Message-ID: <38869BE1.554DFC17@netscapeonline.co.uk>
Date: Wed, 19 Jan 2000 21:23:46 -0800
From: "g3kev" <g3kev@netscapeonline.co.uk>
Organization: Netscape Online member
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re: Antenna Modelling
References: <2000011918370868199@zetnet.co.uk>
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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
>
> <g3ldo@zetnet.co.uk>