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.
...
