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Date: Tue, 04 Jan 2000 15:31:38
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: LF: LF antenna
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Over the last weeks I noticed that Werner, ON6ND, always got a +/- 10dB
better report from UK stations. The fact that he is about 70km closer to
the UK may take 2dB. From his antenna details I could 'guestimate' that the
radiation resistance of his antenna is about 0.05 Ohm while my antenna has
about 0.04 Ohm, so that is another dB. And also the antennacurrent at ON6ND
exceeds mine with about 30%, that is about 2dB better.
But that means that there are still about 5dB 'missing' at my side, so I
took advantage of the holiday period to try to improve my LF antenna.
G3XDV reported an improvement of his antenna when replacing the isolators,
so as a first step I decided to replace the old 'plastic tubes" by real
ceramic isolators (about 10cm long). Although this seemed to have solved
the arcing problem I had during wet weather it did not improve the
antennacurrent or the signalstrength at all.
Next I added (against better knowledge) about 45m of blank galavanized iron
wire to the radial system. A lot of work since for estetical and safety
reasons (traps) only radials that are dug in are approved by the
authorities of the house (= XYL). The result was nil, not a single mA more
of antennacurrent and stil the same reports.
A bit desperate I started my final attempt, again inspired by Mike, G3XDV,
who reported an improvement of his antenna by adding some inductance at the
top of the vertical section of his antenna. Some calculations showed that
0.6mm Cu-wire spaced 0.6mm would give a good weight / loss ratio, very
convenient as I had a 120m roll of 0.6mm stranded Cu-wire with an outer
diameter of 1.3mm (isolation). This wire could be used close-would
simplifying the manufaturing of the coil. I ended up with about 110
windings on a 5 liter bin (the ones they sell purified water in).
Inductance was about 1.25mH. A bit small as the total loadingcoil so far
was 2.9mH, but I'd just give it a try. After solving the mechanical problem
of fixing the coil at a height of 12m just hanging between wires I retuned
the antenna and with 2mH at the base it got the antenna resonant. A test at
full power showed a slight decrease of the antennacurrent (from 1.85A to
1.8A) probably caused by the loss in the extra coil. Also, due to the extra
weight of the coil, I had a 'sag' in the wires holding up the antenna,
reducing the height by about 0.7m. So far not very promising, but as I
spent about a full day in manufacturing and mounting the coil I decided to
give it a try. DCF39 seemed about a S-point stronger, but that could be
just QSB. Also G3AQC who was CQ-ing sounded stronger, but the ear can be
very subjective. I called Laurie, being curious about my report. To my
pleasure I got a 1 S-point better report than I got so far from him, so it
was not just my imagination that despite the lower antennecurrent signals
were stronger.
The same evening I managed to work Finbar, EI0CF, (got 439 / 539) for the
first time after having called him over and over for the last year.
The next days reports from several stations (G4GVC, G3YXM, DK8KW) showed an
increase of 1 S-point.

Based on simple calculations the current distribution should not improve
significantly by the 1.25mH 'top inductance' as my antenna already has an
extensive capasitive toploading. Would interesting to know what
antenna-simulation software tells us (if someone wants to simulate the
antenna I will be happy so send him all antenna dimensions).
Basicaly a 'top inductance' reduces the voltage on the vertical part of the
antenna. Assuming that the losses induced in surrounding objects (ground,
houses, trees, etc...) affect mainly the electrical field this could mean
that by reducing the voltage on the vertical section also the losses caused
by this section are reduced. As my antenna is surrounded by several 100's
of trees, some as close as 5m from the antenna, it could be possible that
the trees (that are mainly vertical) mostly affect the vertical part of the
antenna.
This makes sense to me. 
Any comments ?

Next plan is to increase the 'top inductor' to about 2.5 / 3mH, just
leaving a small inductance for the day-to-day tuning at the base. But I
have to solve some mechanical problems first, with the actual construction
I cannot mount a bigger coil at 12m.

73, Rik  ON7YD