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