Message-ID: <2000011918370868199@zetnet.co.uk>
Date: Wed, 19 Jan 2000 18:37:08 GMT
To: rsgb_lf_group@blacksheep.org
From: "Peter Dodd" <g3ldo@zetnet.co.uk>
Subject: LF: Re: Antenna Modelling
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>
Content-type: text/plain; charset=windows-1252; format=flowed
MIME-Version: 1.0
Content-transfer-encoding: 8bit


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.


-- 
Regards, Peter, G3LDO

<g3ldo@zetnet.co.uk>