit, and once we ran that up to max, we were getting 1000 Watts out.

The Power Output control knob is VERY nonlinear in it's action, and turning it down just a few degrees drops the power from 1000 Watts to 400~500 Watts. We don't really need full power out of the transmitter, and considering the two antennas we'll be using, we don't WANT 1000 Watts, as the tour route gets to within 5 or 10 feet of the antennas, and we have to limit the RF exposure to our guests.

On The Workbench This Week

Only this time it's not something being repaired, but something being assembled.

A few months after I bought my Jeep in the fall of 2006, I decided I wanted a navigation unit for it. I'd just helped my son install a Pioneer AVIC nav radio in his Xterra, so I started looking into an aftermarket unit, as the Jeep came with a pretty generic AM/FM/Sirius radio with a single-disc CD player, and I wanted something more integrated to the vehicle so I wouldn't have to reach over and grab my hand-held GPS when I wanted to see where I was.

WELL......at that time, in order to keep my steering wheel controls, I would have had to add several aftermarket modules, and cut into the wiring harness in several places, and it still wasn't guaranteed that everything would work.

SO....I opted for an OEM navigation unit from an eBay seller who came highly recommended on several Jeep forums.

I bought the radio at a great price, and then got the replacement plastic bezel for it, and installed it in less than 45 minutes!

The first thing I was unhappy with was that it did NOT have a touch screen, and I had to enter the street name and number one.....character.....at.....a.....time.....using the scroll knob.

What a huge PITA!

You simply didn't have enough time at a red light to do anything, and as soon as your speed went over 5~7MPH, it would lock out the front controls, so your passenger couldn't even use it!

Yeah, I understand the safety aspect, but if the car's  computer is smart enough to trip the seat belt warning chime when my dog is riding with me, why couldn't it be "smart" enough to sense a passenger is there, and unlock the controls so the passenger could make adjustments?

Damn lawyers.....

ANYWAY...fast forward 10 years to the present. The joystick/scroll knob used to input data or move the on-screen navigation cursor has been getting intermittent, and the 6-disc CD changer has gotten to where it maybe will/maybe won't play a disc, depending on ambient temperature, the day of the week, and possibly the phase of the Moon. It might be fixable, but I 'd really like a touch screen, and the Sirius/XM tuner has started getting funky, too, dropping signals and suffering "digital breakup" more and more often.

Things have progressed to the point that I was able to get a single box adapter that splices in between the OEM plugs in the car, and the wiring harness for the new radio, and retain full functionality of my steering wheel controls.

This little gem is called a "Maestro RR", made by Automotive Data Solutions, based in Montreal, Canada.


Just a little black box that captures and manipulates the data from the steering wheel controls, and turns it into something the radio can understand.

So, being somewhat of a Kenwood aficionado,  I ordered a DNX771HD from Cructchfield, a Maestro "Rr" interface unit, and all the other wiring harness adapters.

After going through some of the spotty documentation, I finally figured it all out and sat down and soldered the two wiring harnesses together.



Here it is, with the Kenwood on the left, the Maestro RR module in the middle, and the gray plugs on the right  which connect to the existing Jeep OEM wiring harness:

The Maestro RR module also has connections to the OBD II port, allowing various engine parameters to be measured, and displayed:

And it all plugs into the Kenwood radio using the Kenwood OEM connectors:

I used my typical "Good Amateur Practice" from the ARRL Handbook, and soldered the connections together, and covered them with heat shrinkable tubing:

And then we hit some snags.

The M5 screws provided with the Kenwood were too short to go through the thick plastic brackets in the dashboard adapter, so I had to hit the hardware store for some longer ones:

And, of course, some washers to spread the clamping force on the plastic, so it won't crack, which I've had happen before:

Since installing an aftermarket head unit loses control of the OEM Sirius/XM radio, a new receiver module is needed, seen here with the mating plug to the new head unit:

The new receiver has a $70 up-front cost, but when I call to cancel the service on the existing one, and activate this new one, I'll supposedly get a $70 gift card, making the receiver essentially "free". I'm also told that any remaining time on my existing contract will be added to the new contract for this new receiver.

And finally, the last remaining snag that stopped me from having this installed this weekend.....

This radio is an "iDatalink Compatible" model, which means that it's designed with the Maestro module in mind. Besides all the small plugs on the Maestro adapter harness and the OEM harness, which allows the Maestro to tap into the CAN bus of the vehicle, there's two addition connectors on the radio that go to the Maestro module.

