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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