I have seen center conductors migrate in RG8, LMR400 and even RG58. I do broadcast RF work but am also in the two-way radio business. I've been in this field for 47 years and as a vendor, not employed as a broadcast engineer, I get to see a wide verity of different bits of equipment.
You would think something as expensive as a broadcast exciter would have a overtemp alarm or at least a indicator light. How much would that have cost them? A lot less than it cost the station in lost air time.
I see center conductor migration in LMR mobile installations all the time also in transmit combined applications where the peak power levels are much greater than each individual transmitter alone. In your case you had two things against you, one the ambient temperature was excessive from the lack of air movement, also the power level applied to the cable was excessive since the reject was having to pass greater power to the load, so heat from the applied power and ambient high temps could easily cause this. We almost always will use TFE based cables for this type of application but you can still have an excessive power level being applied which might then cause dielectric breakdown and arc-over. Great video though :)
We had a coax fail like that above an equipment rack that lost the cooling fan. We found the fault by using a TDR. About 6 feet of cable had lower impedance up to the short.
I'm surprised the crew here didn't use a TDR. I mean it's *made* to show the impedance across a line and a VSWR is caused by impedance mismatch. Would've probably saved a lot of time as the slightly-above-the-rest corner conductor wouldn't have been replaced for nothing.
Good technical troubleshooting... I've seen a similar issue, albeit somewhat opposite - where a coax (RG-8?) section was wrapped in a loop and anchored down with tie wraps and very tightly wound electrical tape - it was frequently exposed to sunlight...so over time the shield collapsed into the center conductor, eventually shorting it out...in the 2 way radio circles is was called "cold melt" but probably more than one name for this type of fault...
The on line engineer my hobbys are painting pictures and lisining to shortwave and ssb iam thinking about getting my ham license I have 4 shortwave receivers
What may have trip the fan fuse may have been one of the three fans motors was wearing out. It appear the fans run all the time. Also may have been the fans needed to be cleaned. Those fans should have been on a maintenance program to be checked.
My guess would be that one, or more fan(s) failed and created a short, thus blowing the fuse. Wish they would have told us, but then what else could have caused it? Too bad they didn't have a safety switch that would have shut down the exciter if the fans failed, got to stay cool! Still good video.
The video is from 2012 and I'm guessing the failure was at least a year before the video was edited and produced. I'm wondering if there have been any other failures in the combiner since the time of the video.
The explanation of VSWR confuses the reflection coefficient with VSWR. From Wikipedia the VSWR is "ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line." The ratio of the forward to reverse reflected power is actually the reflection coefficient.
RG 214 does not like 90 degree bends, and i dont like using elbows, right angle plugs seem to be the answer, despite the possible loss, good vid, thanks, very informative.
Looks to me like a design flaw - coax is not rated for the carry-current needed. I'd replace it with a bigger coax (with a bigger center conductor) of the same impedance.
Intercomp RG 214/U coax with a "migrated" center conductor? Why would the center conductor melt without the outside jacket not melting or deforming? How much power were you expecting on your reject load? It looks like a 600W load. So you are expecting 600W max, but RG-214/U is only designed to carry 170 Watts at 1 GHz. Generic Name RG214 Flex Type Flexible Impedance 50 Ohm Dielectric Type PE Velocity of Propagation 66 % Jacket Diameter 0.425 in Jacket Material PVC No. of Shields 2 Attenuation at 1 Ghz. 7.32 dB Power, Max at 1 Ghz. 170 Watts
At 5:03 of your video: The bend radius of the 214 going into the load is terrible....You're asking for trouble with that kind of bend/twist. It likely migrated over time through RF heating of the dielectric coupled with the lateral pressure on the center conductor from such a sharp bend in the cable...Raise the load above the line and keep that cable as straight as possible....That bend is also bad for the connector/cable coupling and probably created a high R/L in that cable.....
Very early in my career in 1980,s I was called to help diagnose a Collins fm broadcast transmitter that lost drive to the final cabinet. And this exact thing had happened. The coax in the transmitter was shorted and had migrated.
Wow, domino effect. No temp monitoring on that?! What the heck? Not even an interlock for air movement? I remember they used to use windvanes and microswitches to monitor that stuff. Monitoring temperature is important too but loosing a fan is bad news! Great and instructive report. You had me at the hot elbow! And hot coax? I would have thought the OUTSIDE dielectric would have melted a little. Gez.
