Kent,
Thanks, good tips for antenna builders.
But it all boils down to what one wish to do with the antenna, as far as LEO’s are concerned getting the max from a Yagi antenna is not that important. So Tony does not have to worry to much about the boom.
I agree with your observations and also about Mr LFA claims, as I studied his designs and build a few and found also many flaws that he tends to minimise. However to his credit it is a superb antenna concept.
My philosophy is to avoid (as much as practically possible) any metallic object except the elements as far as possible which makes the design as close as possible to the existing models prediction. Actually I am building a 3M dish and the feed support is all fiberglass, using resins and epoxy glue. Sure the coax or waveguide effect cannot be avoided but taking it at the end for Yagi's and in the centre for a dish minimise negative effect of same (which is quite difficult to model or predict, at least for me).
End-mounting is the best for Yagi’s and I am presently building an array for 70cm (inspired from the Cal-Poly EME experiments) with this configuration, but this is not possible for the long VHF LFA I am also building (7m long).
In my opinion it is all compromises and depending on what you want to achieve within your environment, means, available material in the hardware and building skills, there is no silver bullet.
But what a fun building your own antennas and get better result than commercially available ones at much lower cost.
73
Jean Marc (3B8DU)
On Nov 27, 2020, at 10:26 PM, KENT BRITAIN via AMSAT-BB amsat-bb@amsat.org wrote:
Hi Tony: Thanks for giving me a topic for my next CQ Antenna Column. This topic comes up every few years and also affects the hams who mount a vertically polarized Yagi on a vertical mast. On the antenna range you quickly see that metal (or even plastics) near the tip of a Yagi element moves it down in frequency. When I have a Yagi on the range, I put a small bit of Yagi element on a stick and hold it near each element while watching gain. Get the metal near the tip and gain goes up, Element too Short!Get the metal near the tip and gain goes down. Element too Long! This lets you do a quick look at how well the Yagi is working. So it is the supporting mast near an element tip that does the detuning.With a non-conductive mast what do you do with the coax? Did you run it back down your fiberglass mast? You just #@$%! your non-conductive mast with a conductor. Yes, I did measure a few tenths of a dB degradation, but the alternative is a long loop of coax out the back of the antenna. How much loss do you think is in that loop? So mounting the Crossed Yagi so that both sets of elements are 45 deg to the mast and midway between element sets is the 2nd best solution. Best solution would of course be end mounting. But that his it's own problems if you have a lot of elements! Coax can be run back along the Yagi boom and along the mast with no measurable effects.
Also extend the mast just enough for the antenna clamp to get a good grip. That is, have the mast extend only an inch or so beyond the clamps.
Yes, had an interesting talk with Mr. LFA at Dayton several years ago. I pointed out that the antenna he had on display could NOT have produced that polar plot! He was simplifying his model to get a cleaner plot. .
Kent WA5VJB/2E0VAA/G8EMY PS As an FYI don't think the solution is a Carbon Fiber boom. At Microwave Update I demonstrated a 915 MHz Yagi made using Carbon Fiber Rod for the elements. Gain about the same as Aluminum. Another FYI. Voyager 1 and 2 use a 10 foot Carbon Fiber parabolic dish antenna. They took the antennas out on the antenna range and they meet specification. As a weight saving they were NOT metalized.
Subject: Re: [amsat-bb] Fiberglass Cross-boom Alternative? To: Tony 73guddx@gmail.com, AMSAT-BB@amsat.org AMSAT-BB@amsat.org
Don't dismiss using a metal cross-boom. People say it won't work without actually examining the problem. When antenna guru Kent Britain, WA5VJB looked at the problem he found a metal boom to work quite well. He then tested it on the antenna range and verified it worked. You can see the paper he presented at an AMSAT symposium here: Using Metal Cross Booms w/ Satellite Antennas
When I had a permanent satellite station I used a metal cross-boom, and there were several others in the OSCAR-13 and AO-40 era that did.
73, Gary "Joe", kk0sd
-----Original Message----- From: AMSAT-BB amsat-bb-bounces@amsat.org On Behalf Of Tony via AMSAT-BB Sent: Tuesday, November 24, 2020 6:00 PM To: AMSAT-BB amsat-bb@amsat.org Subject: [amsat-bb] Fiberglass Cross-boom Alternative?
All:
Is there a cheaper non-metallic alternative to fiberglass cross booms? Or maybe a wholesale fiberglass rod distributor that has such a beast at a reasonable price?
Tony -K2MO | | | | Using Metal Cross Booms w/ Satellite Antennas
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