[Fwd: LF: 137 Loading coil]

[Andre Kesteloot]

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Gang,
FYI, a loading coil for 137 KHz
Andé N4ICK

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Date: Sat, 03 Jan 1998 12:16:16 +0000
To: rsgb_lf_group@blacksheep.org
From: drassew2@interalpha.co.uk (Andy Talbot)
Subject: LF: 137 Loading coil
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Reply-To: rsgb_lf_group@blacksheep.org

I've just started to make another loading coil, for 137kHz as I don't like
the idea of having a tapped two band job - there would be a lot of surplus
copper when it is used on 137.  Visited Great Mills DIY for one of their
fermentation bins (see previous writings on 73kHz loading coils) and they
were out of stock!  Called into Homebase and they have a waste bin almost
exactly the same size.  Made by Zetta, this is bright blue and at the much
lower cost of =A37.50.  The fermentation bin is about twice this figure I=
 seem
to recall.   The taper on the waste bin is slightly more but this can be
made use of, to lock the turns more securely.  It also seems slightly
stiffer, but is made of thinner plastic and has a flip top too !  (there
must be a use for this)

As only half the number of turns will be needed as for 73kHz, I will use
1.25mm diameter wire instead of 1mm and incorporate a proper variometer
arrangement instead of the sqeezed in bodge job used on the 73k coil.   I
have an Addis Tupperware type of plastic container 230mm diameter and 100mm
high which will hold 40 - 50 turns of wire.  This fits perfectly inside the
waste bin and can be rotated around a horizontal axis to aid or oppose the
main winding.  Hopefully this will give me so much variation in inductance
that I can cover the whole band in wet or dry weather and leave scope for
antenna experimentation without having to add or remove turns.  The
variometer currently used on 73kHz has only 10 turns and gives me no more
than about 500 Hz of tuning range - just enough for wet / dry weather
adjustment.

Incidently - I am coming round to the idea that the change in resonance when
the antenna is wet is just due to rain on the wire increasing the effective
diameter and capacitance,  water in the air doing likewise.  As soon as it
stops raining, and I knock drops off the wire the frequency rises again.
The increase in load resistance in wet weather is a different matter, and
looks to be due to extra losses introduced into the near, reactive, E field.

Andy  'JNT


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