> Hello All
> Thanks for the replys on my questions about the Loop Antenna I put up.
> I have a question about RF choke reactance value. I use a 30 turn coil
> of 9913F coax wound on a 4.5 inch coil form to act as a 1 to 1 balun.
Hi Dan,
You'd better know what the common mode impedance of the shield is, or
use a much bigger choke. Otherwise you could either be wasting time
with little change, or actually making the system worse.
I'd use a ferrite core that was a somewhat pure and very high
resistance with a very large stack of something like 77 material. I
could also use a choke that is nearly self-resonant on 160 or has
very high reactance, and use a higher Q material like 43 or 61
material or an air core.
Your choke might, through random luck, be OK. But a few hundred ohms
it almost certainly is not a good choke for 160 for your application.
This is the especially true if the length of the feedline happens to
make the feedline shield path look slightly capacitive in the common
mode excitation. In this case, adding the inductance will REDUCE the
isolation of the shield from the antenna and INCREASE the unbalance.
Adding a somewhat pure reactance as a choke-balun, **especially a
small value**, will most likely either have very little effect or
worse yet might make the unwanted shield current INCREASE.
Say the feedline shield, at the antenna, looks back towards the
station with a common mode impedance of 27 ohms -j300. You add a
choke balun of 2 ohms +j300, and the net shield impedance is now 29
ohms j0! That makes the system much worse.
As a matter of fact unless we add significantly MORE than 600 ohms of
choke reactance or significant choke resistance, adding the choke in
this example would almost certainly INCREASE unwanted shield
currents. At 600 ohms you would actually just break even and wind up
with 29ohms j+300 and have no significant change in common mode
current, we'd have to add almost +j900 ohms to really start knocking
down unwanted current.
This is exactly why a Flag or Pennant antenna performance becomes
worse when a choke balun is added to the line near the feedpoint,
unless it is a resonant balun or one that is a nearly pure and
somewhat high value resistance.73, Tom W8JI
W8JI@contesting.com
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