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

To: <towertalk@contesting.com>
Subject: [TowerTalk] Common Mode RF choke on a 50 MHz yagi
From: "Jim Thomson" <jim.thom@telus.net>
Date: Wed, 1 Jul 2020 10:27:28 -0700
List-post: <mailto:towertalk@contesting.com>
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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