I think you are waving your arms around G :-)) There is no "transformation"
the elevated coil merely reduces the voltage in the section below it. This
reduces the current forced into lossy environment near the feed and verticle
(as Mike found) The top section has a remote end with is the high voltage
point, that point drives most displacement current through the "load
capacitance" to ground. The capacitance is a physical thing.....two
plates....it does not vary with frequency !! At the feed point the elevated
coil cancels some of the capacitance so the capacitance must be measured
well below the resonant frequuency, or the inductance allowed for. This can
all be calculated easily. This is why I always recommend measring the
parameters of an untuned aerial . You dont then get confused by the tuning
elements, it is a simple capacitance in series with a resistance (which is
predominantly loss)
If you have a copy of the old Peter Dodd LF experimenters Sourcebook there
is an interesting reprint of an article from 1926 about the Nauen VLF aerial
system.. Short of money they couldnt afford unlimited amounts of copper so
they had to use what they could afford efficiently. The found with a top
load the best point for the ground rods was under the remote end not under
the feed point!!
Alan
G3NYK
----- Original Message -----
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, November 07, 2015 3:53 PM
Subject: Re: Re[2]: LF: Capacitive top hat question
Im thinking of transformation, caused by a top loading coil . That
would appear to offer a higher ground capacity from the top wires ,
after the coil , lowering the feed z at one side and higher at the
other ?
G,
--------------------------------------------------
From: "Alan Melia" <alan.melia@btinternet.com>
Sent: Saturday, November 07, 2015 3:24 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: Re[2]: LF: Capacitive top hat question
Measured ....the only type that matters :-))
Alan
----- Original Message -----
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
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" <alan.melia@btinternet.com>
Sent: Saturday, November 07, 2015 12:32 PM
To: <rsgb_lf_group@blacksheep.org>
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" <chris@chriswilson.tv>
To: "Mike Dennison" <rsgb_lf_group@blacksheep.org>
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:chris@chriswilson.tv
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