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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