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Re: LF: Re: LF antennas

To: [email protected]
Subject: Re: LF: Re: LF antennas
From: "Rik Strobbe" <[email protected]>
Date: Thu, 09 Mar 2000 16:08:31
In-reply-to: <[email protected]>
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At 12:09 9/03/00 -0000, G3XDV wrote:
Hmmm. Well I think we do have to take earth losses into account twice. This is because it not only affects the ERP because of the effective resistance in series with the source (which is also affected by any resistance in the connection to the earth), but it also affects the 'ground gain' which is the result of reflection from the ground after the signal has left the antenna. I am inclined to think that in a practical amateur LF station there is little or no ground gain by reflection, except perhaps where the station is very near the sea. By 'ground' I am including any radial system which in a commercial station dramatically improves the real ground resistance close to the antenna, but is usually very inefficient in an amateur station.
Hello Mike & the others,

I hope that the discussion about the effect of groundloss on antennagain
(directivity) it not too boring, but I think it is very important to
understand this well if we want to find out how short vertical monopoles work.

So here I go again .... ..

1. Assume we have a perfect ground
With any antenna over a perfect ground you will get a (virtual) mirror of
this antenna 'below the ground'. As with images and a glass-mirror the
mirror-antenna will be left-right reversed, or otherwise said in
counterphase to the 'real' antenna. (there must be a better way to explain
that in English)
The 'mirror' antenna will radiate the same power as the 'real' antenna.
This explains why a lambda/4 vertical monopole (Marconi antenna) has a 3dB
gain over a vertical dipole, with the Marconi antenna the second half of
the dipole is formed by the 'mirror' and both parts get full power while
for a vertical dipole the total power is divided over both lamda/4 parts.
This explains also why a horizontal antenna at low height (in wavelengths)
will not work as all radiation is canceled out by the 'mirror' antenna.

2. Assume we have no ground at all (antenna in free space)
Here a short vertical monopole will not radiate at all, as it needs some
'counterweight' (either ground, radials or even the shield of a coax) to
allow the 'return current' to flow.
However a dipole without ground will work perfectly (as it has its own
'counterweight').

3. Assume a lossy ground (real life)
That's a bit like looking in dirty mirror, you will see your mirror-image
but much weaker than with a 'clean mirror'. But the mirror image will be at
the same place regardless wether the mirror is clean or dirty.
For a short vertical monopole over a lossy ground this would mean that only
a (small) part of the power you put in the antenna 'sees a mirror image'
and wil be radiated the other (mayor) part will 'see no mirror image' and
thus will act as for a short vertical monopole without ground : it will not
be radiated.
The rdatio between the radiated and non radiated part is determined by the
ratio of the loss resistance versus radiation resistance. But the part that
is radiated will 'see a mirror antenna' at the same place as in the case of
a loss-less ground and thus it will have the same directivity (= gain).
If that is correct than we have to take the 2.6dB gain into account, even
with a very lossy ground.

I hope the above was not too confusing. This is the way I see it (or try to understand it), it is not meant to be
'the one and only truth'. If there are other opinions, please 'shoot'. We
all can only learn from it.

73, Rik  ON7YD



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