The original question was about replacing ceramic switches - which in our
context are usually used in the tank circuit. PINs used at 50 ohms are a
totally different kettle of fish, and of course can handle very high
powers. With a 3kV HV supply and 1500 watts out, there's around 5kV peak to
peak to handle, and between 10 and 15 amps in the tank circuit - which is
why Rich is always advocating some cooling air around the tank coil. I was
reading in an old IEE HF Conference report of the '60's about about the
problems with variable inductors in a 80kW tx - they used 1/2 inch copper
tube and forced air down it for cooling, while the contact was made up of
80% silver and 20% carbon, which is apparently self lubricating. The
circulating current there was said to be 80amps.
PINs do have a long recovery time, and use this to be able to handle a much
higher RF current than the DC through them. The reason you need a long trr
is so that when the RF current reverses through the diode (on the other
half cycle) the carriers can't be swept out of the junction, so the RF flow
is not interrupted. There's various equations relating maximum RF current
to bias current and trr and frequency. There's some excellent material on
PINs and their applications available from MACOM, much of which originated
when the major supplier was Unitrode. There's also a fair bit on the IMD
performance of PINs when used in attenuators - a lot of that work was done
for the cable TV industry. Because the way the PIN works is to have a long
recovery time, I've found that some brands of 1N4007 do well as switching
diodes (Motorola are good). The trr is about 8 microseconds. However, at
the 100 watt level, they tend to fail after a few months - strange, as they
don't get hot.
There is one way of getting around the ceramic switch problem, and that is
to make your own switch, using fibreglass. Mind, it's not really a project
for kitchen table construction - I think you ideally need some machinery -
at least a vertical drill ( drill press in American) and doubtless a lathe
and mill would make things easier, depending on whether or not you were
stealing the indexing mechanism from somewhere else. Desirable would be
3/16 or 1/4 inch fibreglass, but you can strip PC board material and glue
pieces together. The copper on PC board material isn't thick enough to be
of any use as a conductor at the sort of powers I presume we are talking
about - at 160m, skin depth is about 2 thousands of an inch, and you need a
conductor at least 5 skin depths thick if skin effects are to be neglected.
You would of course need to be very generous on clearances, since if you
had an arc for any reason, the fibreglass board will burn, producing vile
smelling fumes and black smoke........been there, done that in my remote
antenna tuner.
Fair Radio sales <http://www.fairradio.com/tapswi.htm>still have the
ceramic switches from the BC375 TU units at a low price - $3.95 each, 10
for $32 - they are very chunky and good, and can be ganged with a bit of
work, With a bit more work, you can motor drive them, although direct drive
from a stepper motor would need a big motor unless you removed the indexing
mechanism.
73
Peter G3RZP
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