"DaveM" wrote in message
...
"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
Hi Dave, I installed a 40 uhy choke, 1" inch in dia,, closewound with #22
enameled wire, about 1.75" long. My plate choke is #26 as I remember, and
140 uhy. Plate current is 600 ma max, plate voltage supply is 2300V, power
out is 600 w. key down from a pair of 813's in GG. The new choke has not
gone up in flames, and amp tunes OK. 40 uhy gives a reactance of about 880
ohms at 3.5 mhz. 73, Jim.

"JimL" wrote in message
. com...

"DaveM" wrote in message
...
"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
Hi Dave, I installed a 40 uhy choke, 1" inch in dia,, closewound with #22
enameled wire, about 1.75" long. My plate choke is #26 as I remember, and 140
uhy. Plate current is 600 ma max, plate voltage supply is 2300V, power out is
600 w. key down from a pair of 813's in GG. The new choke has not gone up in
flames, and amp tunes OK. 40 uhy gives a reactance of about 880 ohms at 3.5
mhz. 73, Jim.



Thanks for reporting back with the results. Glad to hear that the tuning wasn't
upset, and that everything seems to be going according to theory.
Good luck with the rest of the project.

--
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