Date: Wed, 29 Dec 2010 20:52:22 -0800
From: Jim Brown <firstname.lastname@example.org>
Subject: Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN
On 12/29/2010 7:35 PM, NPAlex@aol.com wrote:
> I think a more accurate description of several coiled turns of coaxial
> cable would be a RF Choke, serving to limit common mode currents on the
> of the coax cable.
That's an Alice In Wonderland view of things. Yes, the coil of coax is
an inductance in the common mode circuit, but the common mode circuit
also includes the series impedance of the transmission line. If that
line is less than 1/4 wave, it is a CAPACITIVE REACTANCE, so the
inductive reactance CANCELS all or part of it and the common mode
current INCREASES rather than decreases. The same thing happens if the
line is between 1/2 and 3/4 wavelengths. Further, the electrical length
is based on a Vf of 0.98, NOT the Vf of the coax (because the common
mode current flows on the outside of the shield). The 0.98 factor
accounts for the slight shortening effect of the outer jacket.
### OK... I'll buy that. OK, now what exactly happens IF the coax is
exactly 1/4 wave long.....or 3/4 wave long.....or 5/4 wave long ????
## W8JI sez if you use a 1/4 or 3/4 wave of coax... on a yagi... and
bond the braid to the top of the tower.... you will end up with a HIGH Z at
DE. [he's talking abt several thousand ohms]. If the coax is taped along the
then you may well have to factor in the C..between the outer braid and the boom.
That stray C could easily be measured with any LCR meter. It could easily be
and MAY affect the overall length of the 1/4 or 3/4 wave of coax. Your .98
VF may have
to be tweaked slightly... if applied in the above case... with coax taped to
boom...and braid bonded
to top of tower.
## The only easy way to see if his concept works..would be to use a clamp on
RF ammeter... and run
it along the coax.... BELOW the top of the tower. I'd also bond the braid to
the base of the tower.
W8JI sez that using a balun in this case..[with a 1/4 wave of coax..with
braid bonded to top of tower]
will make matter's worse..and will not supplement the high Z of the 1/4
wave of coax. I find that
hard to believe, I think they would supplement each other. It seems to me his
concept hinged on
the actual height of the tower VS the band in question.
Later.... Jim VE7RF
The only effective common mode choke is a very lossy parallel resonant
circuit, which is what you get if you wind multiple turns of coax around
a lossy ferrite core (like #31 or #43). All you have to do is use the
number of turns that puts the very broad resonance where it covers the
bands you work.
73, Jim K9YC
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