Dear LF Group,
Getting back to the topic of LF antennas, another useful type of feed
method for LF antennas is the "450ohm unbalanced air line", or, to the
layman, the "bit of wire hanging in the air". The idea is to have a loading
coil at the feed point of the antenna as normal, but with about 90 - 95% of
the inductance required to achieve resonance. This is then connected back
to the shack by a single wire, where the other 5 - 10% of the inductance is
located, which is made variable so that tuning can be performed in the
shack. The feeder can be 10-20m or more, and a couple of metres or so off
the ground without having much effect on antenna behaviour. The impedance
that the feeder is operating at is some hundreds of ohms, comparable with
the Zo of the wire feeder, so loss in the feeder is minimal - analogous to
feeding an HF doublet antenna with open-wire balanced line. Another way of
viewing it is as the same as having an elevated loading coil, but with the
loading coil displaced horizontally instead of vertically.
The advantage is that the single-wire transmission line only carries a
few hundred volts, so normal HF wire antenna type insulation where the
feeder comes into the shack is adequate. This allows you to locate the
loading coil in the best possible position, and greatly reduces the
insulation problems, losses, noise pick-up and fire hazard compared with
having the loading coil inside the shack, while keeping the convenience of
being able to tune the antenna from the operating position without needing
a remote control. Since the feed point is located close to the transmitter,
impedance matching to the TX output can be done in any way you choose
without worrying about matching to an intermediate transmission line. I
used this method successfully for some time witth my first LF antenna a few
years ago. Also, we used this arrangement for the TX antenna on the
Porthcurno expedition recently, with good results.
Cheers, Jim Moritz
73 de M0BMU
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