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Hello Dick, (and group)
I've wound myself a coil on a bucket, the
inductance is a bit low at the moment, the whole thing resonates no lower
than 250kHz at present - so I've some work still to do before I transmit. I
put lots of taps in, as suggested.
But I am puzzled. In the LF Experimenters Handbook,
at the end of the description of your loading coil, it says
"by short-circuiting turns at the top and bottom
end, inductance can be decreased in steps of one turn." Why short-circuit the
unused turns? Surely that is like having short-circuited turns on a transformer
and will make the coil very lossy ?
Why not just connect the vertical part of the
antenna to the appropriate tap, and leave the others open ?
73
Hugh M0WYE
----- Original Message -----
Sent: Tuesday, November 05, 2002 10:18
AM
Subject: LF: Series loading coil
To All from PA0SE
I also use the series coil system
for tuning and matching the aerial to the 136kHz transmitter.
The coil was
made by bolting four 1m long and 12.5cm diameter PVC pipes together and
putting on 200 turns of 1.4mm diameter copper wire with black PVC insulation
as used for house wiring.
Inductance is 4.4mH and unloaded Q about
350.
Starting from the top I made a tap at every 10 turns by putting a
twist in the wire. Heating with a soldering iron melts the insulation that
then easily comes off to bare the wire.
The bottom 10 turns have a
tap at each turn. By short circuiting turns from the top and the bottom the
coil can be adjusted in steps of one turn. For fine tuning a variable
capacitor is in parallel with the coil. As this increases the current in the
coil and thus the loss in it (proportional to current squared!) I adjust the
coil so that only minimal capacitance is needed.
![PA0SE Coil.jpg]()
The
capacitor shown started to arc over at 60W of power. It was replaced by a
vacuum capacitor rated at 5000V maximum. When transmitter power was
increased from 120W to 320W I connected the capacitor over half the coil
in order not to exceed the maximum voltage.
The transmitter is
connected to one of the turns at the bottom end. I don't worry about SWR and
simply select the tap giving maximum aerial current (3A in dry
weather).
A better picture of the coil above can be found on page 62 of
G3LDO's The low frequency experimenter's handbook. The coil with the
vacuum capacitor is shown on the cover of the book.
73, Dick, PA0SE
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