[Elecraft] In Shack Radials and Ground

[JIMMY D HARRIS]

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" <w3fpr at earthlink.net>
>Reply-To: <w3fpr at arrl.net>
>To: "JIMMY D HARRIS" <ab0uk at msn.com>,<elecraft at mailman.qth.net>
>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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