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Re: [TowerTalk] Common Mode RF choke on a 50 MHz yagi

To: Jim Thomson <jim.thom@telus.net>
Subject: Re: [TowerTalk] Common Mode RF choke on a 50 MHz yagi
From: Dave Sublette <k4to.dave@gmail.com>
Reply-to: k4to@arrl.net
Date: Wed, 1 Jul 2020 13:50:31 -0400
List-post: <mailto:towertalk@contesting.com>
The MFJ854 is rated from 1-30 MHz.  Will it work on 5 MHz?  How high can we
"fudge"?

Dave, K4TO

On Wed, Jul 1, 2020 at 1:28 PM Jim Thomson <jim.thom@telus.net> wrote:

> Date: Sun, 28 Jun 2020 15:10:18 -0700
> From: "Richard (Rick) Karlquist" <richard@karlquist.com>
> To: k4to@arrl.net, jim@audiosystemsgroup.com
> Cc: kj6y--- via TowerTalk <towertalk@contesting.com>
> Subject: Re: [TowerTalk] Common Mode RF choke on a 50 MHz yagi
>
> <The simplest way to make a 50 MHz common mode
> <choke is to use 56 inches of coax and connect
> <one end to the feedpoint and connect the shield
> <of the other end to the boom.  Then connect
> <your regular feedline to the end of the
> <56 inch cable. Let the coax hang down in a
> <lazy "U" away from the boom.  Whatever you do,
> <don't be "neat" and tape it to the boom.
>
> <I suspect this configuration is somewhere
> <in the ARRL VHF handbook.
>
> <73
> <Rick N6RK
>
> ##  This is common mode, not  differential mode.
> So its a physical one quarter wave, not an electrical
> quarter wave.
>
> ##  246 /  50.125 mhz = 4.9077  feet long.
> =58.892 inches long.
>
> ##  With coax taped to boom, there is now capacitance
> between braid of coax..and  boom.  This is easily  measured with
> any digital LCR meter.   The CM balun  now has to be  2-3%  shorter.
> =  57.125 inches..... to    57.71 inches  long.   It might even have to be
> shorter.  Using Ricks  56 inches,  thats a 5%  shortening.  No  loop
> required,
> except for normal rotor loop.
>
> ##  Then the Z is sky high at the feed point of the insulated DE.  No
> requirement  for a series  string of   torrid CM chokes.
>
> ##  To test, use a clamp on  RF  ammeter, like a MFJ-854  or  similar.
> The  854  will read CM current  down to as low as 1 ma...or less.
> The 854 is superb to see if a CM choke is really working, or if you want
> to compare different CM chokes.   Just make sure  you measure at the same
> place
> in the coax..downstream.  Mark the coax with some painters green tape
> etc.  You
> can also measure  CM current in  several places if you want. Take the
> readings in
> several marked  places, swap CM chokes,  then they will  either rise, stay
> the same, or fall.
>
> Jim  VE7RF
>
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