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

[Edward Mccann]

Jim-

Referring to your comment:

> ##  This is common mode, not  differential mode.
> So its a physical one quarter wave, not an electrical
> quarter wave.


Can you refer me to a link to a technical description of when to use physical vs electrical length of coax in addressing differential vs longitudinal (common mode) current?  I’m interested in the distinction.

Thank you,

Edward McCann
AG6CX
ag6cx1 at gmail.com

Sent from my iPhone

> On Jul 1, 2020, at 10:27 AM, Jim Thomson <jim.thom at telus.net> wrote:
> 
> Date: Sun, 28 Jun 2020 15:10:18 -0700
> From: "Richard (Rick) Karlquist" <richard at karlquist.com>
> To: k4to at arrl.net, jim at audiosystemsgroup.com
> Cc: kj6y--- via TowerTalk <towertalk at 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
> 
> 
> 
> ##  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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