[TowerTalk] 40M rotary dipole

[Doug Ronald]

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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