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

[Jim Thomson]

##  Mine worked fine, bought last year. I calibrated it with 50 watts into a dummy load.
50 watts  = 1 Amp into 50 ohms.  But the current loop/probe has to be looped around either
the  center conductor.....or the braid, but not both.  Same as clamping on a 60 hz  circuit,
only one conductor passes through the probe.   Otherwise the  opposing fields will cancel.

##  Dunno if it will work on 6M or not.   My xcvr does not have  6M capability.

##  I  dont see any other device available commercially.  The  MFJ-854 will fit  over 
a .405 cable, like RG-213.  It will  NOT fit over  RG-214..which is .425 cable. And it wont fit
over  LMR-600,  LMR 1200db, .5 inch heliax  etc.   In those cases, a short jumper of  213 U, or
RG-393 can be used, like where a lightning arrestor can be temp removed..and replaced with a 
213 jumper.  Or a 213 jumper installed at the remote switch box, or  a bulkhead etc.

## Besides CM choke effectiveness, it can also be used on   radials, and items around the home.
You would be surprised where  RF current ends up, in places you would not have  thought,  like
clothes lines, AC  wiring, alarm wiring, stereo gear, etc.  That list is endless. 

Jim   VE7RF 



From: Dave Sublette 
Sent: Wednesday, July 1, 2020 10:55 AM
To: Jim Thomson 
Cc: kj6y--- via TowerTalk 
Subject: Re: [TowerTalk] Common Mode RF choke on a 50 MHz yagi

Checking the eHam reviews shows LOTS of issues in Quality on the unit.  Unless recent product has been improved, I am hesitant to order one. 

K4TO

On Wed, Jul 1, 2020 at 1:50 PM Dave Sublette <k4to.dave at gmail.com> wrote:

  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 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

    ##  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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