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Drake 2880 Converter



Good afternoon to all,

With a new round of Drake converters apparently on the way I thought
there might be some in the group that would be interested in the fact
that I blew the front end GASFET in another converter while testing
antennas this afternoon.  It apparently happened while swapping the input
cable from one antenna to another exactly like the last time.  (I've been
called lots of things in my life but smart is not one of them!)  I had
planned never to switch antennas with the power on again but somehow I
was unable to comply with that edict.  (I told you I wasn't smart.)  The
last time I speculated about static discharge but after this latest
episode it occurred to me that in the absence of an input (or with a bad
input) the input stage might just take off and destroy itself.  At any
rate whatever the cause it would be wise to take all available
precautions when testing with the converter.

Before I blew up the converter I did manage some antenna tests which
might be of interest.  I built 1 pound and a 3 pound coffee can horns IAW
with the article in CQVHF that Jerry pointed me to last week.  The helix
antenna (21 turns on 1 inch PVC) showed about 3 db better than the 3
pound can and about 6 to 7 db better than the 1 pound can.  If the 3
pound can has around 10 dbi gain and I add 3 db for circular vs linear
polarization (I was using a horizontal dipole for the source)  then that
would give me about 16 dbi for the helix or a couple of db less than
calculations.  Given the fact that I don't yet have a way to optimize the
match and the and my method of measurements is crude at best, the helix
may well be performing up to specs.  At any rate the PVC and the slightly
smaller than calculated size is not a killer.  If the best it gets is 2
db less than calculated then I think I am happy with the helix.  (The
last time I did this I forgot to add the 3 db for polarization mismatch.)
 I also was able to again compare the grounded feed helix with the
ungrounded one and could find no difference so I think I will go with the
grounded feed.

Now all I have to do is get a couple of converters going again!

73 de Jess - W4MVB
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