I think that maybe too much emphasis is placed on specific structures. At
these frequencies any structure of conductors can be resolved into an
equivalent vertical and horizontal arrangement......even a continuously
sloping wire!. As Mike says the horizonal portion does not radiate
appreciably because of the reflection in the close-by ground.
Predominantly horizontal conductors will inrease the capacitance of the
aerial to ground and an extra run of wire will have most effect if separated
by around a metre to reduce interaction between to two. Ball-park figure an
extra 6pF per metre. The actual configuration of connection is unimportant
for the horizonals form a "skeleton" plate. Note whereas Rugby LF station
had originally caged wires between the 850 foot masts to increase the
capacity, after the rebuild the internal area with the masts was "laced"
with straight single wires. This produced more capacity and was easier to
maintain the the high windage cages.
Then the more horizontal wire the higher the aerial capacity, so the smaller
the inductance needed to resonate it ......and lower coil loss. However
another effect not well modelledin aerial synthesis programs isthat doubling
the capacity ofan aerial halves the ground loss. There are mesurements on my
web site confirming this, under spiral aerials. Halving ground loss is very
difficult to achieve with extra groundrods or "radials" at LF/MF unless it
is poor to start with. The only casewhere this is not useful is over very
good ground, a high water-table or possibly sea water.
As to feeding Chris's loop as a loop..... the size is much less than a
wavelength and is too low compared with the wavelenth to radiate
efficiently. Any uncancelled radiation will probably be vertically upwards,
much of which will escape the ionosphere never to return.
Its performance at HF where distance above ground is of the same order as a
wavelength will be totally different. Phase difference round the loop will
lead to a totally different pattern of radiation.
Alan
G3NYK
----- Original Message -----
From: "Chris Wilson" <[email protected]>
To: "Mike Dennison" <[email protected]>
Sent: Saturday, November 07, 2015 11:19 AM
Subject: Re[2]: LF: Capacitive top hat question
Hello Mike,
Saturday, November 7, 2015
Thanks for the info Mike, as always! Is there any real benefit in
having the vertical section centralised within the top hat capacitive
array, be it a horizontal loop, random horizontal wires, or a plain
single wire?
And is there much point in struggling to get one corner or side of a
horizontal top hat loop higher than the rest, or the same for a single
wire? I have some tall trees, but unfortunately not two tall trees
opposite one another across free space, so whatever capacitive hat I
fix up is likely to be significantly higher at one end or one corner.
There's no point in struggling and risking life and limb to get one
end as high as possible if it ends up only as efficient as the lowest
end or point.
To all intents and purposes a short (in wavelengths) vertical
attached to an arrangement of horizontal wires is a simple
(capacitively) loaded vertical.
The horizontal part will radiate, especially if it has a vertical
component rising higher than the feedpoint, but most of that
radiation will be cancelled by its reflection in the ground.
Several unbelievers have tried using purely horizontal transmitting
antennas and have had poor results. The beauty of amateur radio is
that you can prove something to yourself, but it won't change the
laws of physics.
Of course, every electrically small Marconi that is not in free space
performs in a complex way, but the result of that complexity is small
compared to the predominent omnidirectional radiation from the
vertical section. There is very little difference between various
arrangements of capacity hat, so long as you follow the rule to put
up as much wire as possible, as high as possible and covering as much
ground as possible.
Mike, G3XDV
==========
--
Best regards,
Chris mailto:[email protected]
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