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Hello lf-group!
We recently saw an interesting description of
IZ7ATH´s 160-meter EH antenna and with this description came of course the
question of whether this antenna could be thing for 136 kHz.
There have been few comments on the reflector
on this issue, --comments which I as such
largely agree with.
It seems to me, however, that the recent discussion
and earlier discussions on that antenna
too, does not bring any answer to the question what
this antenna actually is.
ON7YD, RIK mentions the antenna as possibly
being "the funny looking top-hat of a vertical".
Now let us see, what it really
is:
The antenna part of it, the two metalic cylinders,
constitute a short (very short) dipole!
The two coils and the two capacitors form a
BOUCHEROT BRIDGE. (This bridge was, by
the way, invented back in the early 1930´ies). A
boucherot bridge performs two functions:
First, it is a unbalanced-to-balanced device
(balun) and secondly, it provides an impedance trans-
formation, in this case low (50 Ohms) - to - high
impedance.
Ideally, there would be no feeder radiation and
further, the antenna, being a dipole, should
not care too much about the earth beneath the
antenna! -- In the real life things are, however,
a bit more complicated.
If we look at the IZ7ATH 160-meter antenna, the
characteristic impedance of the boucherot
bridge is 600-700 Ohm, giving a transformation from
50 Ohms to some 7 kiloOhm (the two
coils ought to be identical and btw., not coupling to each other). There is no doubt in my mind,
that the IZ7ATH antenna works quite well. It is probably a bit
narrow frequencywise.
Now to the question of using this antenna on 136
kHz: Asuming somebody having a large
garden and very tolerant neighbours would try
out this antenna, what would he experience?
If I may guess, it might not be bad at all.
The antenna being a dipole, ground losses would
be considerably lower than with a
ground-reference vertical. There are two coils, who in
addition to providing impedance transformation also
must tune out the antenna capacitance.
I dare not at the present try to estimate how
the losses of the two "airborne" coils
would be compared to
the losses of the variometer of a ground-referenced vertical.
Who is going to try out such an antenna? By the
way, anticipating possible neighbour
reactions, it might be an idea to start building a
cage-like structure instead of sheet
metal cylinders!
73, Niels,
OZ8NJ+
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