Peter Dodd wrote:
G3KEV wrote
> My 3 inverted L antennas are not strung together but spread out around the
> mast so go back to the drawing board. When I progressed from 1 to 3 inv L
> antennas I noticed signals up around 1S point (6 db both on tx and rx, also
> the total base loading inductance dropped to 0.9 mh. Your 1.5 db
> calculation does not correspond with practical observations at this qth.
OK. I accept that the real antenna might differ from the model. This
is because I do not know the detailed structure of the antenna or
your ground characteristics and had to make some assumptions. I went
for 'good' ground rather than 'very good' or 'poor', which were the
other options. You can plug in ground conductivity and dielectric
constant into the model if it is known.
But lets accept your figure for the improvement to the original
inverted L, i.e. 6dB.
If you add 6dB to the original antenna gain figure of -15dB you
finish up with a total gain of -9dB. Impressive!
This means you can achieve 1 W erp with a transmitter output power of
25 - 30 Watts.
> Theory only points one in a certain direction but in reality the practical
> application often conflicts because of environmental considerations, ie
> ground conductivity, number of radials and type etc.
True, but again it can still be modelled by basing some of the input
parameters on the real measurements.
As you are all aware the losses in the antenna are the ground
conductivity (and dielectric constant), coil losses and conductor
I^2 R losses, which are in series with the antenna radiation
resistance. If I know all the physical dimensions of the antenna,
the transmitter power and the antenna current I can calculate the
antenna radiation resistance (and antenna gain), and hence the losses
(by altering the ground characteristics until the power and the
antenna current agree with the supplied data). From that, erp, which
I take to be the power dissipated in the radiation resistance, can be
calculated by using a lossless model.
It may not be perfect but, in the past, the calculated results have
correlated well with observed signal level/distance readings.
Its all guesswork and even if you had all the facts it would only be a rough
estimate.
Forget the modelling and get outside and get your feet wet getting some real
hardware into the air.
--
Regards, Peter, G3LDO
<[email protected]>
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