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[TowerTalk] problems tuning a F12 240N Magnum

Subject: [TowerTalk] problems tuning a F12 240N Magnum
From: Tom_N2SR <>
Date: Mon, 7 Dec 2009 13:14:19 -0800 (PST)
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Hello all, 

I'll try and be brief.  I'm having problems tuning the driven element of the 
F12 240N Magnum.  I've been slowing moving in the right direction because of 
other problems that F12 has been helping in taking care of, but the following 
is my latest concern: 

The VSWR is 1.7:1 @ 7.070 MHz, with the element tips at 34 inches.  The hairpin 
coil is 5T of #10 copper wire, close wound (about 1/4" spacing).   [The coil 
that was sent to me is a 4T coil, so in order to save time, I just made my 

The set up is 1 electrical wavelength of RG-213, and I have taken into account 
bead balun coax length (the balun is one of those DX Engineering piece of 
RG-142 coax with 100 beads over it).  The coax length is around 93 feet or so.  
The measurements are being made with an MFJ-259B, with an AC adaptor (so there 
are no low battery problems).  The element is being raise to at least 50 feet 
in the air, and maybe even as high as 60 feet.  .  The tower is 90 feet of Rohn 
45G guyed in two places (39 feet and 79 feet) with 6700# phillystran, so I am 
pretty sure there is no interaction with the tower guys.  The element at the 
measuring height sits away from the tower at least 6-8 feet.   

The manual says that the element tips should be around 25 inches.  As I said 
previously, mine are set for 34 inches (I used the reflector element tips).  
They are held in place with electrical tape for the time being.  

Questions.  Should I just continue and tune the element for a 1.1:1 VSWR and 
leave the tips where they are (and possabily adjust the tips if there is some 
interaction with tuning the hairpin) ?  

I have spoken with a few people who have had these antennas, and they all tuned 
each element separately, put the antenna together, and the VSWR was fine.   
Force 12 says that I should tune each element separately, then do the final 
tuning (of the driven element)  with both elements on the boom.  That is not 
going to be possible, or at least it's going to be extremely difficult.  

Since the coil is currently close wound (1/4" spacing), if I spread the coil a 
bit, that obviously means that the element wants more inductance.  However, 
I've never heard of anyone using a 6T coil (except on the Delta version).   

Should I put the element tips to 25 inches and start tuning from the beginning? 


Tom, N2SR


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