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Date: Thu, 26 Feb 2009 16:16:38 +0000
Message-ID: <fc7eccec0902260816u68aa7394lc2adc64e2440b62e@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
DomainKey-Status: good (testing) 
Subject: LF: LF Antennas - back to basics
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Having now been encouraged by being decoded now by at least two US
stations,  was wondering how much improvement in efficiency I could
scrape out of my existing antenna.  It is a Marconi Tee, the vertical
element consisting of three wires from the base tapering to 200mm
spacing at the top.  There it is attached to the centre of a a
capacity hat 15m long and 200mm wide made of three parallel wires.

The ground system is a bit haphazard in my 7m wide by 12m long garden,
with about 4 wires meeting at the base of the antenna meandering
around the garden, starting off in different directions from teh base.
 They are all connected to substantial ground rods in heavy clay  at
their far points, a with a few randomly spaced rods in between (I used
several 1m lengths of 15mm water pipe as well as proper rods - there
is probably about 9 or 10 rods in total, mostly 1 - 2m in length -
can't remember that far back!   Some of the earth conductors were also
bonded to the house wiring by a couple of substantial straps, and even
inside the house I added more straps at strategic places.   I also
ensured there was a good fat cable run directly under as much of the
capacity hat as possible.   As a test, disconnecting the PME earthing
bond and measuring the DC resistance between my total set of
connections and the incoming conductor ground/neutral point, the value
was a pretty consistent 3 ohms.  Haven't done this test for a few
years, but at the last check it was still of this order.

Back in the 73kHz days a number of (mostly big!) stations were
surprised my signal was as strong as it was for probably the smallest
antenna in use then; G3LDO put it down to the fact I had concentrated
plenty of earthing wire to run directly under the top-hat.

But, its all 14 years old,  still OK, but hopefully there's a wealth
of experience now, and I wonder if there are obvious improvements that
could be made:

Firstly, bearing in mind there is no scope whatsoever to raise one end
of the capacity hat which is defined by the apex of the house roof,
and the other end would also need some major sugery to raise its heigh
substantially, that only leaves the middle, ie the height of the
actual radiator.    It wouldn't be too difficult to raise this to 10m
or even more with a fibreglass pole, but will I be throwing away all
the advantage by having the top-hat drooping down to 7m?

Secondly, pictures of other peoples antennas aften show a substantial
grounding plate immediately under and around the antenna base.  Just
how far out is is worth going with a really substantial base.   I
could cut more conductors into the ground; while the ground is still
is still soggy in March may be a good time to do this.

Any ideas please ...?


Andy  G4JNT
www.g4jnt.com