WELL.....guess what? One of the cable assemblies that transfers data between the radio and Maestro module was missing from the new, sealed box the Maestro was in!

I went to "the12Volt.com" forum, where the Maestro technical reps hang out, and after a few days, I have a replacement cable on the way. Hopefully it went out Monday afternoon and I'll have it by Friday, but with the weather problems on the East coast, who knows when it will show up.

I'll do another post detailing the actual installation into my Jeep Grand Cherokee and a mini-review of the whole shebang once I get the missing cable.

SHTF Radio Basics

I wrote this up for wirecutter, and he'll be posting it over at his place, but I wanted to see if I could post it in the "odt" formaI. I converted it to a "word doc" for him, as he was having trouble doing a copy-and-paste into his blog window.

Since I run Linux 99.9% of the time (OpenSUSE, to be specific), I use LibreOffice for all my office type documents, and this was a copy-and-paste of the odt file directly into the Blogger window. 

For those that don't know, LibreOffice is a "fork" of the OpenOffice project, which itself came from the StarOffice project way back in the early days of Linux.

While there are many excellent word processing and office-suite type programs available for Linux, LibreOffice/OpenOffice tend to be the "900 pound gorilla" since they're included in most Linux distributions.

Anyway.......

Kenny wanted something that covered the basics of why you might want to have a little radio in your "Git Kit", specifically something that covered more than just AM and FM, so I put this together over the course of a few weeks so he could post it. Seeing as he gets about a bazillion times the hits that I do, I agreed that it would be a good idea for him to post it so that more people could read it.

It's by no means the definitive word on "Emergency Radio" or "Survival Radio", and only covers receiving. One of these days I'll scribble something down about transmitting, as while it's nice to be able to listen, transmitting can be extremely important at times, too.

SHTF Radio Basics

If/when TSHTF, we're all going to want to keep in touch with what's going on around us. This little article is about the first step in communicating by radio, and that's learning how to listen. Radio communication can run from communicating just within your local area, to covering all of North America, to covering the entire world. It all depends on what you need, which determines what gear you need, and how you do it. There's no “magic” to it (well....maybe a little!), and most anybody can learn how to use radio communications effectively.

I'm not going to teach you how to design a radio or to repair one; this is just an intro to using one. If you get really interested, the links provided at the end of this article will allow you to study and learn as much as you want.

Don't get scared off when you see new, unfamiliar words and terms. I put a glossary at the end so you can look up most of the new words and terms you'll be learning.

Keep in mind that I've tried to write this in simple language, for beginners.

Yes, I've taken some liberties with some of the technical terms and descriptions, but they're basically correct, so I don't want to hear “How wrong I am” from anybody with a lot of experience with radio. If you have constructive criticism, or find a glaring error or omission, I'll gladly welcome the feedback, but this 'aint a forum on eHam, so back off a bit!

And if you have some suggestions, by all means submit them. If I think they fit in with this article, I'll include them.

I've been a licensed Amateur Radio Operator since I was 12 years old, almost 50 years now. I didn't always have a transmitter with me in my travels, but I always had a General Coverage receiver with me, so that's where we'll start, with General Coverage receivers.

General Coverage Receivers

Broadly speaking, a General Coverage receiver will pick up radio signals between 3MHz and 30MHz. With today's modern radios, this also includes the AM broadcast band, which covers 540kHz, to 1610kHz, and the FM broadcast band, 88MHz to 108MHz. Some radios will also include “Air Band”, 108MHz to 136MHz which is what commercial and private aviation uses, and possibly the “Long Wave” band, which covers roughly 150kHz to 500kHz. Commercial aviation frequencies are fun to listen to, but not all that important in a SHTF radio. Same with the “Long Wave” band which is mostly used for Non-Directional Beacons (“NDB”), and some types of weather broadcasting. Other, older radios may offer “TV Channel Audio”, but with the television stations going digital, this is pretty much useless these days. Same with radios offering a “Public Service” band or two. Very few Police or Fire departments, especially in large metro areas, have simple VHF/UHF radio systems these days. Almost all of them are on “Trunked” radio systems, and many are using digital audio (“APCO P25”) instead of analog audio. I'll cover other types of receivers, like scanners, in a separate article.

