high/flaky swr in new mobile setup

[danmc]

Hello,

I just purchased my first CB radio. Here is my setup

Cobra 18 WX ST II radio
Cobra HG A1500 antenna (mag mount)
RadioShack 21-534 CB/HF Ham Power/SWR meter

installed in a honda CR-V EX (the EX has a moon roof)

After putting in the radio I wanted to experiment with antenna placement and make sure I was going to be able to get a decent SWR before buttoning up my dash. I used a short (~ 18" or so) cable to run from the radio to the SWR meter and then just ran the antenna cable in through the open window to go to the antenna side of the SWR meter. First check showed ~ 3:1 on channel 40 and worse (maybe 3.5:1) on channel 20 and 1. yuck! But the antenna was stuck up front between the moon roof and windshield. So I moved the antenna to dead center with the most roof metal surrounding it as possible. Slight improvement but still struggling to even get below 3:1 on all channels. I tried various spots and one near the back gave some decent (1.2:1 to maybe 1.8:1) readings without having tuned the antenna yet. I tried a few more spots and they were all bad (3:1 or worse) and was unable to get back to that sweet spot!

A little more playing around showed that if I short out the shield part of the two UHF connectors on the antenna cable and the short cable to the radio with a screwdriver then my SWR drops by quite a bit and I'm out of the red.

This makes me wonder a few things.

1) are all of my SWR readings questionable because of the SWR meter breaking the ground connection between antenna cable and the chassis of the car?

2) is there something I need to be doing to improve the grounding of the radio?

3) any other tricks to getting a reasonable SWR?

Thanks much for any advice.

-Dan

 

[Robb]

I have a CR-V and I run the cable thru the rear side window. I would check the antenna alone with a multimeter and see if there is any inconsistency in the readings - maybe a bad cable or connector.

Did you adjust the height of the antenna - as in raise/lower the whip adjustment? There should be a set screw on the base of the antenna whip to lower/raise the whip in order to fine tune the SWR. That info should be in your installation instructions that came with the antenna - if you bought it new. I have never owned the antenna that you are using - so I don't know much about them. I put my antenna squarely in the middle of the vehicle - on top of course.

I have a Wilson 1000 mag mount antenna; I just put it on and set the whip height a little bit until the SWR came in at 1.15 to 1. Took 3 minutes or less. Some part of your antenna is not right and needs to be thoroughly checked out. It should have been like mine is - unless there is something wrong with th coax or connector. Check it out with the multimeter for intermittent readings. Make sure none of the coax is mashed or cut - could be a big problem if it is...

 

[danmc]

I did mess a little with the antenna height via the set screw like you mentioned. It made a small difference but it seemed like I was starting out too far off. I don't see anything visually wrong with the coax. No cuts, hasn't been slammed in the door or anything either, hasn't been tightly coiled or bent.

As far as checking with a meter, I've verified continuity between the connector center pin and the antenna. Not quite sure what to check on the ground part.

I agree, it seems like something more drastic is wrong. I expected the set screw adjustment step to be tweaking something reasonable into something better and not starting with something bad.

 

[danmc]

I have a CR-V and I run the cable thru the rear side window. I would check the antenna alone with a multimeter and see if there is any inconsistency in the readings - maybe a bad cable or connector.

out of curiosity... do you have

1) a roof rack?

2) a moon roof?


I have both and I realized today that the roof rack is not grounded. The side rails are about 1/8 of a wavelength at 27 MHz. The cross rails are a little less. So not quite up to the 1/2 wave resonance, but perhaps not so great anyway. I'll experiment with grounding the roof rack and see if that makes a difference. As far as the moon roof, I've just tried to get as far away as possible.

 

[Marconi]

Dan are you calibrating the SWR meter every time you read SWR?

Is the roof rack high enough to interfere with the coil at the base of this very small antenna? An antenna that short will likely be very sensitive to its surroundings and the roof rack may well be affecting the tuning coil and for sure if the roof itself is attached physically to the rest of the car using mastic instead of being spot welded really well.

