Using an antenna analyser to tune reflector stubs on a cubical quad? - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 15, 2020, 02:01:04 pm
content: I have most of a Gem two element cubical quad, bought used. The balun toroid is missing, as are the assemblies to make the tuning stubs that go on the reflector elements. I believe I can make an educated guess as to a suitable toroid size and mix, and I think the stub oblongs are 3 inches wide. I will try and upload the manual in a minute.Now, I have not heard of tuning a reflector before, only a driven loop. The driven loops on the Gem are preset by given wire length. But the reflector elements each have a loop of wire as a tuned stub. Instead of the reflector element being a continuous loop it is broken at the base of the square and two vertical legs from each side of the break go up 12 inches, join and return to the other leg of the square. By shorting along these stub loop each reflector element can be "tuned"The instructions say find someone to transmit a local signal on each of the 3 bands (10, 15 and 20 metres) and with the cubical quad turned away from the signal tune the stub of the relevant band for minimum signal.Now this sounds a nightmare, I don't know anyone local willing to send a carrier for ages on different bands, the antenna will be up and down and the tower cranked over like a Yo-Yo and it would take ages. Is there a way to use my AIM4170 antenna analyser to tune the reflector stubs? What am I looking for, a point of resonance? Or something else? I am not sure how a reflector should act... Most designs seem to have fixed reflector loop sizes and tune the driven elements. Thanks for any help!

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 15, 2020, 03:15:46 pm
content: In the manual (page 9) they say "The F/B ratio has quite an effect on SWR."so I wonder if you just can't tune for minimum SWR?

author: David Hess
author_rank: Super Contributor
timestamp: June 15, 2020, 04:47:41 pm
content: I used the top of a hill as an antenna tuning site for 2 meter beams. Even then, at least with the antenna I was designing which was optimized for minimum sidelobes, accurate tuning was limited by reflections from trees and light posts hundreds of yard away.I can imagine that an antenna designed for 10 meters and lower would be much more difficult because of access requirements. Is it possible to climb the tower to gain access to the reflector tuning and then do the testing while on the tower?

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 15, 2020, 05:43:29 pm
content: Quote from: David Hess on June 15, 2020, 04:47:41 pm15 years ago, maybe, but not now David, I have to admit some limitations these daysI feared it could be a PITA, what exactly is the reflector doing when "tuned", RF wise, anything an analyser on a refer to the antenna co-ax could define? Thanks for the replies so far!

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2020, 12:37:51 am
content: Quote from: Chris Wilson on June 15, 2020, 05:43:29 pmAdjusting the stub length changes the phase of the reflection as if the element was larger or smaller but without changing the element size which would be even more inconvenient. An antenna analyzer will see the effect on the impedance but I do not think that adds up to any method to tune for any characteristic except impedance.

author: bob91343
author_rank: Super Contributor
timestamp: June 17, 2020, 02:59:34 am
content: If you can bring the antenna down and work on it within reasonable reach height, you can tune it. For instance, at roof level. Then when you raise it to full height you will be pretty close.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 17, 2020, 07:11:33 pm
content: Right, thanks again for the extra answers. I think I can get it down to a height where ladders will allow me to fiddle with the feed point and the reflector stub (both at the base of the square elements), so that will help a bit!I would like to understand more how the reflector works, as a "boost" to signals to and from the driven element, and as an attenuator of signals from the rear, but struggling to find anything phrased in an easy to understand manner.Finally, the instructions say to trim the reflector for a minimal signal when the back of the quad is pointed at a signal source. Is this beteer or more sensitive than trimming for a maximm signal when pointing the front at a signal source, for some reason? What's the reason?Thanks again.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 17, 2020, 07:15:32 pm
content: Quote from: Chris Wilson on June 17, 2020, 07:11:33 pmI think these are 2 separate settings: one where there is maximum gain for wanted signals and one where there is maximum attenuation for unwanted signals, pick one.Like almost everything nothing comes free.