Jim
Do you expect to reach me with this sort of erp? I am QRV when you are
ready. I expect you to hear me with a few hundred watts
mal/g3kev
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Friday, September 10, 2010 1:58 PM
Subject: LF: Re: UK NoVs
> Dear Jim, Stefan, LF Group,
>
> One obviously cannot make a precise prediction of ERP without
> experimentally-derived data, so you will just have to find out what can
> actually be achieved. But one can make a realistic estimate of the upper
> limit of ERP - here's mine:
>
> Assume 200W max TX power, frequency 10kHz (highest frequency = highest
> antenna efficiency)
>
> The effective height of the antenna depends somewhat on the exact
geometry
> of the wires making up the antenna, and also on shielding effects of
objects
> in the environment close to the antenna. Assume no shielding effect (best
> case for ERP). The effective height will always be less than the physical
> height of the antenna; for a heavily top-loaded inverted-L like this,
maybe
> 80% of the physical height. Again let's assume a "best possible case"
> effective height of 10m. The radiation resistance would then be Rrad =
> 160*pi^2*heff^2 / lambda^2, i.e. 175micro-ohm. You could get a more
specific
> figure for heff / Rrad using the text-book formulae (see ON7YD's antennas
> site), or by doing a NEC simulation using the actual wire geometry, with
> perfect ground and perfect conductors.
>
> You could also estimate the antenna reactance using a NEC simulation; but
a
> good-enough estimate is to assume 6pF/m for a total of 50m of wire, i.e.
> 300pF capacitance, Xc = 53kohm at 10kHz. This reactance is much higher
than
> any likely value of antenna loss resistance, so effectively defines the
> antenna voltage for a given antenna current. If there is no limit on
> transmitter power, antenna breakdown voltage will be the limiting factor
on
> antenna current and ERP. In my experience, antenna voltages of 20kV can be
> achieved with care on "back garden" antennas, maybe more on a dry day...
> This limits maximum antenna current Iant to 20kV/53kohm = 0.38A.
>
> The loss resistance defines required TX power to achieve a given antenna
> current. Rloss varies widely depending on environment and possibly
earthing.
> I have measured figures at VLF between several tens of ohms and several
> hundred ohms, usually more the latter. To this you have to add the loss of
> the loading coil, probably also hundreds of ohms. If we assume total Rloss
> of 1kohm, the TX pwr for 0.38A antenna current is P = I^2R = 144W, so in
> your case antenna voltage will probably be a limiting factor rather than
TX
> power. If Rloss turns out to be very high for your antenna, the maximum
> 200W TX power would limit Iant to <0.38A.
>
> The effective radiated power is Iant^2 * Rrad * D, where D is the
> directivity of the antenna relative to a free-space dipole. For an
> electrically short vertical, D = 1.8. So the maximum ERP you can achieve
> with Iant = 0.38A, Rrad = 175uohm, is 45uW. In practice, this is very much
a
> "best case", and ERP will certainly be reduced by various adverse factors
> (lower Heff, non-zero enviromental loss and so on could easily knock off
> 10dB or more). So 45uW is very much an upper boundary on the achievable
ERP
> from your system. A few uW is probably realistic, not as bad as you
thought!
>
> Good luck with the application,
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
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