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Anybody else used a coax jumper?

R

Radio Bob

I've never seen this mentioned anywhere but would like to hear from anyone who had the same experience. A few years back I was installing an FM translator . I popped a bird watt meter on the transmitter end of the coax and got a good match with little reflections. Then I took the Bird meter off and connected the coax directly to the transmitter. That's when the transmitter showed a bad match maybe 1.3 . I took the bird meter out but left the short piece of coax I had used between the bird meter and the transmitter in. Boom! the match dropped to 1.1.

Jump ahead to yesterday. I was called to a translator that the owner said had a bad match for years 1.3. I tried my trick of adding about 2 feet of coax on the transmitter end but to no avail. But, when I put it on the antenna end ( the antenna was on a rooftop so I could reach it) the match went to 1.0! Anybody had this experience with solid state transmitters before? BTW the signal was noticably improved on the way home.
 
Radio Bob said:
I've never seen this mentioned anywhere but would like to hear from anyone who had the same experience. A few years back I was installing an FM translator . I popped a bird watt meter on the transmitter end of the coax and got a good match with little reflections. Then I took the Bird meter off and connected the coax directly to the transmitter. That's when the transmitter showed a bad match maybe 1.3 . I took the bird meter out but left the short piece of coax I had used between the bird meter and the transmitter in. Boom! the match dropped to 1.1.
Click to expand...
I'm assuming this involves N-connectors. If so, sometimes the center conductor wasn't installed properly on the end connector, or over time it crawled further back into the connector. When you added a jumper, the center conductor penetrated completely and the match was correct. The same can happen on the braided outer conductor, where the amount of braid isn't making a solid connection at one or both ends.
Radio Bob said:
Jump ahead to yesterday. I was called to a translator that the owner said had a bad match for years 1.3. I tried my trick of adding about 2 feet of coax on the transmitter end but to no avail. But, when I put it on the antenna end ( the antenna was on a rooftop so I could reach it) the match went to 1.0! Anybody had this experience with solid state transmitters before? BTW the signal was noticably improved on the way home.
Click to expand...
Adding or removing coax isn't going to fix the problem. Bad connectors or old damaged lines will cause reflected power. That's about it.
 
Kelly A said:
I'm assuming this involves N-connectors. If so, sometimes the center conductor wasn't installed properly on the end connector, or over time it crawled further back into the connector. When you added a jumper, the center conductor penetrated completely and the match was correct. The same can happen on the braided outer conductor, where the amount of braid isn't making a solid connection at one or both ends.

Adding or removing coax isn't going to fix the problem. Bad connectors or old damaged lines will cause reflected power. That's about it.
Click to expand...
That was my first guess but I checked the connectors on the line first to make sure they were fitted properly. The jumper was brand new and the problem returned when I removed the jumper. This wasn't a shorted stub this was just an additional lengthen of the coax. The fact that the problem was fixed at two different sites at different times the same way make me think the length of the total coax run was the issue
 
If the antenna is a perfect match (1:1 VSWR), then cable length won't make a difference. If the antenna isn't a perfect match, cable length will affect VSWR. I've had to add short line sections to get solid state transmitters running into a less than perfect antenna load. Not ideal, but you do what you have to when there isn't a tower crew to tune the antenna.
 
Also make sure that the Type N connectors are for the impedance you are using.

There are 50-ohm versions and 75-ohm versions. The size of the center pin varies between the two impedances.
 
boiseengineer said:
The VSWR may still be there. You've just moved the reflection away from the sample point. That's the gotcha.
Click to expand...
You're right, adding a line just potentially moves the standing wave.
I'll admit in one installation, we discovered that a 6' section of 3-1/8" copper line had developed a pretty warm spot halfway between two elbows. In doing a rough calculation, the hot spot was 1/8" wave at frequency. Rather than add or remove line, because physical space wouldn't allow for it, I chose to 'slug' that section of line, which included installing copper collars clamped to the center conductor that alters the impedance just enough to move the standing wave out of that line section. The slug was placed using a nice, calibrated network analyzer.
Technically I didn't eliminate the standing wave, but moved it somewhere that wasn't creating a hot spot in the building.
 
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