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From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Date: Tue, 4 Jul 2006 18:06:51 +0100
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Subject: LF: Re Antenna tuning
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Status: O

Hi Again, Ok the capacity is is the right ball park. I estimate 1200 to
1350pF, because I am not sure what the effective capacity per metre is at
that height ...I think it tapers off to about 5pF per metre as you go up.

That means you need around 0.9mH to 1mH to resonate it. If your variometer,
or the total loading inductance, is in that area then you are not tryng to
resonate it at a harmonic as I worried about. It should be fairly obvious I
guess now as you would need to only have about 240uH in the variometer to be
resonating at 270kHz. If you dont have an "inductance meter" coving that
range a quick count of the turns and a measuremnt of the diameter pumped
into Reg Edwards SOLNOID3 program will give an esimate of the the inductance
which should be a good guide.

So 1).  Measure the variometer and look for about a mH

The next area where there could be a problem is the hardline resonating with
the tapped portion / link coil. I am not sure about this but I think this
could be a problem if the variometer is not big enough (bigger than would
resonate at the harmonic ) but if the antenna is off resonance it will
present a odd impedance to the end of the coax. Now 75m of 50 ohm coax is
going to look like 7500pF. That would only require about 45uH of inductive
reactance to resonate at 270kHz.. So too many tap turns and you might get
misleading results. I think it is only possible to calculate the tap point
or link turns if you know the resistance you are trying to match......and
most of this is ground loss so a bit of an unknown.

I dont know what best to suggest now other than to test the antenna at the
load coil so you are not working over a long length of coax. I would use a
small simple bridge to do this. With a resistive bridge the null will be
deepest when the antenna is resonant, then look to find a 50 ohm impedance
tapping point. One problem with LF bridges on antennas is that the antennas
do pick up a lot of signal from BC stations and beacons which are way of
frequency so you really need a tuned detector rather than a simple wideband
RF milli-voltmeter as most simple bridges use. When a bridge is working
properly the nulls are excrucatingly narrow, it is very easy to miss them
and often very difficult to set the pot in the right position. If the nulls
are wide and poorly defined then the antenna is either not resonant or you
have a big received signal.

Sorry to hear about your sympoms, I have lived with that in my family and I
know the effects. Keep the "little grey cells" working as hard as you can
and try and stave off the development. It seems to be like physical exercise
that strengthens the muscles and helps stop us "oldies" falling over and
breaking things.

I hope that is useful. It is difficult to help at this sort of distance with
only text as a guide.  Possibly other can add useful comments or
ideas....most of us have fought with LF antennas in some way, before we
finally got then "tamed".

Best Wishes
Alan G3NYK