[TowerTalk] 40M rotary dipole

[Jim Thomson]

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