Wednesday, July 25, 2018

Antenna Planning

After looking over all the various antennas that I feel will comfortably fit on our lot atop a 30' tower, I've pretty much narrowed it down to a Mosley TA-53-M. This antenna has a 14 foot boom (the big pipe that the elements mount to) with a longest element of 27 feet that won't over power the lot the house sits on. It weighs 55 lbs fully assembled, and has a wind load of 6.7sqft.

This is a conventional 3-element Yagi-Uda design, with three elements on 10, 15, and 20 Meters, and also three elements on the 17 and 12 Meter bands. These last two bands are called the "WARC Bands" after the World Administrative Radio Conference held in 1979 which made them a world-wide Amateur Radio frequency allocation.

The 17 Meter band, which runs from 18.068 to 18.168MHz, is particularly important for me, as it's one of my favorite bands to operate. It's a "Gentleman's Band" where people hang out to make contacts and actually stick around and chat for a while. Propagation is similar to both 20 Meters (14.000~14.435MHz)  with the band being "open" quite often and 15 Meters (21.000~21.450MHz) in that as you approach 30 MHz, atmospheric noise start to decrease, making for a "quiet" band where weak signals can be more readily heard. And by "Gentlemen's Agreement", NO contesting takes place on the 12 and 17 Meter bands, which makes them a nice refuge for non-contesters on contest weekends.




I was considering it's Big Brother, the TA-54-XL which has four elements per band, but the 21' foot boom and 29-1/2 foot longest element and 85 lb assembled weight with an 8.7sqft wind load kind of puts me off........


And the Big Bad Boy Pro-57-M with it's 24 foot boom is definitely too big for the yard. Interestingly, the longest element on this one is also 27-1/2 feet. It weighs in at 92 lbs all-up with an 11sqft wind load.





One other thing to consider in picking one of the three is what's called the "Wind Load" of the antenna. This is basically the square footage the antenna presents to the wind, and the bigger the antenna, the stronger the supporting structure (tower, roof, or building) has to be.

Out here the wind load can be more important than keeping the dead weight down. The 2 Meter antenna I'll be putting at the top, an M2 2M9ssb, has a minuscule wind load of 1.2sqft.

I can't find the windload figures for the Alfa-Spid rotor I'm planning on using, so I'll guess 2sqft.





These are worm-gear driven rotors, and not the "Bell Rotors" of days gone by. These don't need a mechanical/electrical brake to hold the antenna in place so it won't windmill.


The tower I'm planning on buying, a "Universal Towers" model #35-30, is a 30 foot, free-standing tower. One thing I want to do is keep it small enough (height and wind load) and robust enough (good design and fabrication) so that I don't need guy wires/ropes. If you use metallic wire, you have to make sure no individual wire comes close to being a resonant length, otherwise they guy wires will strongly interact with the antenna causing myriad problems. In Ye Dayes Of Olde, you used big honking ceramic insulators to break up the lengths so none of them would resonate and act like an antenna, but these days synthetic materials have replaced good old "Stainless Steel Aircraft Cable" or even older galvanized wire rope.

ANYWAY...this tower is rated for safely supporting 34.5sqft @ sustained 80MPH winds, 25sqft @ 100MPH, and 19sqft at 110MPH.

With the M2 2M9SSB at the top, the mast, and the rotor/cabling I'll have about 12sqft hanging in the breeze on the tower, well within it's capability.

10 comments:

  1. Your figures sound about right, from what I can recall of my days in the County Planning Dept, years ago. And I'm sure your details are more up-to-date than my fuzzy memories. Go for it!

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  2. The only building code provision I have to worry about is the mandate that if it comes down it has to land on my own property.

    Stunningly simple concept of personal responsibility allowed by law.

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    1. I wasn't referring to the building code, but that's correct. The base of any tower was required to be its own height from the property line, plus 10 feet. What I was referring to is your calculation of the wind loads, and the prudent decision at which you arrived. Well done.

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    2. yuk-yuk.... just apply my "150% overdesign rule", and go from there.

      I'd rather spend another $1k on the initial installation of a "robust" tower than buy a smaller tower and have to spend at least that much on synthetic guy ropes, along with the special hardware you to belay the ends and tension it.

      And then you have to start worrying about the condition of the guying ropes after 5 years or so....no thanks! I've seen way too many towers come down when a guy fails.

      Once one fails, it cascades astoundingly fast and you're soon left with a pile of very expensive junk steel and aluminum.....

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  3. Replies
    1. Depends on what you mean by "back-up power".

      I have a 2kW Honda generator, and 200 Watts of solar panels feeding a 150 AHr Optima "Yellow Top" deep-cycle battery.

      The solar stuff will let me keep my portable/"emergency" radio gear online indefinitely. The generator would be used sparingly "as required". I only have about 10 gallons of fuel, plus whatever we could siphon out of the cars.

      The in-laws have generous amounts of gas and diesel, properly stored, for their construction and farming/ranching activities that we can draw on if needed.

      Most of my radio gear can run on "12 Volts", and this particular antenna rotator has a DC motor, so I'll even be able to rotate the big antenna if I need to.

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  4. Looks like a very good setup, and a Mosley TA-53-M is great as the anchor of the station. My setup is similar, except for having the tilt over capability. I ordinarily only do that for hurricanes, so probably more of a concern for me than you, but it's handy for maintenance or repair. Like now, when my Yaesu rotator is behaving badly.

    My upper antenna is a 4 elements on 6m, but I've been thinking of replacing that with a 50-500 LPDA. I currently don't use 2m and 432 very much, but part of that is not having a good antenna for them.

    When I sold my Cushcraft A3SS and switched to my HF LPDA, it was unusual to have gain on 17m. It's a great band.

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    1. The only thing "bad" I've read about this antenna is that it's deficient in F/B ratio. When they designed this on a 14' boom, they optimized it forward gain, and not F/B. So while it has very good gain (6.7~7.9dBd), it "only" has 8~10dB F/B.

      The Pro-57 blows the TA-53 away, but I can't justify swinging a 24' boom on a "suburban" lot.

      And besides, the Pro-57 really deserves to be up at 40' or more, and that's out of the question here.

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    2. So while it has very good gain (6.7~7.9dBd), it "only" has 8~10dB F/B. Physics is a bitch. She simply will not compromise with us.

      A while back, I got the idea that I wondered if I could make my 15-30 MHz short Log Periodic (12' boom) work on 30 meters by trapping the back element.

      Lots of software and time was involved. EZNEC for modeling - first I had to come up with a model for what I have, then calculate traps, then model the new version. The end result was, yeah it could be done, but the whole thing is compromised by being too short. Too much signal going up. I don't think it would be worth the effort.

      Someone once told me "if you suddenly get a chunk of money and you have to decide on a better antenna installation or better radio, always go with the antenna" May not be true if you're using a piece of junk, but it sure is solid advice.

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    3. Yep, the consequence of going for max forward gain on such a short boom resulted in a major sacrifice of F/B.

      And just like tracers "work both ways", so do antennas. An investment in antennas always pays big dividends "both ways", in transmit and receive.

      This will be fed with some new 1/2" Heliax from my stash, with either DavisRF "BuryFlex" or DX Engineering DX400MAX coax doing the rotor jumper and inside-the-shack duties.

      The 2M9SSB will get fed with 7/8" Heliax and similar jumpers. I'm also putting one of my 2M SSB Electronic low noise preamps on it, so I should have a pretty nice 2M weak-signal station.

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Keep it civil, please....