Afternoon Carter,
Did you test the central vac pipe in a microwave oven to make sure it's
not the kind of plastic pipe that heats up in the presence of RF?
I didn't but I will. They haven't gotten hot (or even warm) with the
amplifier on 75 meters running just under 950 watts output.
For those of us that don't have access to the 1990 article, what is the
value of the variable cap and its voltage rating? How is it connected?
The original article (with updates) is located at
http://www.vcnet.com/measures/bbat.html. The QST article was an edited
version with a picture of a working unit and prettied up pictures. The
pictures I have up show a variable capacitor taken from an old Command
transmitter. I first used a capacitor from TEN-TEC
(http://www.tentec.com/tkit.htm#model1207) but changed it out for the
Command unit to get a lower minimum capacitance. The TEN-TEC 1207 with a
wider spacing will handle higher power. Some day when I get the time I'll
re-install the TEN-TEC. Make sure to add an insulator to the shaft of
whatever variable you use. RF burns are less than fun.
Any idea if the inductance values presented will work for a 130 foot
ladder line fed dipole rather than your 950 ft loop?
I built the input balun and the two coils from information and pictures
from Measures' article and a couple of other pictures and places I found on
the web. Somebody posted measurements on either here or qrp-l of various
coax type baluns within the last year. The information (from all the
sources) seem to be all about the same with some slight variation. I would
think it would work with a dipole.
I used #10 wire for the two coils. If I had it to do over I would use #14
instead. It took several tries to get the spacing correct and the #10
wound tight enough to stay in place. It happened that an electrician
friend of mine had an end of a role of solid #10 kicking around. #14 house
wire would likely be more common (and cheaper).
Finally, if I do use it with my dipole, it is a given that due to the
limitations of the real world, the dipole is *not* electrically
symmetrical. Therefore, when you tune this, do you just go by being on the
same tap for each coil, or do you tune for equal current in each leg
(conceivably resulting in being on different taps on each coil)?
If you want to get really into this I suppose you could put a couple of rf
current meters in line but not having a pair of those I would accept
what tuning I managed to get. Measures does some testing in his article
but into a balanced antenna if I remember correctly.
Dave
VE1VQ
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