Fascinating, that cross section of 214 cable. I suppose the center conductor could easily move if the poly becomes semi fluid and the cable was positioned in a less than straight manner; combined with heat from power loss. That could be a good reason to use TFE dielectric cable. BTW, a comment on any 90 degree coaxial "elbow" is that you can never consider them perfectly 50 ohms because of the mechanical design vs electrical character. And especially at UHF & above. They are convenient and often necessary but generally to be avoided because of loss.
What probe setup are you using for the Agilent E4418B? Is this a special build with custom firmware? I thought they only spec'ed out to +44dbm, sensor dependent.
If you knew you had a transmission line fault why didn’t you start your troubleshooting with a spectrum analyzer and a cable sweep? Would have been easier than pulling the elbow out of that hardline.
I know the insulator to be a pretty tough material, but I didn't factor in the heat actually making it melt, I guess causing the core to move towards ground and short out. Great video.
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In 47 years as a broadcast engineer (now retired), I never saw a center conductor migrate like this. Fascinating.
I have seen center conductors migrate in RG8, LMR400 and even RG58. I do broadcast RF work but am also in the two-way radio business. I've been in this field for 47 years and as a vendor, not employed as a broadcast engineer, I get to see a wide verity of different bits of equipment.
I never see this migration cable....Never listen.....incredible...!!! ( But i am simple elettronic amatour)
Wow! That picture of the center conductor off center was awesome. I'm so glad you showed that. I would never have expected that kind of fault.
You would think something as expensive as a broadcast exciter would have a overtemp alarm or at least a indicator light.
How much would that have cost them? A lot less than it cost the station in lost air time.
I see center conductor migration in LMR mobile installations all the time also in transmit combined applications where the peak power levels are much greater than each individual transmitter alone. In your case you had two things against you, one the ambient temperature was excessive from the lack of air movement, also the power level applied to the cable was excessive since the reject was having to pass greater power to the load, so heat from the applied power and ambient high temps could easily cause this. We almost always will use TFE based cables for this type of application but you can still have an excessive power level being applied which might then cause dielectric breakdown and arc-over. Great video though :)
We had a coax fail like that above an equipment rack that lost the cooling fan. We found the fault by using a TDR. About 6 feet of cable had lower impedance up to the short.
I'm surprised the crew here didn't use a TDR. I mean it's *made* to show the impedance across a line and a VSWR is caused by impedance mismatch. Would've probably saved a lot of time as the slightly-above-the-rest corner conductor wouldn't have been replaced for nothing.
would be good to add an alarm when the fans stop working, so you can avoid problems before overheating the equipment
If only all problems were explained like this or better.
I have heard tell of "migration" but didn't understand it until now. What a clear picture you have given. Thanks a lot.
The center conductor has migrated in a RG214 ???? I never seen that in over 25 yeras of experience !!!
Good technical troubleshooting... I've seen a similar issue, albeit somewhat opposite - where a coax (RG-8?) section was wrapped in a loop and anchored down with tie wraps and very tightly wound electrical tape - it was frequently exposed to sunlight...so over time the shield collapsed into the center conductor, eventually shorting it out...in the 2 way radio circles is was called "cold melt" but probably more than one name for this type of fault...
As a Ham Op I see it much to often. Tight bends or in flexible foam dielectric cable.
You won't see it at all if you respect the minimum bending radius and maximum power capability of the cable.
I had this same thing happen on a Dansk Transmitter in Antarctica. High current in RG8 caused melting of the dialectic.
I have witnessed conductors move under intense amperage cables jump.
Right angle turns not good either.
That looked like a mfg defect in insulation
The on line engineer my hobbys are painting pictures and lisining to shortwave and ssb iam thinking about getting my ham license I have 4 shortwave receivers
What may have trip the fan fuse may have been one of the three fans motors was wearing out. It appear the fans run all the time. Also may have been the fans needed to be cleaned. Those fans should have been on a maintenance program to be checked.
The on line engineer me and my cousin are going to a Swap meet in September 11 Sunday morning at 8 am in Milwaukee 2022
Well it can happen with enough power and the cable in a loop that pushes the center outward. I was surprised when I saw it.
That's such an esoteric problem. Nice detective work!
So what caused the fan fuse to blow?
