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 <[email protected]>
To: <[email protected]>
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.
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