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Re: Re[2]: LF: Capacitive top hat question

To: <[email protected]>
Subject: Re: Re[2]: LF: Capacitive top hat question
From: "Alan Melia" <[email protected]>
Date: Sat, 7 Nov 2015 15:24:25 -0000
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Measured ....the only type that matters :-))
Alan
----- Original Message ----- From: "Graham" <[email protected]>
To: <[email protected]>
Sent: Saturday, November 07, 2015 2:31 PM
Subject: Re: Re[2]: LF: Capacitive top hat question


doubling the capacity ofan aerial halves the ground loss

Actual or  effective  capacity  ?

G,

--------------------------------------------------
From: "Alan Melia" <[email protected]>
Sent: Saturday, November 07, 2015 12:32 PM
To: <[email protected]>
Subject: Re: Re[2]: LF: Capacitive top hat question

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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