Message-ID: <2000020710145268199@zetnet.co.uk>
Date: Mon, 7 Feb 2000 10:14:52 GMT
To: rsgb_lf_group@blacksheep.org
From: "Peter Dodd" <g3ldo@zetnet.co.uk>
Subject: LF: Re:  LF Propagation and modelling
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 IK1ODO wrote:

> >What is the vertical radiation pattern of our shortened
> >verticals? Is it dependent of ground resistance and nearby
> >objects? may we model it?

 ON7YD wrote
> From what I found in old handbooks (60's and before, when LF was more
> 'fashionable' for the commercials) the radiation pattern of a short
> vertical is independent of its length (height).


This seem to be true. According to EZNEC the polar diagram of an 
electrically short antenna is the same which ever way you configure 
it. So an inverted L or V or a T antenna all have the same polar 
diagram - a sort of half doughnut.

As the height of a vertical is increased to a quarter wavelength the 
polar diagram remains the same although the radiation resistance 
decreases, which means that the 1W erp can be achieved with less power.


> But if we want to make 'DX-QSOs' with 1 Watt ERP we definitely need
> ionospheric wave propagation, so producing some higher angle radiation
> might be useful.

True. If the near quarter wave has a large horizontal  component (as 
the OH1TN and MM0ALM antennas) then the polar diagram changes so that 
some of the radiation angle is high. We have noticed interesting 
propagation effects with both these stations and they both have good 'ears'. 

As regards modelling the effect of ground EZNEC is quite useful in 
this regard. In fact if you select 'High-Accuracy Ground' 
calculations it produces what is known as a Sommerfield-Norton 
interpolation table of ground characteristics based on the ground 
complex permittivity. This is described on page 6.10 of the 'LF 
Experimenter's Source Book'. In short this allows modelling of the 
ground effect of antennas for 136kHz provided any horizontal 
component of the element is over 10m high.
In general, the ground 'quality' has little effect on the shape of 
the polar diagram although it has a marked effect on antenna efficency..
I have not seen a method of modelling electromagnetic obstacles such 
as buildings and trees.

-- 
Regards, Peter, G3LDO

<g3ldo@zetnet.co.uk>