Comments are made after the earlier material from Steve and Rik:
Rik Strobbe wrote:
At 18:24 7/03/99 +1100, VK2ZTO wrote:
>...
>The antenna configuration I have decided to attempt first is a
>capacitively-loaded vertical mounted on the roof of the house. This is the
>only tree-free area on the block. The loading coil I want to place as far
>up as possible to maximise the current in the vertical part of the antenna.
> Elevated radials will cover the roof as far as possible around the base of
>the antenna and connect to surrounding metal roofs, sheds, chook runs and
>an aviary and also to ground.
>
>A picture of this can be found at:
>
>http://www.zeta.org.au/~ollaneg/images/AXSO_ant.gif
>...
Hello Steve & LF gang,
I had a look at the picture on your web-page and it reminded me of an
(unsuccesfull) attempt to improve my LF antenna. At my QTH it was not the
house but some smaller (5m high) trees under the antenna that I tried to
'screen' with some elevated radials. In the beginning I was very confused
by the result, as putting radials over the trees increased the
antennacurrent from 0.72A to 0.90A but decreased (!) the signalstrength by
about 2dB instead of the increase of 2dB that could be expected based on
the higher antennacurrent.
Trying to find an explenation for this 4dB 'deficit' I came to the
conclusion that elevated radials not only screened the lossy trees (and
increased the antennacurrent) but also elevated the hight of the 'virtual
ground' (and lowered the effective height of the antenna). So in my case
these elevated radials gave a gain of 2dB by increasing antennacurrent but
at the same time also a loss of 4dB by lowering the effective height of the
antenna, so overal 'gain' was -2dB.
Of course it is not sure that you will come to the same result when
screening the house but I just want to warn that an increase in
antennacurrent not always means an increase in ERP.
73, Rik ON7YD
This is very good experimental evidence that elevated radials do reduce
"ground losses", but simultaneously reduce effective height. The
theoretical efficiency depends on EFFECTIVE HEIGHT SQUARED so for an
amateur situation with realistic limitations in the height of the top
loading, then elevated radials that go over local trees will inevitably
end up losing more in effective height (and the penalty is height
squared) relative to the reduction of losses in the ground (much more
flux goes on to the elevated radials than via the ground).
There is also a matter of nuisance value of elevated radials, as they
can be a tripping or collision hazard. The general feedback from those
who I know have tried elevated radials is that they soon "get the
message" from the XYL and others that "they are in the way, get rid of
them ...".
Lots of buried radials take a lot of beating for an amateur LF antenna.
Regards,
Bob ZL2CA
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