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#1
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R.F. Choke Design
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. |
#2
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R.F. Choke Design
"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 |
#3
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R.F. Choke Design
Watch out for possible core saturation when using a ferrite device
for that application. Pete |
#4
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R.F. Choke Design
"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. |
#5
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R.F. Choke Design
On Feb 11, 7:13*pm, "JimL" wrote:
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. The requirements for the safety choke are pretty tame compared to a plate choke. The impedance level around it is 50 ohms (contrast it with thousands of ohms around a plate choke), so you don't have to maintain a particularly high RF impedance. In other words, the same choke you used for the plate choke would work just fine. |
#6
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R.F. Choke Design
"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 |
#7
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R.F. Choke Design
"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. |
#8
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R.F. Choke Design
1000 ohms reactance at 3.5 MHz would be almost invisible to the rf output
tuning. I have a drawer full of old surplus rf chokes, wound with large wire. Generally one section. About 1/2 the size of a golf ball. I have thrown those in and they work fine. For lower power, a standard 2.5 mh choke is ok, but I like larger wire size for higher voltage and current. The old tuning units from the BC-375 would have a handful of such chokes, with nice ceramic stand-offs. 73, Colin K7FM |
#9
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R.F. Choke Design
"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 |
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