Adding to what Paul says I have found that NEC needs to be treated with
caution even higher in the LF range. Another thing to remember is that the
skin depth at these VLF frequencies is quite significant. So the apparent
ground plane can be many feet below ground level, and this adds to the
"effective height" of the antenna.
The near-field zone is around a half wavelength radius from the radiator,
and in this area or sometimes called the induction zone, the E-field drops
off (from memory) proportional to the inverse cube, and the H-field
proportional to the inverse fourth power.
Alan
G3NYK
----- Original Message -----
From: "Paul Nicholson" <[email protected]>
To: <[email protected]>
Sent: Monday, December 21, 2015 2:28 PM
Subject: Re: LF: VLF in VO
Jim AA5BW wrote:
> Would you tend to allocate another 10db-15dB or so of
> loss for waveguide coupling efficiency given horizontal
> polarization
The radiator, although a horizontal wire, is practically a
vertical polarised Hertzian dipole with the ground as one pole
and the wire as the other. Horizontal polarisation suffers
very high attenuation at VLF. I've never got anything sensible
from NEC at VLF and tend to use a quasi static calculation to
estimate the effective height, eg with
http://abelian.org/lcng
and then treat it as a vertical Hertzian dipole.
At long range the received power density is inversely
proportional to range because the transmitted power is trapped
in the Earth-ionosphere cavity. On top of that 'geometric'
loss, the attenuation will only be 1 or 2dB per 1000km at
night over water.
Dimitris VK2COW/VK1SV wrote
> attenuation over distance does seem to be a lot more
> pronounced than you would expect and I think this has to do
> with the fact that at a kilometre or so distance from the
> transmitter, we are still in the near field of the antenna!
Near field strength falls away very rapidly but don't let
that put you off. Once the far field takes over, VLF signals
propagate very well. You might not be able to hear your near
field signal beyond a few km, but a GPS locked carrier can
be integrated for a long time to produce a clear detection at
great distance.
My suggestions:
- Announce your transmissions, even if you don't expect much.
It's fun to look for the weak signals anyway and often we
are surprised;
- Long unmodulated carrier so that a simple long coherent
integration can be used;
- GPS or rubidium locked so that the transmitted phase stays
fixed. A very good OCXO is also worth a try, especially
if you can calibrate it so we know the exact frequency.
- Avoid multiples of 50 Hz or 60Hz depending on the reception
area. Place the carrier in between the mains harmonics;
- Avoid tampering with the loading coil tuning once
you've started because this upsets the phase;
- Don't rule out daytime tests. In Europe, daytime often does
better up to ranges of around 2Mm because the receiver
doesn't have the strong nighttime noise from South America.
My receiver in Todmorden records E and H continuously to catch
natural radio signals. It is no trouble at all to look for
a weak amateur signal so there's no harm in trying!
There are also very good natural radio receivers in Germany,
Italy, and North America which run continuously and make their
signals available on the net. You can aim for these too.
http://abelian.org/vlf
Bielefeld, Cumiana, Hawley and Forest are GPS timed so are
capable of long integrations and their signals are recorded
on about a 20 day loop which means you can announce your tests
afterwards.
--
Paul Nicholson
--
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