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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|