[TowerTalk] Directivity and F/B Ratio was Re: FW: Force 12

Jim Thomson jim.thom at telus.net
Tue Apr 15 12:37:38 EDT 2014

Date: Tue, 15 Apr 2014 06:11:45 -0400
From: Pete Smith N4ZR <n4zr at contesting.com>
To: towertalk at contesting.com
Subject: [TowerTalk] Directivity and F/B Ratio was Re: FW: Force 12
Delta 240 2 Elem 40M Yagi

Just one small addendum. If you set the electrical length of the 
reflector at, say, 7200 KHz, you may well find that the antenna reverses 
direction down at the low end of the band, presumably because the 
reflector is so much too short that it becomes a director at that 
frequency.  I found this experimentally by using a remote signal source 
(an MFJ259B) a few hundred feet away to determine the point of maximum 
F/B, and then moving it to 7050 where it would not reverse until well 
below the band.  N6BT points out in Array of Light that the resonant 
frequency of the driven element is only important for matching and 
losses, not for gain or F/B.

73, Pete N4ZR

##  Put a relay box on each ele. Then you can resonate the eles wherever you want to.
I tossed the F12 junk box with the 12 ga thnn wire and dpst relay..and replaced it
with  3 vac relays and made each coil from .75 inch wide silver plated cu strap.
Each coil is  aprx .5 uh.  Coil #1 is tapped at the .33 point..and also  between the 
.33 point and the far end of the coil.    Coil #2 has only 1 relay across the entire coil.
Then I end up with a total of 7 x segments.   7000,  7050, 7100, 7150, 7200, 7250, 7300  

## Problem solved.   Then you can have your cake..and eat it too.   I got the idea from
Steve Babcock,  VE6WZ...u can see it on his website.  

Jim  VE7RF

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