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

[Dave Sublette]

For anyone interested, here is the link

http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htm

MIght even put a photo up if it works.  I am trying to adapt a large clamp
for holding glued joints to use as a spring mechanism to open and close the
Ferrite.

Dave

On Thu, Jul 2, 2020 at 11:28 AM Dave Sublette <k4to.dave at gmail.com> wrote:

> I have since started building one based on plans in RSGB by G0SNO.  Looks
> like it will be a fun project.
>
> 73,
>
> Dave, K4TO
>
> On Thu, Jul 2, 2020 at 11:07 AM Jim Thomson <jim.thom at telus.net> wrote:
>
>> ##  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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>>>