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@gmail.com
Sent from my iPhone
> On Jul 1, 2020, at 10:27 AM, 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
>
>
>
> ## 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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