Coax, null or not?

[Crawdad]

The good thing about using resonant lenght of coaxial is that the coaxial will not be part of the antenna ( will be invisible) when using an antenna analizer. ( like if you had the antenna analizer directly attached at the antenna feed point).

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.

 

[fourstringburn]

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???

 

[Greg T]

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.

 

[Cable Guy]

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.

 

[linearone]

In theory, it sounds great. Has anyone ever been able to show that the wrong length actually shows a loss in some way?

It might help someone who is driving Chinese transistors on the edge of failure in order to make reflected power low as possible?

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.

 

[Greg T]

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.

I agree. Some cheap, garbage antennae will use the shield as a counterpoise and cutting the excess will whack out the match. I've been there as well. 99.999% of the antennae I have installed have been 108" whips or ground planes and have found the best jumper is the shortest jumper.

 

[linearone]

Oh .yeah one other observation is that the problem with wierd length coaxes is more evident with a reactive load. Alot of issues go away when you are using a totally non reactive load like a dummy load/purely resistive load

 

[nomadradio]

Years ago I had Galaxy 2100 in my VW bus. The 102-inch steel whip had a low SWR, but the radio's final would oscillate when feeding it. Changed the coax to a measured half-wavelength and it stopped misbehaving. Weird but true.

73