Is all of this stuff new and have you ever seen the radio, meter, coax, and antenna work under any other circumstances? The match you are describing to us is not even close to tune and tuning will not likely fix the problems you have. I would first check out the meter and the radio on a system that is known to work good.

Keep us posted on your progress and don't try too many different things at one time trying to find out what is going on.

 

[danmc]

I am calibrating the SWR meter each time.

Is the roof rack high enough to interfere? Not really sure, but the coil is pretty much at the base of the antenna. Most of the roof rack is metal but the part which actually connects to the car is plastic. I have verified both visually and via a dc meter that the roof rack is floating electrically.

Yep, every last bit is new so I have never witnessed any of it working in any other conditions. Nothing like too many variables.

It is helpful to have confirmation of my gut reaction that something is wrong beyond just not having it tuned yet.

 

[Robb]

No; I don't have a moon roof or a rack on my CR-V; I didn't want the alloy wheels either. I don't believe that the moon roof or the rack is going to adversely affect the SWR. Try the antenna on another car if you can temporarily install the radio.

I'm pretty sure that there is something wrong with your antenna, and/or the jumper piece of coax between the SWR meter and your radio is bad. If I couldn't get that antenna to work, waste no more time and get yourself a Wilson 1000 mag mount - and never have to look back! But I hope you find the problem with what you've got.

Are there any truck stop/CB repair shacks close to where you live? You might have them look over your system - if all else fails...

 

[mackmobile43]

Hello,

I just purchased my first CB radio. Here is my setup

Cobra 18 WX ST II radio
Cobra HG A1500 antenna (mag mount)
RadioShack 21-534 CB/HF Ham Power/SWR meter

installed in a honda CR-V EX (the EX has a moon roof)

After putting in the radio I wanted to experiment with antenna placement and make sure I was going to be able to get a decent SWR before buttoning up my dash. I used a short (~ 18" or so) cable to run from the radio to the SWR meter and then just ran the antenna cable in through the open window to go to the antenna side of the SWR meter. First check showed ~ 3:1 on channel 40 and worse (maybe 3.5:1) on channel 20 and 1. yuck! But the antenna was stuck up front between the moon roof and windshield. So I moved the antenna to dead center with the most roof metal surrounding it as possible. Slight improvement but still struggling to even get below 3:1 on all channels. I tried various spots and one near the back gave some decent (1.2:1 to maybe 1.8:1) readings without having tuned the antenna yet. I tried a few more spots and they were all bad (3:1 or worse) and was unable to get back to that sweet spot!

A little more playing around showed that if I short out the shield part of the two UHF connectors on the antenna cable and the short cable to the radio with a screwdriver then my SWR drops by quite a bit and I'm out of the red.

This makes me wonder a few things.

1) are all of my SWR readings questionable because of the SWR meter breaking the ground connection between antenna cable and the chassis of the car?

2) is there something I need to be doing to improve the grounding of the radio?

3) any other tricks to getting a reasonable SWR?

Thanks much for any advice.

-Dan

If there is'nt a problem with the shielded portion of the coax then your best bet would be to acquire an antenna with a larger magnet for better capacitive coupling to ground characteristics and one other thing if your radio is not ground IE the chassis of the radio then the shielded portion of the coax is ineffective.

 

[danmc]

If there is'nt a problem with the shielded portion of the coax then your best bet would be to acquire an antenna with a larger magnet for better capacitive coupling to ground characteristics and one other thing if your radio is not ground IE the chassis of the radio then the shielded portion of the coax is ineffective.

I tried a different antenna and it easily tuned up for an SWR which is < about 1.2:1 or 1.3:1 over channels 1-40. This time it went much more smoothly and took about what I expected. The only challenge was waiting for a day with no rain in Atlanta (Atlantis?).