VE7RF wrote:
>### Im assuming that the top of the tower..or a qtr wave back from the
>feedpoint of a yagi, say on the boom itself on a 10 or 6m yagi, would
be a
>low Z point ?
>I may well be wrong, depending the height of the tower, and how well
>bonded it is to the ground....say via 3 x 8 foot copper clad grnd
rods,
>and cadwelded 2 ga cu wire to each tower leg.
It isn't clear what you're aiming to achieve here, Jim. If you connect
the coax shield to the boom at a point that has a low impedance down to
the ground at the base of the tower, that will *maximize* unwanted
radiation and noise pickup by the boom, mast and tower. Surely our aim
should be the exact opposite?
1. Minimize unwanted common-mode current on the outside of the coax
shield
.AND.
2. Minimize crossover of that CM current onto the boom, mast and tower
(to prevent them becoming an unwanted part of the transmitting antenna,
and also a receiving antenna for noise).
#1 is best achieved by a high impedance common-mode choke (aka "feedline
choke", "line isolator" or "current balun") at the feedpoint. I think
most of us here on TT understand that... I hope...
#2 is the problem area, if we also wish to connect the shield to the
metalwork of the boom and tower for protection against lightning. To
minimize the crossover of CM current at the bonding connection, the
optimum location would be at a minimum in the CM current on the outside
of the coax shield and also a maximum in the impedance of the boom, mast
and tower down to the ground below. But where is that optimum point?
Does it even exist? And when we change bands, won't it move to somewhere
else?
It seems to me that the best strategy is to focus intensively on #1, and
aim to reduce the CM current on the feedline to a very low level. Issue
#2 - the location for bonding - then becomes far less critical.
But there are also some horrible examples of badly installed CM chokes
that may make the whole situation even worse than before, so perhaps we
should also add a third requirement:
#3. Don't make mistakes!
One of the most common mistakes when installing a CM choke is to tape it
to the boom. There are many criticisms of reactive air-cored chokes but
worst of all is the way they are so often just taped to the boom.
Remember that the shield is connected only to one side of the driven
element, so the capacitance between the shield and the boom will
UNbalance the feedpoint! CM chokes of all descriptions need to be spaced
away from the boom by a block of insulating material, or suspended
inside a plastic enclosure.
There are serious doubts about CM chokes in metal enclosures. As well as
unnecessary cost and weight, a metal enclosure raises a whole series of
unnecessary questions about unbalanced stray capacitances, and how or
where to connect the box to the shield and/or the boom. Those issues can
be completely avoided by using a plastic enclosure; or in many climates,
no enclosure at all (neither coax nor ferrite require weatherproofing).
Another bad practice is to pass the coax through the boom at the
feedpoint, which seems to have become fashionable for 6m antennas
recently. Once again, think about the unbalanced capacitance.
73 from Ian GM3SEK
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