Joe Subich, W4TV wrote:
> >> But the 3-5 k has to go at the feedpoint, and not in the form of a
> >> line isolator at the entrance to the shack?
> >
> > That's right, because the feedline acts like any other antenna.
> > Choking it at the shack simply puts an open circuit at that point,
> > but depending on its length, that can make it a very effective RX
> > antenna, which then couples to the main antenna.
>
> Correct if one is interested in keeping the omnidirectional noise
> from coupling to the antenna.
>
> >> Put another way, is their any benefit to installing a 5 K choke
> >> balun at the entrance to shack... [for noise elimination] ?
> >
> > Probably not for noise isolation, but it is definitely a good thing
> > to kill interaction with other antennas, especially wire and some
> > verticals. See my discussion on using chokes as the equivalent of
> > "egg insulators" in the feedline.
>
> There may be ... if the omnidirectional noise can couple to the
> antenna in one direction, it should also be able to couple into
> the receiver in the other direction (particularly if the equipment
> is not properly grounded and decoupled as you have documented
> elsewhere).
>
> I'm a believer in placing a broadband 5K common mode choke on the
> equipment side of the feedline(s) at the common (entry) ground.
> All equipment should be bonded to the common ground point with a
> separate, large conductor and not rely on the shield of the coax
> for that purpose. Even if the common ground is relatively poor
> at RF, it will still be fairly low compared to choke's impedance
> and result in about 30 dB in reduction of common mode noise on
> the shield.
>
So, hypothetically.. say you had a big metal plate with bulkhead
feedthroughs. Say you took a big stack of toroids to make a sort of
tube, that you'd make a "shroud" on the *inside* of the plate,
surrounding the coax going to the equipment.
That would essentially decouple any RF that's "on the plate" (e.g.
transients, noise picked up on the outside of the feedlines coming from
the other antennas,etc.) from being carried down the outside of the coax.
This is a lot like what the feedthroughs on a screen room at work look
like (except that they stick out of the outside of the screen room wall,
and the inside is flush.
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