'elevated coil merely reduces the voltage in the section below it'.
'load the best point for the ground rods was under the remote end'
Quite so , what go's up must come down, lower voltage at the tuner , more
amps in the vertical , higher voltage
at the top/end .. same capacity , but more voltage , more power transfer
to ground
Which was my point , adding additional top wires, more capacity , has the
same effect as adding a top loading coil, which in turn increases the
apparent capacity to ground , same result ?
G, ..
--------------------------------------------------
From: "Alan Melia" <[email protected]>
Sent: Saturday, November 07, 2015 4:23 PM
To: <[email protected]>
Subject: Re: Re[2]: LF: Capacitive top hat question
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" <[email protected]>
To: <[email protected]>
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" <[email protected]>
Sent: Saturday, November 07, 2015 3:24 PM
To: <[email protected]>
Subject: Re: Re[2]: LF: Capacitive top hat question
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 a
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