Don,
I'm not sure we disagree. I seems like we are both are agreeing to stay
away from quarter wavelength ground runs (wires) and use half wavelength.
That is what I intended to say. Usually ground systems are not effective RF
grounds. The connecting wire may be a relatively effective radiator or an
element in tuning an antenna system.
Jim, AB0UK
k2/100 S/N 4787
From: "Don Wilhelm" <[EMAIL PROTECTED]>
Reply-To: <[EMAIL PROTECTED]>
To: "JIMMY D HARRIS" <[EMAIL PROTECTED]>,<[email protected]>
Subject: RE: [Elecraft] In Shack Radials and Ground
Date: Mon, 22 Jan 2007 22:27:08 -0500
Jim,
Sorry to disagree - consider what happens on a quarterwave wire: It has a
low impedance at one end and a high impedance at the other end. Think
about
what will happen if you connect the far end of a quarter wave wire to a
good
ground (low impedance) - the other (near) end will have a high impedance at
that frequency, and will not serve as an RF ground at all (in fact quite
the
opposite).
A halfwave wire however can have a low impedance at each end, so grounding
the far end of a half wave wire will make the near end at a similarly low
impedance.
A grounded radial and a counterpoise wire are two different things - the
counterpoise wire creates a low impedance (about 35 ohms) by nature of
having the far end ungrounded, whereas a grounded (or buried) radial forms
a
screen or reflector - yes, the counterpoise will radiate because it becomes
a part of the antenna system. The counterpoise controls the radiation
instead of having it wander willy-nilly around the shack and other places
where it should not be present.
I do understand that this is not intuitive - we have to think in terms of
antenna theory when dealing with RF grounds - what works fine at DC and low
frequency AC does not necessarily work at RF.
Ground rods can be a good RF ground, but the wire connecting the ground rod
to the shack may not behave as expected - a 16 foot connection to the
ground
rod will present a high impedance to 14 MHz RF at the shack end - but
should
be a good RF ground for 10 meters since it is a halfwavelength away from
the
low impedance ground rod.
73,
Don W3FPR
> -----Original Message-----
>
> There was mention about ground rods not being a good RF ground. For the
> most part I agree with that. However, the wiring to the ground rod is
in
> fact a radial that is some part of a wavelength long. As we know
quarter
> wavelength radials can tune out RF. By the same token other fraction
of
> wavelength ground runs (radials) can create RF in the shack when used in
> conjunction with a poorly designed antenna system. Stay away from
ground
> runs that are halfwave wavelength (or near) or multiples thereof of
> frequencies your antenna system is designed for.
>
> 'nough said......
>
> Jim, AB0UK
> K2/100 S/N 4787
>
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