Yagi design involves a compromise between forward gain, F/B and SWR. The
best gain occurs at a different setting than the best F/B and obtaining a
flat SWR curve effects the F/B and perhaps also the gain. Personally, I
adjust my antennas for the best gain while maintaining a reasonable SWR and
F/B ratio and I've been happy with the results.
John KK9A / P40A
To: "Geoffrey Way" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Reflector element question...
From: "Stan Stockton"
Date: Fri, 10 Sep 2010 16:18:19 -0500
Geoff,
The length and the diameter and length of each diameter (taper schedule)
will affect the performance. You would not want to look at the overall
length dimensions for elements of a 6m design in the handbook and then
use tubing of the same length that is twice the diameter. With the
right dimensions, however, you can make what you have work well -
perhaps even better than what you would have had using the smaller
diameter tubing the article calls for. I have no idea where that
particular antenna has its best gain or F/B but you can certainly
adversely affect both by making the reflector a different electrical
length than what the design calls for.
I just looked at a design of a 5 element 6M Yagi that had 31+ dB of F/B
at 50.1. Increasing the length of the reflector by 2 inches overall
dropped the F/B by about 6 dB. Generally speaking increasing the length
will help the F/B at a lower frequency than where it was previously
best. For example, while it dropped 6 dB at 50.1 it got better at 49.9
by about 4 dB.
Stan, K5GO
----- Original Message -----
From: "Geoffrey Way" <wayg@cape-vision.com>
To: <towertalk@contesting.com>
Sent: Friday, September 10, 2010 3:14 PM
Subject: [TowerTalk] Reflector element question...
> I'm converting an old Moseley CL-33 tribander into a 6m beam.
> It has a 12 foot boom, so I'm following the suggested layout in the
> ARRL antenna book for a 5-element design shown on p. 18-27 of
> the 2007 (21st) edition.
>
> My question is, if I have some extra length on the reflector element,
> would I get any additional front to back ratio if it were longer? And,
> is there a length that becomes too long, having some adverse effect?
> Or is it just a case of no further benefit? Perhaps some other effect
> that I am unaware of?
> --
>
> Geoff Way, KA1IOR
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|