RE:RF Ground

[Richard Kent]

Question Ron. I have seen/heard of a grounding system that uses coax. The
inner conductor and shield are shorted together at the rig(I think). The
inner conductor is attached to the grounding rod. The shield is floated at
this end. Would this accomplish what you are describing??

Richard Kent WD8AJG / K2 /#5296

Sorry if I sent this off list.

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[Ron D'Eau Claire-2]

Richard, WD8AJD asked:

Question Ron. I have seen/heard of a grounding system that uses coax. The
inner conductor and shield are shorted together at the rig(I think). The
inner conductor is attached to the grounding rod. The shield is floated at
this end. Would this accomplish what you are describing??

--------------------------------------------------

Hi, Richard -

I've not seen that myself, but it should work. All that's needed for an RF
ground using a 1/4 wave length of wire is to have the conductor 1/4 wave
long and floating (insulated) at the far end. That shield would accomplish
that just like a separate wire would provided:

1) The far end is insulated and

2) It is electrically 1/4 wave long.

The second item means you have to take into consideration the velocity
factor of the coax. Physically, it'll be a bit shorter than a "free space"
1/4 wavelength. For coaxial lines that is typically somewhere in the range
of 85% to 95% of the "free space" length. I'm not sure whether distributed
capacitance between the shield and center conductor will also effect the
length or whether that's part of the velocity factor compensation, but the
length is usually not really critical. So you'd cut the shield off at a
length of 1/4 wavelength from the rig and the center conductor would
continue on as needed to the earth ground.

The fact is that a wire running around the shack baseboards or somewhere
else close to other objects will be electrically 1/4 wave long when slightly
shorter than the expected "free space" 1/4 wavelength, thanks to the loading
effect of the nearby objects. Either sort of ground can be pruned for
optimum length by monitoring the current flowing into it at the rig. That
requires an RF ammeter, but it can be as simple a small flashlight bulb in
series with the wire right where it attaches to the rig. Keeping output
power constant, prune the wire for maximum brilliance. Maximum brilliance
indicates maximum current which results when the impedance and RF voltage is
at minimum.  However, in most cases such fine tuning isn't really needed to
keep the RF voltage on the rig down to a level that doesn't cause trouble.

Ron AC7AC

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[Stuart Rohre]

If the center conductor and shield are shorted at the rig, ONLY the shield
has any effect, and velocity factor does not enter into the action of this
type of counterpoise.

There is another type using coax, where a capacitor shorts the shield and
braid for RF purposes, (0.001 mf ceramic disc cap) at the far quarter wave
away end.  However with that implementation, I would predict not velocity
factor, but the effect of shield to center capacitance or the bigger
diameter of the shield to affect the length needed for quarter wave
equivalent.

Stuart
K5KVH


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