[Elecraft] Re: RF Coupleing

[Ron D'Eau Claire]

Lennart's comments fit my experience. There isn't a "ground loop" formed by
using coax and it does eliminate much of the RF currents floating about at
random inside the rig.

With the RF current flowing at the center pin connected to coax, the RF
energy must propagate along the *inside* of the coax, with RF currents
flowing on the outside of the center conductor and on the *inside* surface
of the shield. There is only one significant path for the RF: through the
coax. 

Since the center conductor, carrying 1/2 of the RF current, is electrically
isolated from the rest of the rig, there is nothing to encourage RF current
to flow from the connector outer conductor through the metal cabinet to some
other terminal. As noted, RF isn't DC or low-frequency AC. Indeed, it's
useful to consider a wire (or coax) carrying RF as a sort of "wave-guide";
not the classic stuff used at microwaves, but nevertheless a guide that
directs the RF energy. In that respect, the RF energy flows through the
space between the conductors while inducing RF currents along the surfaces
of those conductors. In coax, the RF energy flows through the dielectric
between the center and shield. That's why dielectric has such a huge effect
on the electrical length of a piece of coax. It retards the RF wave
propagation. 

If a bare wire is used, the RF will flow between it and some convenient
conductor connected to the shielded side of the conductor, such as the
cabinet, pc board ground, etc. That produces an RF field that fills the
space between them which might include lots of other circuits in which the
RF field induces currents. Those are currents that can cause mischief if
they get into an RF-sensitive circuit. 

In any design where I want to isolate the RF I use coaxial cable on all
internal RF connections. Generally it's "overkill" but that's the luxury of
building a one-only homebrew project in which a few extra dollars in parts
is insignificant. That's not the case in something that must be manufactured
at a competitive price and at a profit. That requires engineering expertise
that can evaluate exactly where additional shielding is needed and how much.

As I often note, engineering is all about making compromises. Great
engineering is about making great compromises. 

Yes, a "hood" on back of a female SO239 might be good, but I've found little
need for one at HF. It might be needed in this case if extreme isolation is
wanted, but I'd be surprised if that were the case.

Ron AC7AC


-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Lennart Michaëlsson

Vic,
I would not go for that solution because the insulated part of the coax
inside the cabinet would reradiate anything coming on the coax from the
outside. Best solution is short piece of coax grounded on the inside of
cabinet AND at the antenna tuner. That is my opinion, humbly yours Len
SM7BIC

That's certainly part of the problem. But just adding a ground wire might
create a ground loop. I guess the best way to do it would be to insulate the
SO239's from the chassis, and use hoods on them with coax!
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco _______________________________________________
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