Since we all pretty much know about “AM and “FM”, I'll just briefly cover those, and then get to the frequency coverage (“Bands” or “Wavelengths”) that makes these little radios much more useful than just a typical AM-FM portable radio.

AM Radio

Good Old AM (Amplitude Modulation) radio has been with us since the 1920's, over 90 years now. It might be said (tongue firmly in cheek!) of radio that “In The Beginning, There Was AM, And It Was Good”. These days, AM has radically changed from the “Top 40” format I grew up with, to being mostly talk-radio, all-news, religious, and foreign language. And that's its strength. If you want local news, you get it during the day, out to several hundred miles depending on the power of the station. If you want news from more distant areas, then listen at night. There are “Clear Channel” AM stations in all the major cities, and you can usually receive them coast-to-coast at night.

Clear Channel stations, are by U.S. and International regulation, the only station on that frequency 24 hours a day. There may be other low-powered local stations, but when the sun goes down, the locals shut down. Clear Channel stations generally run 50,000 Watts of transmitter power, although some of the Mexican stations run 150,000 Watts. Since they're the only station on that frequency, they'll be listenable all over North America at night. This allows them to be useful for gathering information from well outside your local area.

The current list of Clear Channel stations is below:

540 CBK Watrous, Saskatchewan

540 CBT Grand Falls, Newfoundland and Labrador

540 XEWA San Luis Potosí, San Luis Potosí

640 CBN St. John's, Newfoundland and Labrador

640 KFI Los Angeles, California

650 WSM Nashville, Tennessee

660 WFAN New York, New York

670 WSCR Chicago, Illinois

680 KNBR San Francisco, California

690 CKGM[7] Montreal, Quebec

690 XEWW Tijuana, Baja California

700 WLW Cincinnati, Ohio

710 KIRO Seattle, Washington

710 WOR New York, New York

720 WGN Chicago, Illinois

730 CKAC Montreal, Quebec

730 XEX Mexico City, D.F.

740 CFZM[8] Toronto, Ontario

750 WSB Atlanta, Georgia

760 WJR Detroit, Michigan

770 WABC New York, New York

780 WBBM Chicago, Illinois

800 XEROK Ciudad Juárez, Chihuahua

810 KGO San Francisco, California

810 WGY Schenectady, New York

820 WBAP Fort Worth, Texas

830 WCCO Minneapolis, Minnesota

840 WHAS Louisville, Kentucky

850 KOA Denver, Colorado

850 XETQ Ixhuatlancillo, Veracruz

860 CJBC Toronto, Ontario

870 WWL New Orleans, Louisiana

880 WCBS New York, New York

890 WLS Chicago, Illinois

900 XEW Mexico City, D.F.

940 silent[9] Montreal, Quebec

940 XEQ Mexico City, D.F.

990 CBW Winnipeg, Manitoba

990 CBY Corner Brook, Newfoundland and Labrador

1000 KOMO Seattle, Washington

1000 WMVP Chicago, Illinois

1000 XEOY Mexico City, D.F.

1010 CBR Calgary, Alberta

1010 CFRB Toronto, Ontario

1020 KDKA Pittsburgh, Pennsylvania

1030 WBZ Boston, Massachusetts

1040 WHO Des Moines, Iowa

1050 XEG Monterrey, Nuevo León

1060 KYW Philadelphia, Pennsylvania

1060 XEEP Mexico City, D.F.

1070 silent[10] Moncton, New Brunswick

1070 KNX Los Angeles, California

1080 WTIC Hartford, Connecticut

1080 KRLD Dallas, Texas

1090 KAAY Little Rock, Arkansas

1090 WBAL Baltimore, Maryland

1090 XEPRS Rancho del Mar, Rosarito, Baja California

1100 WTAM Cleveland, Ohio

1110 KFAB Omaha, Nebraska

1110 WBT Charlotte, North Carolina

1120 KMOX St. Louis, Missouri

1130 CKWX Vancouver, British Columbia

1130 KWKH Shreveport, Louisiana

1130 WBBR New York, New York

1140 WRVA Richmond, Virginia

1140 XEMR Monterrey, Nuevo León

1160 KSL Salt Lake City, Utah

1170 KFAQ Tulsa, Oklahoma

1170 WWVA Wheeling, West Virginia

1180 WHAM Rochester, New York

1190 KEX Portland, Oregon

1190 XEWK Guadalajara, Jalisco

1200 WOAI San Antonio, Texas

1210 WPHT Philadelphia, Pennsylvania

1220 XEB Mexico City, D.F.

1500 KSTP Saint Paul, Minnesota

1500 WFED Washington, D.C.

1510 WLAC Nashville, Tennessee

1520 KOKC Oklahoma City, Oklahoma

