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Re: LF: LF Antennas

To: [email protected]
Subject: Re: LF: LF Antennas
From: "john sexton" <[email protected]>
Date: Wed, 4 Apr 2001 12:49:32 -0700 (PDT)
Reply-to: [email protected]
Sender: <[email protected]>
Hi Peter, hi All,

I was particularly interested to read about Peter's new receive loop, which
shares several features with the loop that I built, primarily its size and
the use of thick Litz wire.
Like Peter I am really impressed with the results. During initial trials
last year I was at first taken aback by the high noise level, but now
understand that this is a direct result of its superb sensitivity. During
the transatlantic tests, I found my antenna really delivered the goods.

Our experiences seem to confirm that this sort of size (i.e. of the order of
a couple of metres in diameter) and particularly the use of low resistance
Litz is optimum.
The number of turns is 10 in Peter's case and 36 in mine and I am still
unsure whether more or less is better. I am not convinced by the theoretical
arguments that have been put forward, preferring an ounce of experience to a
pound of theory (or gram, kilo respectively).

Peter suggests that multistranded PVC covered wire would be a good
substitute where Litz is not available. I cannot agree with this - I first
wound my loop with this type of wire and was very disappointed with the
results. Measurements of the resistance explained why. Perhaps really thick
copper wire or lots of insulated turns in parallel rather than in series
might work.

Regarding the use of a high loading coil, Mike (XDV) comments:

                    It is also worth saying this is effective when the
part beyond the coil is quite
                    small (as in your straight vertical diagrams). For an
antenna with substantial
                    top loading, the gains and losses (caused by the much
large inductance
needed) can cancel each other out.
Perhaps I have misunderstood your point Mike, but this sounds wrong to me.
The less top loading, the greater the inductance required for resonance.
With little top loading above the high coil, a very large inductance is
required to have any effect, since the capacitive reactance will be
extremely high. A small coil will have little to no impact on this
reactance.

It would appear to me that the reduction of high voltages in those parts of
the antenna that are near ground, trees, buildings, etc. is probably where
the most gain is made. Moving the current node up the antenna obviously
makes a difference, but the reduction in losses because of the lower
voltages is probably much more significant. The high voltages are still
there of course, but they are now confined to the top load, which hopefully
is well in the clear.
Best 73, John, G4CNN





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