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Re: [TowerTalk] Motor Driven vac used in 40M DE?

To: "Jim Walker" <>, "'Jim Thomson'" <>
Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?
From: "Gene Fuller" <>
Date: Tue, 29 Dec 2009 11:42:27 -0500
List-post: <">>
All the vac cap in the driven element does is reduce the coax loss caused by 
a little hhigher SWR, whereas a tuner at the station end may find additional 
uses beyond the 40 meter application. Also, putting the vac cap in the 
driven element creates a hard to get to additional possible failure  point.
Gene / W2LU
----- Original Message ----- 
From: "Jim Walker" <>
To: "'Jim Thomson'" <>
Cc: <>
Sent: Tuesday, December 29, 2009 11:17 AM
Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?

I have the two-element version of the same Force 12, 40 meter antenna.

I tuned mine in conjunction with the original instructions which provided
for its best performance...both SWR and F/B.  My lowest SWR is found at
7.050 and am able to work between 7.0 mhz to about 7.1 mhz without a tuner.
Beyond 7.1, such as when working split DX SSB, I use my Palstar tuner.

I still have directivity in the SSB portion of the band, but not as much as
when I am the lower portion of the band.

Conclusion:  I agree with Rick.  While you do eliminate the need for a tuner
by using the Jennings cap, to retain full F/B performance, you need to
adjust the dimensions of all three elements.


-----Original Message-----
[] On Behalf Of Richard (Rick)
Sent: Tuesday, December 29, 2009 9:47 AM
To: Jim Thomson
Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?

Jim Thomson wrote:
> Has anyone tried this b4 ?    I have a F-12 340N, used, of recent vintage.

> OK... here's the deal.     I'm seriously thinking of installing a
Jennings  ceramic  10-1050 pf  @  15 kv  motor driven vac cap in series with
the hot leg of the DE.
> On paper, after doing the math's... this should work... and result in a
flat swr across  the entire band.    The stock swr curve is ok.... but I
want to be able to operate at the opposite ends of the band.   F-12
measures the swr curve  in the shack, using the FT-1000MP's  internal  SWR
meter.... and  400' of  LMR-400  to the yagi.     The real  swr curve,
measured at the DE, is of course, higher.  [worse]
> Have any of you tried this b4.. or seen it done ?    The idea was to tune
the DE to the bottom of the band... or even a little lower.   Then  with
1050 pf dialed in.. [ 21.7 ohms Xc]..... swr should be flat  at  7003 khz.
As I move up the band, less pf required. .. [ more XC]

I did some antenna range tests with my MonstIR, varying the driven
element length while leaving the director and reflector fixed.
You can improve the SWR bandwidth doing this, but the gain and
F/B drop off as you get away from the optimum frequency
to which the parasitic elements are tuned.  You really need to
tune all 3 elements as is done in the MonstIR.

Rick N6RK

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