SWR tuning

[lectric80]

This is mainly for Pauly, but anyone with experience can chime in.

I have a Cobra 19 Ultra III compact radio, and a 4' fiberglass antenna from Radio Shack. The antenna is tuneable, but when I try to dial in the SWR, I can't even get close to 2 on channel 20. At channel 40 I am at 2.4, at 20 3.1, and at channel 1 I am off the scale on the high side.

I have run the adjustment slug all the way to the top, but it started going backwards on the meter, so I went back in. I have it set as I said above, but not sure why the curve isn't the way it should be. Any ideas guys? I really need to get this tuned before I take the meter back to my dad.

 

[readymix]

Sounds like you don't have a good enough ground on the antenna.

 

[lectric80]

That is possible. I will try to run a ground wire from the antenna base to the inside of the truck. Should be able to do it when I am working on the wiring and new parts.

 

[W7NB]

Mount?

How is the antenna mounted? Where is it? What kind of coax did you use, and most important PLEASE tell me you did not use one of those crappy radio shack crimp on connectors.

 

[lectric80]

How is the antenna mounted? Where is it? What kind of coax did you use, and most important PLEASE tell me you did not use one of those crappy radio shack crimp on connectors.

The antenna is mounted to the mirror on the drivers side, the coax is 12 ft of prebuilt cable that did come from RS, but no crimp on connectors. It has full rubber connectors with stress relief for bending them a little.

 

[pauly383]

From experience , if the mirror is not a chrome one , and doesn't make a good enough contact with the body ground , it will be a bear to tune .

betcha if you touched it while keying up ( low power just for a second ) , the swr would drop .

Try an L-bracket out from the hood area , I did this on my 93 pickup with the older style mirrors ( the same mirrors as a K5 , but black painted post and isolated from ground ) . Run the coax through the fender and up in the gap by cowl/fender , there is room on a K5 .

BTW , my 11 meter cb antenna on the pickup is an Antennex with an NMO mount , I like those better than the 3/8 stud , and these antennas are better than Wilsons , and under 30 bucks on ebay .

But you get the idea on mounting with the pic , use an l-bracket , some self tapping screws , and drill the bracket for your 3/8 stud

It also keeps the wind from moving your mirror on the freeway !!

 

[lectric80]

My mirrors are a bit different, and I don't have a cowl like that, but I have considered this option. Initially I mounted to my mirror because it was convenient, but it really being a pain for tuning. I may move it to the side of the hood, but I will need to make me a bracket, not that hard. I am going to be doing some work later this week, installing hubs and a few other things, so I will try to move it, or at least ground it with a small wire running into the cab.

My mirrors are chrome, and mount to the door skins, but there is a small black plastic section between the mirror and the door. Plus there is another black plastic bushing for the mirror arm, so that may be the issue. I will give it a try and let you know.

 

[lectric80]

Thanks to everyone who posted up. I ran a wire from the bracket into the cab and grounded it to the dash, then retuned the antenna. It is now at ~1.8 at channel 20 and down to ~.8 at 1 and 40. Next step is to run the cable in and just ground it to the door under the door panel. But, at least it is working correctly now.

 

[TerryD]

FWIW: If you don't have 18 feet of cable (or an odd multiple of it) the SWR you read at the radio is incorrect. The best place to take SWR readings is at the antenna end of the setup. On my jeep the SWR was inverted at the radio end (depends on length of your cable as to what phenomenon you will experience). Showing ~0.2:1 SWR on 1 and ~0.5:1 on 40. When checked at the antenna, I have 1:1 on 1 and 1.5:1 on 40. Verified with two different meters. 102" S/S whip, RG8X coax and a Cobra 18WXSTII.

I just put my meter in-line, put my CB on the channel I want to check, and key up the mic with a small rubber band. Then walk to the back of the vehicle and take my reading. Go back and take the rubber band off, change channels (record previous reading if need be), re-key via rubber band and take my reading. Then make what adjustments are needed. You can key a radio into a bad SWR for a short time w/o hurting it, even a cheap radio like the 18 and 19 cobras.

 

[lectric80]

I didn't check it at the antenna, although it would be easy to do since my antenna is on the mirror. I did however do the ground trick and the SWR came in line with what I would want. I don't remember what the length of the cable is, but I think it was 12'.

I haven't dealt with the CB much since it was installed, since I have been working on the interior audio/video, axle swap, winch bumper, batteries, and the list goes on. I do know that it works pretty good to about 10 miles, as least according some of the trucks on I-80. My main goal was to have a fairly universal comm system for the trails.