My guess would be that one, or more fan(s) failed and created a short, thus blowing the fuse. Wish they would have told us, but then what else could have caused it? Too bad they didn't have a safety switch that would have shut down the exciter if the fans failed, got to stay cool! Still good video.
The video is from 2012 and I'm guessing the failure was at least a year before the video was edited and produced. I'm wondering if there have been any other failures in the combiner since the time of the video.
The explanation of VSWR confuses the reflection coefficient with VSWR. From Wikipedia the VSWR is "ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line." The ratio of the forward to reverse reflected power is actually the reflection coefficient.
RG 214 does not like 90 degree bends, and i dont like using elbows, right angle plugs seem to be the answer, despite the possible loss, good vid, thanks, very informative.
Yes. And I really don't like that kink in the coax going into the load. That's asking for trouble.
Looks to me like a design flaw - coax is not rated for the carry-current needed. I'd replace it with a bigger coax (with a bigger center conductor) of the same impedance.
Intercomp RG 214/U coax with a "migrated" center conductor?
Why would the center conductor melt without the outside jacket not melting or deforming?
How much power were you expecting on your reject load? It looks like a 600W load. So you are expecting 600W max, but RG-214/U is only designed to carry 170 Watts at 1 GHz.
Generic Name RG214
Flex Type Flexible
Impedance 50 Ohm
Dielectric Type PE
Velocity of Propagation 66 %
Jacket Diameter 0.425 in
Jacket Material PVC
No. of Shields 2
Attenuation at 1 Ghz. 7.32 dB
Power, Max at 1 Ghz. 170 Watts
At 5:03 of your video: The bend radius of the 214 going into the load is terrible....You're asking for trouble with that kind of bend/twist. It likely migrated over time through RF heating of the dielectric coupled with the lateral pressure on the center conductor from such a sharp bend in the cable...Raise the load above the line and keep that cable as straight as possible....That bend is also bad for the connector/cable coupling and probably created a high R/L in that cable.....
Very early in my career in 1980,s I was called to help diagnose a Collins fm broadcast transmitter that lost drive to the final cabinet. And this exact thing had happened. The coax in the transmitter was shorted and had migrated.
What caused the fan box fuse to trip?
Wow... I look forward for more videos from you.
The on line engineer I like your utube videos
Don't you hate it when you fix the obvious problem and nothing changes! Excellent troubleshooting! Bravo!
Wow, domino effect. No temp monitoring on that?! What the heck? Not even an interlock for air movement? I remember they used to use windvanes and microswitches to monitor that stuff. Monitoring temperature is important too but loosing a fan is bad news! Great and instructive report. You had me at the hot elbow! And hot coax? I would have thought the OUTSIDE dielectric would have melted a little. Gez.
Fascinating, that cross section of 214 cable. I suppose the center conductor could easily move if the poly becomes semi fluid and the cable was positioned in a less than straight manner; combined with heat from power loss. That could be a good reason to use TFE dielectric cable. BTW, a comment on any 90 degree coaxial "elbow" is that you can never consider them perfectly 50 ohms because of the mechanical design vs electrical character. And especially at UHF & above. They are convenient and often necessary but generally to be avoided because of loss.
What probe setup are you using for the Agilent E4418B? Is this a special build with custom firmware? I thought they only spec'ed out to +44dbm, sensor dependent.
wow.. center conductor migration to the shield is a very special problem. thanks for highlighting this.
If you knew you had a transmission line fault why didn’t you start your troubleshooting with a spectrum analyzer and a cable sweep? Would have been easier than pulling the elbow out of that hardline.
Funny seeing Radio Shack thermometers in a professional environment. Nice video.
Excellent sleuthing, better than Sherlock Holmes with a DVM.
I know the insulator to be a pretty tough material, but I didn't factor in the heat actually making it melt, I guess causing the core to move towards ground and short out. Great video.
*8VSBspectrum out of band emission/ACLR*
Very rewarding to find the actual problem(s) and not just shotgun it our - thanks for sharing.
Is just simple magic for me :) Excellent checking work!
That's the oddest thing. I've never seen 214 do that before.
sometimes, the smallest malf-outs can cause the biggest problems
20 years in the industry and never seen that before, awesome find.
Very interesting
even though most went over my head.
A really nice, logical instructional video. Thanks!
A really good experience. Thanks for sharing.
Thank you
That cable must have been wrong , in the first place ............. but well done ...