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>
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