1520 WWKB Buffalo, New York

1530 KFBK Sacramento, California

1530 WCKY Cincinnati, Ohio

1540 KXEL Waterloo, Iowa

1540 ZNS-1 Nassau, Bahamas

1550 silent[11] Windsor, Ontario

1550 XERUV Xalapa, Veracruz

1560 KNZR[12] Bakersfield, California

1560 WQEW New York, New York

1570 XERF Ciudad Acuña, Coahuila

1580 CKDO[13] Oshawa, Ontario

Alaskan class A (former class I-N) stations Freq.

(kHz) Callsign City of license

640 KYUK Bethel

650 KENI Anchorage

660 KFAR Fairbanks

670 KDLG Dillingham

680 KBRW Barrow

700 KBYR Anchorage

720 KOTZ Kotzebue

750 KFQD Anchorage

770 KCHU Valdez

780 KNOM Nome

820 KCBF Fairbanks

850 KICY Nome

890 KBBI Homer

1020 KOAN Eagle River

1080 KUDO Anchorage

1170 KJNP North Pole

FM Radio

FM (Frequency Modulation) radio was invented my Major Edwin H. Armstrong, one of radio's true pioneers and visionaries. He received the patent for Wide-Band FM on 26 December 1933, although widespread use of FM didn't really catch on until the early 1950's. FM radio is virtually immune to static and noise from natural sources, and most man-made interference. As a result, it's well suited to broadcasting High Fidelity music. It's also capable of excellent voice reproduction with a smaller bandwidth than an AM signal, making it the choice for Public Service agencies, like Police and Fire Departments, who NEED clear, easy to understand voice on their radios.

If there's a “problem” with FM radio stations, it's that their range isn't as great as AM stations.

This has nothing to do with the type of modulation (Amplitude vs Frequency), but rather the frequency that the station broadcasts at. The 88-108MHz frequency range is pretty much limited to what's called “Line-of-Sight”, meaning that if you don't have a fairly clear path to the station, or it's over the horizon, you can't receive it.

This means FM stations are local in nature, and another good place to listen for local information.

In between the AM radio band that “hugs the ground” during daylight hours, called “Ground Wave”, and the FM/VHF/UHF bands that are limited to “Line-of-Sight” propagation, are the shortwave bands, and that's where we'll go next.

Short Wave Radio

Short Wave radio, also called “HF” for High Frequency, covers the frequencies from 3MHz to 30MHz. Frequencies below 3MHz, along with the AM broadcast band, are called “Medium Wave”, or “Medium Frequency”. Many old tube type radios had the bands labeled as such, calling them “LW” for Long Wave, “MW” for Medium Wave. And “SW” for Short Wave. Many of the better receivers had more than one Short Wave band, and they'd label them “SW1”, “SW2”, and so forth.

Besides being labeled with frequencies, short wave bands are also referred to by their wavelength, measured in meters. Thus, the 7MHz band is also called the “41 Meter” band, 11MHz to 12MHz is called the “25 Meter” band, and so on. The higher the frequency, the shorter the wavelength.

The types of modulation you'll find on HF radio are pretty much limited (for our discussion) to AM, and SSB. There are various digital modes used, but that's beyond this article.

Why bother with Short Wave, and have to learn a bunch of “new” stuff? Well, when the SHTF, local news may very well be heavily controlled (i.e. censored), or even “blacked out”. Having an alternative source, from many miles away, might actually be more useful. The BBC (British Broadcasting Corporation) was always good at providing unbiased news from around the world, along with CBC/Radio-Canada, and so was the VOA (Voice of America), who specialized in beaming Pro-American programming to other parts of the world.

Unfortunately, many of these radio services have been severely cut back in recent years, replaced by lower-cost streaming Internet “radio” services.

While a “Short Wave” radio can receive many different frequencies, broadcasters use specific frequencies agreed upon by international treaties, commonly called “Bands”.

The most commonly used shortwave bands are below.

