Message-ID: <000301c31785$af1230c0$3ce4fc3e@l8p8y6>
From: "hamilton mal" <g3kev.ham@virgin.net>
To: rsgb_lf_group@blacksheep.org
References: <000e01c316ce$89bf5e40$fe79a8c0@PCVONWALTER>
Date: Sat, 10 May 2003 16:13:39 +0100
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Subject: Re: LF: Other transmission lines
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----- Original Message -----
From: "Walter Staubach" <walter.staubach@fen-net.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, May 10, 2003 9:31 AM
Subject: Re: LF: Other transmission lines


Hallo Jim and group,

     what you described is exactly my antenna configuration.
The PA is in my shack followed by a small coil 0,268mH with taps, then a 6m
long insulated wire feeds the 2,7mH rubbish-bin coil, located outside under
a roof of glass. The vertical wire follows.  Additionally to the taps I can
make a fine tuning by moving a bundle of four "steam radio" ferrite rods in
the small coil. All this by comfortably sitting in an armchair, no remote
control required.

Hi Walter.
This would not work for me because my antenna and loading coil are 50 metres
away from the shack.
My vertical system has the loading coil and matching coil all located
beneath the vertical drop under the antenna.
Once set up it does not need a lot of attention except I need to change
frequeny. It seems very stable and does not change much with wx variations.
Frequency changes are not often necessary for such a small band only 2 khz
wide approx and I always search the band after a CQ for others on a
different frequency, and I am sure others do the same.
73 de Mal/G3KEV

73  Walter DJ2LF

-----Ursprüngliche Nachricht-----
Von: James Moritz <j.r.moritz@herts.ac.uk>
An: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Datum: Freitag, 9. Mai 2003 18:29
Betreff: Re: LF: Other transmission lines


>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
>
>
>