Message-ID: <000001bf5ace$a807a7a0$0c29f7c2@oemcomputer>
From: "John Rabson" <word.factory@zetnet.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <29188.200001071140@gemini> <3875F52A.3AF2D67@bellatlantic.net> <00dc01bf594c$7f1a9ba0$0600a8c0@main> <3876554B.72D98BF3@bellatlantic.net> <3877AF3D.47A84B20@netscapeonline.co.uk> <3877A137.5F0D@xtra.co.nz>
Subject: Re: LF: Re: AMRAD Antenna ?
Date: Sun, 9 Jan 2000 10:12:18 -0000
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We use this technique of grounding  both ends of a wire in Cave Radio work
as it enables us to communicate to a greater depth than does the
conventional method, which uses resonant loops a metre or two in diameter.
It has also been used  on 73 kHz at a location in  Oxfordshire - the station
was heard at a distance of 250 kilometres. This technique has been referred
to as an earth bipole.

I understand from Graham Naylor that during the recent cave rescue operation
in south-western France, where this technique was used, the rescue party
developed a new variation at the surface station. Instead of feeding the
wire in a symmetrical manner at its centre, the two halves of the bipole
were strapped together and fed against an earth connection near the
mid-point of the wire. This seemed to produce equally good results as far as
underground communication was concerned and produced a useful reduction in
the level of interference from LORAN.

This technique has been dubbed the earth tripole.

73 de

John Rabson G3PAI

----- Original Message -----
From: vernall <vernall@xtra.co.nz>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, January 08, 2000 8:42 PM
Subject: Re: LF: Re: AMRAD Antenna ?


> Hi all,
>
> The "dipole" with outer ends each grounded is used by some ZL stations
> for LF receiving.  It has been called a ground loop, after the name
> given to it by Andrew ZL2BBJ in a local article.  The LF ground path
> goes "underground" as skin depth is many metres at LF in typical soil,
> so the loop area is significantly more than the visible area above
> ground.
>
> The ground path (actually multiple paths) between "dipole" ends is lossy
> compared to copper wire, so the circulating current is generally lower
> than if ground wires were used, however the aperture is greater if no
> ground conductors are used, and aperture is what is good for receiving
> (the noise figure is basically set by signal to external QRN ratio).  In
> use for transmitting, the ground loop can be expected to be fairly lossy
> compared to higher Q antennas (DX QSOs need absolute radiated power).
>
> Bob ZL2CA
>
> g3kev wrote:
> >
> > Hello All.
> > Your comment about your antenna maybe operating like a LOOP is probably
correct.
> > I would suggest it is performing like a grounded quad. Similar systems
used on 160
> > and 80 metres where one cannot get a full size quad up.
> > At 1600 ft long and 50 ft high, think that is what you said, its natural
resonant
> > frequency used as a grounded quad would be 296 khz. I expect there is
some sort of
> > loading to resonate on 137 khz.
> > It would be interesting to check whether it radiates better as a loop or
a long wire
> > with the grounded far end disconnected. Judging by experiments in the
past using
> > loops v verticlals, I think the vertical/long wire approach would be
better for low
> > angle.
> > I have tried a variety of loops in the past for 160 metres ie 40 m loop
resonated on
> > 160 and although it was quieter than my full size quarter wave on 160,
it was not as
> > sensitive and did not pull in the long haul low angle dx, in fact there
were signals
> > that  I could not hear that I was able to copy solid on the vertical,
although at
> > times probably noiser. Small loops for short/medium distances of several
hundred
> > miles are acceptable but for low angle long haul poor on mf/hf.
> > A full size loop ie quad or delta etc resonant at the operating
frequency and
> > preferably at least a quarter wave above ground is a totally different
story.
> > In the UK stations using loops have poor signals compared to those using
verticals,
> > even low verticals heights with top loading. A couple of stations that
have been
> > using loops have changed over to verticals and although not very high
made a hugh
> > difference to their signals received at my qth.
> > The so called long wire, just a few feet above ground and fed with a
drop wire is
> > really a top loaded vertical or inv L.
> > The above comments are a result of experiments and observations,
especially on 137
> > khz and 1800 khz bands
> > 73 de Mal/G3KEV
> > Andre' Kesteloot wrote:
> >
> > > Wooops,
> > > I guess I did not express myself quite clearly enough.
> > > The far end of the wire terminates in a field , (and specifically near
a pond)
> > > visited by many cows.  In order to avoid any possible unpleasantness
(wire
> > > falling on the ground if broken by the wind, etc.), we decided to
ground that
> > > end.
> > > It may well be that the whole thing operates as a loop of sorts, as
there is a
> > > non-zero resistance between the two grounds (the one at the Tx site,
and the one
> > > at the pond end)
> > > 73
> > > Andre'
> > >
> > > Dave wrote:
> > >
> > > > Surely the Voltage gradient is just the same but the other way
round? High
> > > > current point at the earthed end and high Voltage point at the TX
site as it
> > > > is about a quarter wave....
> > > > The "earthed at the far end" idea has been used with topband
antennas for
> > > > years in order to get the current into the vertical drop.
> > > >
> > > > 73 Dave G3YXM.
>
>
>
>