120 m2300–2495 kHz tropic band

90 m 3200 – 3400 kHz tropic band

75 m 3900 – 4000 kHz shared with the North American Amateur Radio “80 meter” band

60 m 4750 – 5060 kHz tropic band

49 m 5900 – 6200 kHz

41 m 7200 – 7600 kHz shared with the Amateur Radio “40 meter” band

31 m 9400 – 9900 kHz currently the most heavily-used band

25 m 11,600 - 12,200 kHz

22 m 13,570 - 13,870 kHz substantially used only in Eurasia

19 m 15,100 - 15,800 kHz

16 m 17,480 - 17,900 kHz

15 m 18,900 - 19,020 kHz almost unused, could become a DRM band

13 m 21,450 - 21,850 kHz

11 m 25,600 - 26,100 kHz may be used for local DRM broadcasting

Many, if not most, older radios that included shortwave coverage would add labels to the bands like “Mexico”, “London”, “Berlin”, indicating the approximate place on the dial where broadcasts from those locations could be found. They'd also label the bands “Evening”, “Afternoon”, “Morning”, or “All Day” to indicate what time of day these stations could be heard.

Why did they have the time of day listed? Well, it has to do with what's called Radio Propagation. Propagation is the term used to describe how radio waves travel through space. Propagation is a highly variable thing. How far shortwave signals will travel depends on what frequency they are, the time of day, what season it is, and where we are in the current sunspot cycle.

I'll cover Radio Propagation in another article.

Buying A SHTF/Emergency Radio

I could probably write an entire article on just this subject!

I suppose the first thing to ask yourself is “How much to I want to spend?”, and the second would be “How much radio do I need?”, along with “What do I want to listen to?”. A decent, reliable, easy to use radio will cost about $100, maybe less if you do your shopping carefully. Some of the more expensive radios are actually poorer performers than some of the less expensive ones. Do you want to be able to listen to Hams, or just International Broadcasters? Once you've decided on how much you want to pay, start looking at the reviews on eHam.net, under “Reviews/Categories/Receivers/General Coverage”. Keep in mind that 99% of these reviews are written by Hams, and what they decide makes a “Good” receiver is probably quite different than what you'll need. When reading reviews, I always like to look at the most negative reviews first. Some of them are hilarious, as in people who gripe about what color the radio is I also tend to steer away from a review that only has a few entries, compared to a review that has 20, 30, or more entries. Anybody can get a “dog” radio, just like any other product, and a glowing review score of “5” from one reviewer could also be a fluke.

As a general rule, I don't care for those hand-crank/solar-cell “Emergency” flashlight/strobe/cellphone charger/radios you see advertised everywhere. If it's all you can afford, by all means get one, but most of them are very limited in what they'll receive, and their pretty marginal in their sensitivity. They're OK in metro areas where there's lots of stations, but they're pretty weak out in the boonies.

I've had several of them, and was really disappointed in their performance, but then I guess I'm a little jaded about what a radio should be capable of doing.

I currently own a Grundig G3 “Globe Traveler”, and a TechSun PL-660, along with some other, older radios. Both of these little guys are available for under $100, fit in your coat pocket, and besides covering AM and FM, they also have “Air Band” coverage, and continuous tuning from 150 kHz to 30 MHz. They also both have a TON of memories to store favorite frequencies in, a Synchronous Detector for helping to eliminate what's called “Selective Fading”, and a BFO (“Beat Frequency Oscillator”) which allows them to receive Single Sideband (“SSB”) signals from Amateur Radio operators, and various utility stations in the MW and SW bands.

They both run on 4 “AA” batteries, and will charge NiMh batteries with their included AC adapter/charger, or run just fine on alkaline or Lithium batteries.

These things are absolute marvels of Engineering. They receive more things, and do them better, than my first Amateur Radio receiver.

For larger, “base” or “table top” type receivers your options for buying new are quite a bit more limited, simply because most manufacturers stopped making medium range receivers. Currently, Icom sells the IC-R75, a very good little radio, and Alinco sells the DX-R8T, another decent radio. Most of the other radios available are either computer-controlled, or very high-end models that start at $800, and go up from there.

I won't go into buying older radios, or tube-type radios as that's beyond the scope of this introduction. If you're really into vacuum tube gear, you probably already have some, and are already ahead of the curve on this.

For good reviews on the currently available, small, SHTF radios, check out the reviews pages on eHam.net under “Reviews Categories>Receivers:General Coverage”.

Some Basic Tips Not Otherwise Covered.......

If you're not going to be using the radio for a while, like when you store it,

====> TAKE THE DAMN BATTERIES OUT! <====

One of the most depressing things is to get your SHTF or “emergency” radio out, and not only finding dead batteries in it, but seeing all the glop those dead batteries have leaked all over the battery compartment, corroding everything metal within sight. Yes, you can clean them up if they're not too badly corroded, but why take the chance in the first place?

Your radio will thank you. I just checked the batteries in all of our remote controls, and was ready to kick myself in the rear end.....ALL of them had gone dead and leaked!

Looks like I'll be spending some time cleaning up a whole lot of little battery contacts in the near future.

BTW...”TARN-X” silver cleaner works wonders at getting minor battery corrosion off the contacts. Brush the contacts with a small brush to get the big chunks off, and then apply the TARN-X sparingly with a cotton swab. You'll see any residue foam up, and the green corrosion will disappear. Flush any remaining residue away with a swab and some clean water. I've done this several times on equipment that had dead batteries left in it, and it really cleans up the contacts. If the contacts are really bad and have to be replaced, you might be able to find replacement contacts at Mouser, DigiKey or Keystone Electronics.

Batteries- Try and buy a radio that uses a “common” battery size as the rest of your SHTF gear. If your radio is the only piece of gear you have that uses “C” or “D” batteries, that's one extra battery size you'll have to stockpile. Think of it like you do ammunition calibers, and try to 'standardize' one more thing.

NiCad and NiMH batteries are good in that they can be recharged, BUT they only put out 1.2 Volts per cell, while alkaline and lithium batteries put out 1.5 Volts per cell. If your radio takes four batteries, the NiCad and NiMH cells will only have 4.8 Volts instead of 6 Volts.

Will this “hurt” the radio? No, but you'll probably see the “Low Battery” display all the time, and if the device has a low battery shutoff, the batteries might not power the device very long before it shuts off even though the batteries have plenty of juice left in them!

Lithium batteries have very long shelf life, and are capable of putting out a burst of high current, which is nice for things like flash units, or walkie-talkies, but cost considerably more than alkalines.

And BE AWARE that Nicad and NiMh batteries require different charging methods! Most of the inexpensive chargers for sale will either have a switch to select which type of battery is being charged, or will auto-detect the battery type. Mixing battery types in any device is a BIG no-no, and can result in damage to the device, or in extreme cases, a fire.

If your SHTF radio has a connector for “External Power”, see if you can get a cable or “Cigar Plug/Cigarette Lighter Adapter” that will let you power it from 12 Volts DC.

Some people will ask about how to protect a solid-state (Transistorized) radio from an EMP event. I keep both my little Grundig G3 radio, and my Elecraft K2 HF transceiver in metal ammo cans. The Grundig fits into a small 30 cal can, along with some extra AA batteries, and the AC power supply. The K2 is a bigger radio, so I store that one in a larger 20mm can. If you're really paranoid about EMP, then seal the edges of the cans with some aluminum tape (NOT duct tape), making sure that all the seams are covered. I also keep a power supply for the K2 in a separate can, although if we ever have an EMP event, the power grid will most likely go down, so the power supply might very well be useless.

And I always toss a couple of desiccant bags in the cans before I close them.

If you're really that concerned about having some radio gear survive and EMP, then look in to getting some older gear that uses vacuum tubes. There are some vacuum tube General Coverage receivers that run on batteries, but the replacement battery packs haven't been available for DECADES. The Zenith Trans-Oceanic is one, Hallicrafters made a few different models, and I'm sure there are others. You'll have to cobble together your own battery packs, and that's WAY beyond this article, and you'll have to pay “Collector” prices for these sets.

Antennas- Most of the newer radios have acceptable sensitivity with the built-in whip antenna. If you think you need more, you can string some insulated wire up, and either wrap a few turns around the whip (AFTER you stripped the insulation off!), or use a clip-lead to attach it to the whip. A lot of the new little radios have a jack to connect an external antenna to, but be careful! Some radios will overload with an external antenna, especially if you live near a high-power radio station. My little Grundig G3 has a “Local/DX” switch that adds some attenuation to the antenna circuit, making it less sensitive, and cutting down on overloading when used with an external antenna.

The rule-of-thumb for short wave antennas is get it as high as you can, and make it about 50' long. Going much over 50' will pick up more noise than signal, at least in an urban area. Just be careful that the antenna is well clear of any power lines, and that if it breaks, the pieces of it won't fall on any power lines.

