From: "James Moritz" <J.R.Moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 19 Jan 2004 14:40:40 -0000
Organization: University of Hertfordshire
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In-reply-to: <C75A388845B4B54B9CF5E2ADB589B0E3031AEFDE@btss.siemens-pse.sk>
Subject: LF: RE: Test failed...
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Dear Rich,

Sorry to hear about your problems... Is it possible that something silly
has happened - e.g. the mast insulator has been shorted to ground for
lightning protection? That would make it hard to tune up!

I guess the capacitance of the mast will be about 1500 - 2500pF, so
loading coil inductance should be in the several hundreds of uH. If the
antenna is resonated with a simple series L, the resistance component of
the antenna impedance will probably be only a very few ohms - if your TX
does not like low impedance loads, it may not deliver much output to
such a load. When I operated from the old Decca site, I found the
simplest solution was to make a custom matching transformer on the site
- this had about 20t primary on a large ferrite ring connected to the TX
output, and I then wound on the secondary (about 6t) for a reasonable
match with the series coil adjusted for resonance.

You will get very strong received signals with a big mast (Marconi was
using antennas this sort of size to receive trans-atlantic LF sigs on
crystal sets)- so signal strengths will still be high if the matching is
not very good - even if there is something like an open or short
circuit. But if you heavily attenuate the RX input, there should still
be a sharp resonance peak when the antenna is correctly tuned.

Hope you have another opportunity soon-

Cheers, Jim Moritz
73 de M0BMU