Thoughts on mounting and troubleshooting CB antenna

[v7guy]

I was pretty surprised. The difference was striking.
Even after the CB was hardwired and the antenna was grounded the readings were bad. In fact they were actually worse. We keyed the mic and had the SWR readings shoot into the red where that wasn't a problem before.
Oddly enough while Matt was adjusting the antenna I noticed the reading would plunge to fantastic as the tip of the antenna got close to the roof... maybe it was reflecting off the body??? I dunno.
We sat in the garage for awhile, I set myself on fire, and then we figured we'd go ahead and uncoil the cable. So we cut the zip ties and uncoiled the wire that was neatly wrapped in a loop under the dash.
The first test we left it uncoiled in the street. Our readings instantly got fantastic.
The second test had us looping it in a clumsy figure eight, again the readings were fantastic. Unchanged from uncoiled.
The third test we stuffed it up under the dash. We figured it was a possibility that maybe the wires under the dash were causing interference. Still the readings remained consistent with the uncoiled results.

Like many on here I was skeptical that the coiled cable could be a problem. But I'm a very firm believer now.

Cliff notes...
Figure 8s seem good, unwound cable seem good, circular coils are bad.

[KE7WOX]

There is a lot of BS floating out there about coax length, much of which is discussed in essays like this: http://www.stu-offroad.com/misc/myth-1.htm

I believe part of that myth stems from cophased antennas, which a lot of truckers seem to be running. I did some reading into co-phased antennas for 2m and you do have to be very careful with how you cut, coil and run the cables, as the impedance to both antennas has to be the same and some more stuff that was mostly EE talk, maybe even some jmagjnary stuff.

[SaltWaterDrinker]

It turned out to be the wire being in a coil. Once we removed the coil we had between 1-1.5 on channel 1,20 and 40. We also tried it in a figure 8 and it worked fine.

WIN.

[Trail X]

So we were tuning Matts CB and were getting crazy high readings. 2.5 plus. We then noticed that when he bent the antenna the swr reading dropped to below 1.

I'm not totally up on radio terminology, but are you sure that you're reading your SWR meter properly?

If I understand it correctly, SWR should never be below 1, as a reading of 1 is perfect.

[v7guy]

So we were tuning Matts CB and were getting crazy high readings. 2.5 plus. We then noticed that when he bent the antenna the swr reading dropped to below 1.

I'm not totally up on radio terminology, but are you sure that you're reading your SWR meter properly?

If I understand it correctly, SWR should never be below 1, as a reading of 1 is perfect.

The needle plunged all the way to the left when the tip of the antenna was brought close to the roof. I don't know if it was a false reading or some kind of feedback created by putting the tip of it close to the roof or not. I don't understand why it helped.
It's possible that we did something wrong with the meter I suppose. But we got consistent and repeatable results. It seems like it would be hard to mess up. Flip the switch on the meter, key it up, adjust to SET, continue keying, switch to REF see what the reading is.
http://www.firestik.com/Tech_Docs/Setting_SWR.htm

After the coax was uncoiled we got readings of 1 or 1.1... which is supposedly perfect on the lower channels and then a bit higher on the higher channels. I'm guessing that the exceptional reading is from the antenna being 9 1/2 feet in the air and everything being hardwired and grounded.

Without a doubt the coiled coax was the problem.

[KE7WOX]

When the tip was brought close? I wonder if that was shorting / creating some sort of arc between the roof and the antenna.

[v7guy]

When the tip was brought close? I wonder if that was shorting / creating some sort of arc between the roof and the antenna.

Unfortunately I haven't been able to find an answer while reading around. But I wouldn't be surprised. If anyone has any references I can read it'd be great.
In a link posted earlier a guy talks about the cable length not being important and he references a guide of significant length. I need to do a search and see if I can dig it up.
I can't even give a reason for why the coiled coax was causing problems. I just know that it did. That was the only variable that changed. Mostly I'm glad it works now. It's one more thing checked off the list of things to take care of before Tecore.

[djthumper]

If you were touching the antenna when you were getting a reading it could be going to ground through the person. Hence why no one should be hear the antenna when you are taking a reading. Coiling the coax creates a choke coil so you lose your forward power. I have mentioned this in previous posts. I also stated that you could alternate the loops to cancel out the choke effect.

[KE7WOX]

Slightly unrelated, but do you know if coiling the coax on a receiving device will also cause issues? Something like a TV or a Scanner.

+1 on the person, I didn't think of that, but either way, the testing methodology was flawed.

The only thing I found mentioning specific lengths was here http://www.signalengineering.com/ultima ... asing.html and that's for co-phased antennas, I don't think you really want to go there, I've seen plenty of LE agencies and truckers using those, but for most people it's not even worth the trouble.

[v7guy]

Larry, your post and others are why we gave it a try. But we've also had a couple guys say that the coiling of the coax shouldn't make any difference because of the shielding. So which theory is correct? Is the cable really causing a field to interupt itself? It would seem so. But there are as many people saying that doesn't happen as there are people that say it does during my limited research.
I was lead to believe the sudden improvement in readings had more to do with the proximity to the roof because he just happened to bend it down while I was keying the mic. I didn't realize he was about to adjust it and it was really just dumb luck.
Given that the antenna is mounted to the roof rack it was entirely above Matt while he stood on the sliders inbetween adjustments. While waiting for the reading he was holding onto the rack. The nearest objects, trees, houses etc were about 25-30 feet away which is about as good as we can do in this area excluding a shopping center parking lot. After I asked what he just did and he said he bent the antenna over we were able to repeat the results. The readings changed when the antenna was bent over but not when he was touching it. Obviously we had mistakes while sorting it all out. There may still be a little tweaking needed. But I was curious why the antenna reacted as it did when the tip was brought closer to the roof.
We're going to go ahead and retest it either tomorrow night or Wednesday night to duplicate our earlier results. We'll take more time and make sure he's on the ground away from the antenna when we're checking the readings. But the final numbers posted was with Matt standing beside the truck on the ground. Looking through the open window at the readings while we checked each channel amazed it worked.
Obviously this is all a moot point now as it appears to be sorted out and working but I'm still curious about the roof and antenna tip results we saw.

[v7guy]

I had difficulties installin my CB tonight and getting the swr reading reasonable. I believe I've found the issue but I won't know until tomorrow. Here's some info I've found helpful.



http://www.off-road.com/trails-events/t ... 53469.html