A Dual Coax Cable RF Feed System for High Power Rife/Bare Systems
Many Rife researchers who are running their systems at high RF power levels have, at one time or another, had the unfortunate occurrence of burning up a coax cable. The usual point of failure is in the section of RG8A/U coax cable that connects the output of the antenna tuner to the balun assembly at the Rife/Bare tube. This cable has a tendency towards failure, since it is always operating in a condition of high VSWR, due to the on-off cycle of the Rife/Bare tube.
A second cause of cable failure is that because of the impedance transformation of the balun used to match the RF power to the Rife/Bare tube, the average impedance presented to the coax cable is well below the desired 50 ohm value for best operation of the cable. The combination of these problems causes the coax cable to see excessively high currents during some parts of the Rife/Bare tube operating cycle.
This problem normally becomes serious when operating the Rife/Bare system at power levels above 200 watts. Below that power level, the coax cable can usually dissipate the heat without serious damage. The RF power lost in the coax cable is a loss, and never gets to the Rife/Bare tube. Decreasing the cable loss will prevent cable failure, and also increase the output of the Rife/Bare tube.
This high current overheats the inner conductor of the cable, and eventually causes the plastic insulation inside the cable to melt. This softened insulation then allows the center conductor to move inside the cable and short out against the inside of the cable shield braid. Serious overheating can actually cause a blistering of the outer plastic jacket of the cable, or, in a really serious case, it can actually burn through the cable, releasing quite a bit of smoke and even flames! Here's what failed cables look like:
This section of polyfoam coaxial cable overheated enough to melt the outer jacket!
A cross-section of the cable clearly shows that the center conductor has moved inside the cable and shorted out against the shield braid. This is a sure way to send your amplifier to the repair shop!
Oops! Here we've heated the cable enough to actually melt the polyethylene inner insulation and cause it to flow out between the shield braid interstices.
My last smoke bomb! This section of heavy-duty cable failed after 30 minutes at 500 watts average power. Since the failure point was hidden from view when it failed, I thought the smoke and smell was from the amplifier. Needless to say, I was quite relieved when I discovered that the damage was confined to the cable!
After some thought, I decided to modify the RF feed system slightly to try to avoid another coaxial cable failure. Since one of the main problems is the incorrect RF impedance as seen by the coax cable, it seemed that one possible solution might be to use a cable having a lower impedance than the 50 ohm RG8A/U cable.
Since you can't just go to your local electronic parts store and buy some low impedance cable (they don't make it) I figured that I could use an old trick of paralleling several lengths of 50 ohm cable to make a section of cable having an effective impedance of 25 ohms. I thought that a 25 ohm cable would probably be a better match and run at a lower loss. I did, and it does.
My unit uses a cable of about 32-36 inches between the shielded enclosure which housed my Rife/Bare tube and the output of the antenna tuner. I cut two 36 inch sections of regular RG8A/U cable and fitted them with standard UHF-type PL-259 connectors (278-188a) from Radio Shack.
Note that you should only use the silver plated connectors for your cables, especially the ones between the tuner and the balun. The cheaper nickel plated connectors have a much higher RF loss. I have burned up several of the cheaper connectors at power levels as low as 200 watts in extended time sessions.
Now that I had two sections of cable to use, I had to connect them in parallel. I headed back to my local parts store for four UHF elbows and two UHF tees. I then fastened two of the elbows to each tee and carefully screwed them tightly together with pliers. Then, I attached one of the elbow/tee adapters to the tuner output, and the other adapter to the balun at the Rife/Bare tube. Lastly, I connected the new cables between the adapters, and tightened everything securely with the pliers.
After completing the modifications, I fired up the system and found that I needed to do only a slight adjustment on the antenna tuner to get the best VSWR reading.
Here's a look at some of my handiwork:
This picture shows the adapter system attached to the output of the antenna tuner.
The results are excellent. The coax cables get only mildly warm, even at power levels as high as 500 watts for 45 minutes. The tube lights slightly brighter, so it is apparent that more RF power is getting to the tube. The modification was effective and worthwhile.
It seems there's always a weak link somewhere! Now that the cables don't burn up, I have found that the balun - actually my Twisted Transmission Line - gets so hot at high power levels that it has actually unsoldered itself from the coax connector! It gets so got that the insulation in the coax connector melts, and the heat has even melted the insulation in the elbow adapter on the back of the tube enclosure. So, I'll be working on a way to eliminate the balun-to-tuner connections, and try to devise a matching arrangement at the tube. That would allow me to use a coax cable directly from the amplifier to the tube housing, and eliminate the antenna tuner. I'll post results as soon as I figure something out.
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