"JimL" wrote in message
. com...

"DaveM" wrote in message
. ..
"JimL" wrote in message
. com...
Greetings- can someone provide data on their home made choke for connecting
across the output of a 1 KW, 3.5 mhz , transmitter working into 50 ohms.
The so called safety choke to prevent the HV plate voltage from appearing on
the antenna in case of failure of a blocking capacitor. Normally a
commercial 2.5 MHY RFC is used. Perhaps something wound on a ferrite rod.
Thanks, Jim.


You might try calculating the inductance of a choke that presents about 500
to 1000 ohms of inductive reactance of the transmitter frequency. Use wire
that can handle about 2 to 3 times the plate current of the final, or 2 to 3
times the HV fuse rating.
The math is left as an exercise for the OP.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra
Thanks. That calculation is trivial. A more difficult calculation is
determining what effect say, a 25 uhy choke, would have on the pi network to
which it is connected. Evidentally 1 or 2.5 mhy has a negligible effect,
since it never is considered in the literature I have seen. Perhaps the same
is true for the 25 uhy., but I am too lazy to do the math.Quicker to just try
same. Jim.



My reasoning for recommending those values is this:
Since the circuit impedance around the RFC is 50 ohms, I figured that if the
inductor presented a reactance around 10-20 times that, it wouldn't upset the
tuning (well, maybe just a tad, but insignificant). The wire in the coil should
be big enough to carry the HV current until the HV fuse blows, without damaging
the coil itself.
If you figure the physical size of a 25-50 uH coil, using wire that can handle
the current, it might be a bit large. A 1-2.5 mHy coil would be very large
(lots of turns of large wire).
Considering the power levels involved, using a magnetic core in the coil would
probably waste a good bit of heat.
If you're going to experiment with inductor sizes, I'd be interested in hearing
your results.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra