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.
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.
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 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 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 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 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 Windsor, Ontario
1550 XERUV Xalapa, Veracruz
1560 KNZR Bakersfield, California
1560 WQEW New York, New York
1570 XERF Ciudad Acuña, Coahuila
1580 CKDO 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 (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!
Monitoring Times Magazine- One of the best out there. Covers everything in this article, along with beginner to expert articles and columns. HIGHLY recommended.
Nuts & Volts- A very interesting magazine. Covers a wide range of topics, and has absolutely KILLER ads for the hobbyist/experimenter.
Popular Communications Magazine- I haven't read this in years, so I can't vouch for it. Some people love it, some hate it.
Dxing.com- Home of the “Modern Shortwave Receiver Survey”. Also has ratings for older radios.
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.
eHam.net website- Has product reviews and forums geared mostly towards Ham Radio, but has forums for Shortwave listeners.
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.
Phil's Old Radios- A marvelous site to learn about restoring old radios, and admire his patience and craftsmanship.
RadioIntel website- A nice site with tons of info and links to other websites.
Ham Radio Outlet- Sells new and some used radios. Also sells antennas and other neat stuff.
Amateur Electronic Supply- One of the oldest places around for buying new and used gear.
Universal Radio- Another seller of new and good quality used radio equipment, books, and accessories,
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.