[TowerTalk] 40M rotary dipole

[Grant Saviers]

OUCH!

Grant KZ1W

On 9/21/2017 11:06 AM, Doug Ronald wrote:
> The RF current your receiver responds to flows on the inside of the outer
> shield of the coax transmission line, so what difference does it make what
> currents may be flowing on the outside of the shield?
>
> Doug
> W6DSR
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim
> Thomson
> Sent: Thursday, September 21, 2017 08:04 AM
> To: towertalk at contesting.com
> Subject: [TowerTalk] 40M rotary dipole
>
> Date: Wed, 20 Sep 2017 16:59:06 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] 40M rotary dipole
> Message-ID:
>
> On 9/20/2017 3:29 PM, Jim Thomson wrote:
>> Date: Wed, 20 Sep 2017 16:42:42 -0500
>> From: Stan Stockton <wa5rtg at gmail.com>
>> To: Dave Sublette <k4to at arrl.net>
>> Cc: Dan Maguire via TowerTalk <towertalk at contesting.com>
>> ##  Easiest way to see if the CM  choke is  doing the job, or to compare
> no choke, lousy choke, good choke,
>> is to use a clamp on RF ammeter, like the deluxe version that MFJ sells.
> MFJ-854.   Measure it at the base of the tower,
> ?Sorry, that's not valid, because like any conductor carrying RF current,
> the current varies along the length of the conductor depending on its
> boundary conditions ( the mathematical term for its terminations).? These
> boundary conditions are sort of obvious -- current is forced to near zero at
> the end of an unconnected wire and at a choke with very high Z. But just
> because the current is near zero at the top doesn't mean it's near zero
> anywhere else along the length, because it's an ANTENNA!?? SO -- a current
> measurement at the tower base does NOT tell us about current at the top next
> to the choke.
>
> 73, Jim K9YC
>
> ##  Agreed. Im not arguing with you.  EZNEC  will show you what kind of CM
> current  you can expect at any point on the coax.  But by taking a  current
> reading  at some convenient point on the coax,
> and perhaps in  2 or 3 places,  it will tell you if your CM choke is up to
> the task.    IE:  compare no CM choke
> at all vs  a lousy CM choke,  VS  a superb CM choke....and  then compare the
> RF current readings.   You will
> see at a glance that the better CM choke..placed at the  feedpoint,   will
> always result in a lower RF current reading
> downstream on the coax.  The reading on the clamp on RF ammeter is relative.
> Call it... X.   Then  swap CM chokes
> at the feed point,  then measure the differences.    The caveat is, take all
> RF ammeter readings at the same exact place
> on the coax  each time.
>
> ##  For you folks with  dipoles ..and no CM choke  at the feedpoint,
> depending on coax length, under some conditions you can end up with more
> current flowing down the coax braid vs one of the two legs of wire that make
> up the dipole.
>
> ##  You can get an eye opener with the clamp on RF ammeter.  You will find
> RF current flowing on stuff you would not  expect.
> Everything from   AC wiring, to clothes lines, guy wires, DC wiring,  copper
> tubing in the home, vdsl inputs and outputs, phone lines, etc.
>
> ##  By placing a  2nd or 3rd  CM choke  downstream, then re-measuring the rf
> currents on the coax in the same places,  you can see where the most
> effective  insertion points are.
>
> Jim  VE7RF
>
>
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