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Fortunately it was not half as bad as it looks.
During the transmission I heard the fan speed up as the current from the power
supply dropped, opened the roof window where the coil is sitting,
noticed the little flames eating the white plastic bag, and blew them out
just like a birthday candle. The fire had probably started in the
greenish foam insulation where the wire from the bottom of the outer coil runs
back up to the top of the inner coil. Afterwards the whole coil was still
intact DC-wise (4 ohms) but showed slightly lower inductance and very
high RF loss resistance (about 53 ohms), apparently a short circuit between
adjacent turns. After separating the injured turns with
my fingernails, the RF resistance went back down to the normal 7 ohms, so I'm
confident that this coil can be repaired.
But I decided that I want to build a more
substantial coil using Decca RF litzwire, the "secret weapon" which serves so
well in Stefan's beautiful large glass coil. However as this insulated
litz is much thicker than the old one (4.5 mm od instead of 1.5 mm), the
only way to get a compact 5 mH coil is winding in multiple layers.
Here's my first attempt
http://df6nm.bplaced.net/LF/pictures/topload_doubled/decca_fourlayer_coil_29.jpg ,
using 180 turns in a staggered four-layer
winding, about 28 cm diameter and 19 cm long in five
segments. The inductance was then adjusted by removing 18 turns
from the bottom end. DC resistance is only 1 ohm, but at 137 kHz the
loss is a disappointing 6.5 ohms, hardly better than the thin litz wire
used before! This is clearly due to proximity effect, ie eddy currents induced
within each inner filament by the magnetic field created by
the outer layers. For a given strand diameter, there must be an optimal
total thickness of the layers, and using more copper than that reduces
the Q factor again. Guess I will try to rewind this coil on a larger
former in three or two layers, which will require a longer wire
but probably reduce AC losses.
Antenna pulled in to low position now, watching LF
but no transmissions from here tonight.
Best 73,
Markus (DF6NM)
----- Original Message -----
Sent: Saturday, March 30, 2013 8:29 PM
Subject: LF: Re: Re: Re: DF6NM DK7FC in CT
Markus
Sorry to hear of the mishap ... pix of the damage
are always appreciated! Wonder if your neighbors got to see the 'show'?
If you haven't started a fire while
transmitting LF you're not running enough power ;~) .
Jay W1VD WD2XNS WE2XGR/2
----- Original Message -----
Sent: Saturday, March 30, 2013 10:12
AM
Subject: LF: Re: Re: DF6NM DK7FC in
CT
Here's the result of 13 minutes at 1W
ERP
...more work to do :-(
Best 73,
Markus
----- Original Message -----
Sent: Saturday, March 30, 2013 1:56 PM
Subject: LF: Re: DF6NM DK7FC in CT
...
At the end of a 15 minute transmission, the
temperature at the inside of the litzwire loading coil had gone up to 80°C, so
I seem to be limited by current rather than voltage now.
...
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