On 7/5/16 11:01 PM, Jim Brown wrote:
On Tue,7/5/2016 5:02 PM, Jim Thomson wrote:
Somehow, you seem to miss the point, which is NOT gain, but rather
keeping RF picked up on the feedline from coupling to the antenna, and
from there via the feedline in differential mode to the receiver.
I wouldn't think of it as a gain thing, but more that there's coupling
between the feedline and antenna, and then (the shield of) the feedline
becomes "part" of the antenna system.
I don't know that it would change gain all that much, but it could
certainly ruin a null.
And it might provide a path from a near field noise source (some low
level RFI source that's very close to the feedline), but then, you'd
really want a choke near the noise source.
## the yagis at the time all used gamma or omega matches... an no
CMCs used.
Just because you never saw it doesn't mean that it's not good practice.
It just means you never saw an installation where someone was smart
enough to do it. :) Good practice is to do whatever the mfr of the
antenna designed for matching, and then ADD a common mode choke.
When I first published my work on using ferrite chokes at the feedpoint
of antennas, guys in our contest club started doing it and found that
their antennas were quieter. Guys in multi-transmitter stations found
less interaction between stations. This includes everything from
dipoles, to monoband and multiband arrays of aluminum, to SteppIRs, and
is independent of how they are fed. Even verticals benefit from a coax
choke at the feedpoint.
I think that's generally the whole "if you avoid large conducting things
near your antenna, and particularly, connected to your antenna, they
work better".
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|