At 21:48 6/02/00 +0100, IK1ODO wrote:
I have a question for antenna modelling gurus (G3LDO?)
What is the vertical radiation pattern of our shortened
verticals? Is it dependent of ground resistance and nearby
objects? may we model it?
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). Any any antenna less than
10% of the wavelength is consired 'short' (so even the 7S6SAJ guys were
using an electrical short antenna). A short vertical above a perfect ground
has a gain of 4.8dBi, again independent of the antennalength.
Regarding the groundresistance the rules was : the lesser the better. No
only for the sake of antenna effeciency but also to avoid radiation at
higher angles. This 'high angle radiation' was for the commercials, who
were only interested in surface wave propagation (stable 24h/day, 365
days/year) not only pure waste but also a source of possible interference
(via ionospheric wave).
But if we want to make 'DX-QSOs' with 1 Watt ERP we defenitely need
ionospheric wave propagation, so producing some higher angle radiation
might be usefull.
So far I have not found too much information about ionospheric propagation
on LF in literature (except for how to avoid it). It would be interesting
to find out what radiation angle is optimal, if the angle is too low then
the signal will bend around the earth (surface wave) and won't 'escape'
into the ionosphere, if the angle is too high then the 'hops' will be
short. Maybe the interesting contributions of OH2LX can help us with that
(tnx Vaino).
One remark regarding antenna length :
Despite the fact that radiation pattern and antenna gain are independent of
the lenght a longer antenna has still a big advantage : a higher radiation
resistance. Doubling the length of a short vertical antenna will quadruple
the radiation resistance, this is a 6dB 'gain' due to a better ratio of
radiation resistance versus loss resistance (assuming that the losses are
much higher that the radiation resistance). But you can get exactly the
same effect by quadrupling the TX-power, so in theory you can reach the 1
Watt ERP limit with any antenna if you use adequate power. And, despite
what some try to make us to believe, you cannot hear the difference between
1 Watt ERP from a big antenna with low power or a small antenna with high
power. Of course with a big antenna and high power the 1 Watt ERP limit can
be exceeded significantly, but that is against the legal limitations and I
am sure a real ham will not do that.
As far as I know the only station that got a special permit to run higher
ERP is 7S6SAJ. Congrats to Chister and Co. for the '1kW experiment', they
got a lot interest from all over Europe. Let's hope that this will result
in some more countries becoming active on 136kHz.
73, Rik ON7YD
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