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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