Don't worry about getting the antenna length matched to the wavelength you want to receive. This is more important for transmitting than receiving, and since most of the portable radios we're concerned with are made to use the somewhat less than optimum whip antenna that's built in to them, adding some extra wire that's up and in the clear is the important thing.

Headphones- Ah yes, a good set of 'cans' for those times when you don't want to disturb others, or don't want anybody else to hear what you're listening to, or that you're listening at all. I prefer the 'over-the-ear' type that cover the entire ear, and seal out background noise. There might be times when 'ear buds' are appropriate, but for digging out really weak signals (I'm talking “ESP weak”!), NOTHING beats a good set of headphones. Watch out for the “open back” type that seal to your head, but allow sound to radiate out the back of the earpieces. And you don't need $600 “audophile” quality 'phones. Human speech covers roughly 20Hz to 20kHz, and unless you're young, or have exceptionally good hearing, you probably can't hear 20kHz anyway. Radio Shack has some decent ones for under $50, as does Best Buy.

We're looking for “Communications Quality” audio here, NOT something to listen to a $3000 stereo system with!

Other Resources

Monitoring Times Magazine- One of the best out there. Covers everything in this article, along with beginner to expert articles and columns. HIGHLY recommended.

http://www.monitoringtimes.com/

Nuts & Volts- A very interesting magazine. Covers a wide range of topics, and has absolutely KILLER ads for the hobbyist/experimenter.

http://www.nutsvolts.com/

Popular Communications Magazine- I haven't read this in years, so I can't vouch for it. Some people love it, some hate it.

http://www.popular-communications.com/

Dxing.com- Home of the “Modern Shortwave Receiver Survey”. Also has ratings for older radios.

http://www.dxing.com/rx.htm

QST- The official magazine of the American Radio Relay League (AARL). This is geared towards Amateur Radio (“HAM” Radio), but has wide variety of articles. Considered one of the premiere radio magazines in the world.

http://www.arrl.org/

eHam.net website- Has product reviews and forums geared mostly towards Ham Radio, but has forums for Shortwave listeners.

http://www.eham.net/

Radio Reference website- For scanner users, this is a MUST website! They have current, ACCURATE databases of all non-military radio frequency assignments in the U.S., and excellent forums. Several of the applications used to program modern “Do Everything” scanners are capable of extracting the data from the Radio Reference website, and will send it directly to your scanner, saving HOURS of time entering frequencies by hand.

If you have a question about scanners, you'll probably find the answer here.

http://www.radioreference.com/

Phil's Old Radios- A marvelous site to learn about restoring old radios, and admire his patience and craftsmanship.

http://antiqueradio.org

RadioIntel website- A nice site with tons of info and links to other websites.

http://www.radiointel.com/ref.htm

Ham Radio Outlet- Sells new and some used radios. Also sells antennas and other neat stuff.

http://hamradio.com/

Amateur Electronic Supply- One of the oldest places around for buying new and used gear.

http://www.aesham.com/

Universal Radio- Another seller of new and good quality used radio equipment, books, and accessories,

http://www.universal-radio.com/index.html

Glossary

AC: Alternating Current. A current which changes polarity (or direction if that makes it easier for you to understand) with respect to time. The voltage out of your wall socket is A.C.

AM: Amplitude Modulation. A form of modulation in which the information applied to the carrier wave causes it to change in amplitude. An AM radio signal consists of a Carrier Wave, and two Sidebands, one above, and one below the carrier wave.

Amateur Radio: A NON commercial radio service used by licensed individuals for message exchange, experimentation, self-training, and emergency communications.

Antenna: A device for capturing radio signals from the air. Can be a length of wire, a “whip”, a “discone”, or a directional antenna, like the old TV antennas we all used to have. Generally speaking, antennas are most efficient when their physical length corresponds to the electrical wavelength of the signal being received.

BFO, or Beat Frequency Oscillator: A circuit in a radio receiver that allows it to receive Single Side Band radio signals, and properly demodulate them.

Carrier Wave: A radio signal without any information applied to it. Sometimes just called a “carrier”, or “dead carrier”.

Current: The “flow” of electricity through a circuit, measured in Amperes. Think of it just like the flow of water in a hose.

