Larry Menzel wrote:
...
So, what to do. According to Paul, W0AIH, the solution is really very
simple. He inserts a lamp holder, like from a boatanchor radio dial light,
and puts a #47 bulb in line with the receive antenna line. Simply take a
piece of coax, cut it and solder the braid and center conductor to the lamp
holder tabs, and VIOLA, a cheap and totally reliable fuse in the receive
line. Paul insists that using a WIDE spaced lamp element, as with the #47
bulb, is very important, and NOT to use a "grain of wheat" style light bulb
which has the two poles of the filament holder too close to provide enough
protection from arcing across. Although I submit that if one is only running
100 watts or less, that that would be sufficient. But running 1500 watts is
a whole 'nother kettle of RF. HI.
...
A #47 acting as a _fuse_ isn't going to do much. It takes milliseconds
to get hot and melt. Your front-end can blow in a few microseconds.
If you look at the schematics, you will see that the Orion front ends
are protected by (1) a 1 mH RFC to ground, (2) a spark gap to ground,
and (3) diode clamps to +/- 5 volts. For 160 M work, I expect #3 is
your defense. The question is how much juice can you dump into the
front end before those diodes blow?
Back to back diodes at the Rx input aren't a great idea, because they
will conduct and cause IMD and harmonic problems if there is any strong
out of band RF in your area. The only "good" answer is a switch ahead of
the Rx that is shut off _before_ any RF is generated.
73 Martin AA6E
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