[Lf] [Fwd: LF: LF Antennas]

[Andre' Kesteloot]

john sexton wrote:

> 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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