[Fwd: LF: Re: AMRAD Antenna ?]
Sat, 08 Jan 2000 16:32:02 -0500
> 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
> 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.