On 26/11/2014 21:50, Wilson wrote:
3. As for balance, there’s a good bit of impedance in a wavelength or two of
coax shield, so I doubt if the shield end at the top is actually very near ground.
Length of the coax is no guarantee of a high impedance. Any length of
wire a multiple of a half-wave long and earthed at one end will exhibit
a *low* impedance at the other end!
Suppose the coax were inside the mast?
It would make no difference as far as the level of conducted common-mode
current is concerned. To prove it to yourself just apply Ohm's law at
the feedpoint.
What about an antenna floating in space? Would balance be an issue? As long as
current flows in the element and nowhere else, won’t it work as expected?
Any current flowing back along the braid is current "robbed" from one
side of the driven element. A beam floating in space wont have equal
currents in the two legs of the driven element because it's not attached
to anything. The moment you attach coax to it there is a path for CM
current and the potential for imbalance
I’m not competent to make analytical arguments about which choke is best, but
rather I maintain that using the ARRL choke and getting the Z of the choke close to
ten times the feedline Z
is likely to be all anyone needs.
You may get by with low choke Z as far as achieving an acceptable level
of imbalance is concerned; for example 250 Ohms may be enough with a
balanced 50 Ohm antenna. Now run 1500W into that load and you could have
160v across the choke - that would mean a choke dissipation of 100W !!
The required choke impedance is often determined by the need to limit
dissipation rather than what is required for good balance. That old "ten
times the load impedance" rule of thumb is seriously flawed.
Steve G3TXQ
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|