DC: Direct Current. A current which does not change polarity, or direction. Your car battery (really, ANY battery) is a source of Direct Current.

Demodulation: The process of recovering the information that was put on to a radio wave.

DRM: Digital Radio Mondiale. A digital audio broadcasting format used in certain shortwave bands. It requires special equipment to receive, but has excellent (“FM Quality”) audio quality. Most receivers can be easily modified to allow a PC sound card to decode the audio.

FM: Frequency Modulation. A form of modulation in which the information applied to the carrier wave causes its frequency to change.

Frequency: The number of times per second an electromagnetic wave changes polarity, expressed in “Hertz”, abbreviated “Hz”. One thousand Hertz is one kiloHertz, kHz, and one million Hertz is one MegaHertz, MHz. Frequency and wavelength are inversely related. The lower the frequency, the longer the wavelength, and the higher the frequency, the shorter the wavelength. The AC power out of your wall socket changes polarity 60 times per second, so it's called “60 Hz A.C.”

Ground Wave: Radio waves that primarily travel along the surface of the Earth.

Ionosphere: A region of the upper atmosphere, consisting of charged (“ionized”) particles starting at approximately 50 miles above the surface of the Earth, and extending out to approximately 375 miles. The particles are ionized by energy from the Sun. The ionosphere is divided into layers, with the “D” layer being the lowest, and the “F” layer being the highest. The heights of the layers, their thickness, and their density, determine which radio frequencies are reflected back to Earth, and which are absorbed. The layers all change with the time of day, the seasons, and the sunspot cycle.

Line-of-Sight: A term used to describe the “path” that a radio signal follows. This is determined mostly by the frequency used. Lower frequencies, like those used for AM radio, tend follow the nap of the Earth better than the higher frequency signals like those used for FM and Television. An AM radio station can be heard well past the “Line-of-Sight” to it, where you need a “clear shot” to a TV or FM station to hear it well.

Modulation: The process of putting information (voice, music, pictures, data) on to a radio wave. The two most common forms are AM, where the amplitude is varied, and FM, where the frequency is varied.

NiCad: Short for “Nickel Cadmium”, and early type of rechargeable battery. Still pretty good for uses that require a burst of high current, like for transmitting. They suffer from the “Memory Effect” which means if you don't FULLY run them down before you recharge them, they'll 'remember' they still have a little charge in them, and won't charge back to to a usable 100%. Most modern NiCads, and modern chargers, can overcome this, but be aware of it.

NiMH: Short for “Nickel Metal Hydride”, and improved type of rechargeable battery, similar to a NiCad, but using a different metallic compound in place of Cadmium. Much more forgiving then NiCads on charging/discharging, and pack more useable power in the same size than NiCads.

Radio: The art and science of using electromagnetic radiation to communicate or exchange information over long distances.

Receiver: A device that converts radio signals into a usable form. The output can be video, as in a TV set, audio, as in a radio, or data, as in a radio modem.

Selective Fading: Also called Frequency Selective Fading. A type of signal distortion where one of the sidebands of an AM radio transmission becomes attenuated more than the other sideband.

Single Side Band, Single Sideband, or SSB: A special type of AM radio signal that eliminates one of the sidebands, and the carrier wave. This allows approximately four times the transmitter output power for a given size of amplifier in the radio.

Sky Wave: Radio waves that do not primarily travel along the surface of the Earth, but go into space, to be reflected back by the ionosphere. Depending on the frequency, the signal may be reflected back to Earth a few hundred miles away, or many thousands.

Synchronous Detector: A special type of demodulation circuit used to minimize the effects of selective fading.

Trunked Radio System: A type of radio “Party Line” where all radios receive a “Control Channel” along with the regular channels programmed in to them. When one radio calls another radio, the Control Channel tells both radios what frequency is clear to use, and they both tune to it automatically. This allows multiple groups, like Police, Fire, and Civilian users to share the same radio system. It can allow the Police and Fire to inter-communicate, while keeping the Civilians “locked out” of those channels. If you hear about big cities having major problems with their radio systems, it's a Trunked System!

Voltage: The “pressure” that drives the electricity through a circuit, measured in Volts. Think of it as the pressure of the water in a hose.

Wavelength, or Wave Length: The length, generally stated in Meters, of a radio wave. A “40 Meter” (7 MHz) radio wave is about 134 feet long, and a “10 Meter” (30 MHz) radio wave is about 31 feet long.
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