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
>>
>> _______________________________________________
>>
>>
>>
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>
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