At 10:04 25/01/2002 +0000, you wrote:
...So nearly all the energy that goes into the ground is dissipated and
does not return to the feedpoint. Therefore it cannot reinforce the
radiation pattern. In that case, does the theoretical gain still hold?
Gain is only obtained from directivity. Directivity can be calculated
from physical considerations but the equation to obtain gain from
directivity is G = e*D , where G = power gain, D = directivity, and e =
radiated power/total power...
Dear Walter, LF Group,
Not sure if I'm an expert, but...
I think the losses that the signal is subject to between transmitter output
and receiver antenna can basically be divided into 2 types:
Firstly, the losses that are incurred in producing a current in the antenna
in the first place. In the case of a vertical it is necessary to generate a
huge voltage gradient between the antenna element and earth in order to get
useful current to flow, and this field gives rises to large losses in
materials in that field; the earth itself, buildings, trees; the antenna
wire and it's immediate environment can be modelled as a lossy capacitor.
The theorists say that it is the RF current in the wire that is actually
causing the propagating E and H fields, so the losses due to the electric
field are not really "radiation" losses and make up the "e" part of the
gain formula. These are the losses which make the feed point resistance of
the antenna greater than the radiation resistance - the power dissipated by
the radiation resistance is that part which is being radiated away, and
wether it is absorbed or carries on propagating, it does not come back and
affect the impedance of the antenna
Given that a particular current is produced in a wire of a particular
geometry, the radiation pattern (and so the "D" part of the formula) can be
calculated either analytically or a numerical result obtained using NEC or
something similar. "Image" currents in a ground plane are taken into
account. The radiation pattern for a monopole over lossy ground uses a
model for the ground which, as I understand it, is a plane surface with a
reflection coefficient of magnitude < 1 to account for the energy "radiated
into the ground". This type of modelling is one way of producing ground
loss curves. As PA0SE pointed out some time ago, The lossy ground model
produces different radiation patterns at different distances from the
antenna. The default radiation pattern displayed by EZNEC and shown in the
text books is the "far field" pattern, mathematically the radiation pattern
at a distance tending to infinity. For lossy grounds this inevitably has a
null at ground level, because over very large distance, the ground wave
will be attenuated very much more than the signal propagating through
space, while a monopole over perfect ground has maximum signal at zero
elevation. However, as the distance decreases, the attenuation of the
ground wave is reduced, until at short distances, where the excess loss due
to ground losses is negligible, it turns out that the monopole over lossy
ground has much the same radiation pattern and D as the perfect ground case.
This actually seems to be close to the truth - calculating the field
strength produced by my piece of wet string with a certain current in it by
using a NEC model consisting of perfectly conducting wire over a perfect
ground gives results that are within a few dB (usually a few dB higher) of
reality, over distances between 1 - 10km. It seems reasonable that the few
dB's additional "site loss" that occurs could be explained by the obstacles
close to the antenna which the radiated signal runs into - since higher
antennas that reach above this local clutter have reduced site loss, at
least on the basis of the very limited data that is available.
So in summary: There are "antenna" losses, the power dissipated by the
antenna and its near environment whilst a certain current is flowing, which
are responsible for the efficiency "e" part of the gain formula, and which
determine the loss resistance of the antenna.. The directivity D is
affected by the distance from the antenna - close in, D is much the same as
an ideal monopole, far away the ground losses modify the radiation pattern.
Due to the very low values of e for all amateur antennas, G will always be
a large negative number of dB, however. Calculating the radiation
resistance and measuring the antenna current is the best easily done way of
determining what ERP is being radiated, since it does not require knowledge
of the antenna losses, only D, which is much the same for any electrically
small vertical antenna, ie. 1.8 relative to a dipole in free space. So ERP
= D.Iant^2.Rrad. In practice, the actual ERP may be lower by a few dBs
"site loss"
There are lots of loose ends to this - it means that the radiation pattern,
gain, and so by definition the value of ERP, depends on the distance
at which it is measured where ground losses are significant. Also,
modelling the ground as a 2 dimensional plane is not very satisfactory -
currents and fields exist within the ground, as the cave radio guys well
know! Significant LF radiation from "earth" antennas has been demonstrated.
NEC just does not seem to model real grounds well where one part of the
antenna actually has a ground connection. More work needs doing on the
"site loss". The lobed radiation pattern of verticals shown in the
textbooks is fine at HF, but at LF the "upwards" components of the radiated
signal will be bumping into the ionosphere and coming back down long before
the ground wave has decayed. One could spend a whole lifetime on this sort
of thing...
Cheers, Jim Moritz
73 de M0BMU
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