[TowerTalk] Fair rite materials for choke baluns

[jimlux]

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".