[amsat-bb] Fiberglass Cross-boom Alternative

[KENT BRITAIN]

 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 at gmail.com>, AMSAT-BB at amsat.org <AMSAT-BB at 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 at amsat.org> On Behalf Of Tony via AMSAT-BB
Sent: Tuesday, November 24, 2020 6:00 PM
To: AMSAT-BB <amsat-bb at 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 
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Using Metal Cross Booms w/ Satellite Antennas


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