swr question

The SWR is in the antenna not the radio.

 

What he^ said.

 

Well depends sometimes. I have seen SWR/antennas needing to be tuned when going from a low powered stock radio to a hgiher powered radio. Not all the time though. Don't quote me on this but I think it has to do with pushing the limits of the setup. In a perfect world with proper RF grounding, good groundplane and no possible problems with reflect from body or vehicle parts being too close to the antenna then you should beable to tune the the system and beable to switch back and forth from radio to radio without having to retune.

A perfect world and factory (unmodifed) trucks do not belong in the same sentance, when you are talking factory coax antenna setups for Cb radios.

A prime example would be the 379 that I am currently running. I could mount up a stock CB radio to the factory coax with multiplexor etc and tune the antennas to an almost perfect SWR across the band. Take the cobra down and install a higher powered radio and SWR would go up to 2.0 or higher. Replace the coax, seperate the multiplexer out of the CB setup, add proper grounding to the doors and mirror brackets and boom the SWR does not change from radio to radio.

My current setup now nets me and SWR of a tad under 1.5 across the board no matter what radio (box stock Cobra 29 with a Palomar 450, Cobra 29 Stealth with a palomar 450 or my stand alone Connex 4600 Turbo) I have mounted up.

I added extra grounding between the frame and the cab,between the door post and the door, between the frame and the hood and cleaned up the grounds going to the motor and cleaned up the joints on the mirror brackets.

Antennas are dual wilson 5000s with 10 inch shafts mounted on the very end of the mirror mounts to get them as far away from each other as possible. Bull horned just slightly to keep them away from the stacks as they flex back when going down the road.

I tried a single but I had loss of transmit signal to one side due to the trailer I pull.

 

"Well depends sometimes. I have seen SWR/antennas needing to be tuned when going from a low powered stock radio to a hgiher powered radio."

The SWR is always a antenna problem ....What you might have seen is a dirty radio due to mods and tinkering with the power output ....

IF the radio is dirty the meter will show high SWR due to reflected trash that is outside the range of your antenna.

 

Rat also mentioned having had to silver solder and re-sheathe some antennas in the past cuz of crappy connections near the feedpoints. That's one of the scenarios where things are "at their limits" and a grossly higher power level will make a weak point show its ugly little head.
In the already-acknowledged-unlikely-perfect world, with everything shiny clean and the radio putting out all its power on the desired channel and in its designed bandwidth, the SWR won't change; it's a function of the antenna system.
A quick check is to put a low pass filter before the SWR meter and check two things: SWR and power output, both compared to the readings without the low pass filter. If the power is lower with the filter, and the SWR is better, then the radio is putting out harmonics -- back to the bench.
Low pass filters are pretty inexpensive; I have one on each of my CB mobiles and base, and one on the HF ham system, which runs on many bands.
At the risk of seeming to have strayed from Tony's question, the easy answer is, "Once the SWR is set with a stock radio or antenna analyzer, it'll be the same for any radio on the same frequency."

 

The resoldering was due to the wire breaking off from the flexing of the antenna after a short period of time.


I do know that the factory setup on my 379 was not that great even with better antennas.

I had set the swr using a box stock cobra 29 and when I went to my Connex 4300 HP that I had at the time. The SWR went up remarkably if I had the Rf power turned up.

I seperated the factory multiplexer box (This is used so that the AM/FM radio can gets its signal from the CB antennas eliminating the need for a seperate antenna) out of the CB system by installing new coax just for the CB. I then went through all the grounding on the truck and added some grounding to the doors etc. Once I had done this then everything was fine no matter what radio I put up.

As for the AM/FM, well I have not had time to do a proper install of a seperate antenna. I needed something quick to pickup the local HS basketball games so I threw something together. 1/4 inch aluminized brake tubing with with the factory ends, have the same threadsize etc as a CB antenna. I took an old bolt on mount and bolted it to the right side mirror bracket. Cut off a breakline that I had for another project ( I build scale downed Rc buggy tube frames with this stuff). I slide in a peice of stainless rod into it and crimped it down for a tight fit. Installed it into the old antenna mount. Turned the radio on and slide the stainless insert up and down till I got the best reception. It works for now but I plan on installing a regular antenna in the center of the roof this summer.