Date: Wed, 20 Sep 2017 16:59:06 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@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@gmail.com>
> To: Dave Sublette <k4to@arrl.net>
> Cc: Dan Maguire via TowerTalk <towertalk@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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