Or use an analyzer that allows calibration with a Open, Short, Load set that makes any length feedline invisible to the analyzer. Same as analyzer connected directly to antenna feedpoint.
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Coax, null or not?
- Thread starter Cable Guy
- Start date
I did once watch a YouTube video on coax jumper lengths for amplifiers. Normally I would call BS on this too but the video made a reasonable valid point on jumper lengths while running the typical C.B. class C amps. According to the video host, nearly all of those type of amplifiers DO NOT have tuned 50 ohm inputs like they should have so making jumpers in 1/4 wavelengths so the amplifier sees a 50 ohm impedance sounds plausible in this instance. Another video host started with a 9 ft piece of coax hooked to an analyzer and kept trimming it until the analyzer shown a 50 ohm impedance. According to one host, many operators have damaged their amplifiers using 3 ft jumpers between the radio and amplifier because of the lack of a 50 ohm input.
What do you guys think of this???
What do you guys think of this???
Greg T
WDX-945 (Jazz Singer) Upper Peninsula of Michigan
All I can tell is my experience. I have no analyzer so this is just real world. I have a radio, jump[er to meter, jumper to amp, jumper to meter, jumper to antenna switch and then antenna. My SWR is flat for a very wide range. I have had this configuration in many different [positions on the bench and have used many, many different length jumpers over a few years. NO difference in antenna match at the radio or the amp. NONE. I had a few 3' and a couple of 6' jumpers and decided to use just what I needed and no more. So, I have a 12" going from radio to meter, 8" from meter to amp, 6" amp to meter, 9" from meter to switch. Still lays in perfectly and works like a charm. At different times I even had a 12' between the radio and the first meter. Makes NO difference.
I have observed about the same, I have the shortest jumpers where they can go, especially when routing through my switching matrix before going to either antennas or load. Through the changes made over the years, no difference discerned.
Now, I still hold that it does have practicality in certain situations. For example, bad antenna, horrible placement and radio swr detection (some radios) blinking an additional light and/or reducing output.
For instance, if a new antenna isn't an option, and the antenna HAS to be facing forward in a 45 degree incline to look cool and bolted to the plastic mirror frame (truckers, ya know) with a pigtail grounding the mount to the frame and a driver who won't listen to reason (worst case scenario), well if cutting the coax into pieces makes sense to him, and fools the radio into not blinking it's aws light, while getting a hard headed driver on his way out the door, okay. I'd rather sneak his radio inside and clip the swr circuit or light while keeping him distracted looking at the dogs. "Will they bite?" "Well, they have teeth don't they? So yes, they may bite"
Other than that odd situation, I see no need to cut the coax. But, I don't tailor to truckers too much, I mostly work on base stations or mobiles used as base stations, with competent folks who are serious about the hobby and antennas, towers and feed lines in their installation, so making do with subpar elements is rare.
it doesnt show a loss but it will show itself. especially with certain watt meters or swr meters. plus, with alot of cb amps that have rf sensing keying circuits in them, the boxes will exhibit problems with keying or getting hung up or requiring too much drive to actuate the relay.