Thoughts on mounting and troubleshooting CB antenna

by **v7guy** » Wed Oct 03, 2012 10:45 am

I was helping tune a CB the other day and we didn't have any luck. Admittedly I have virtually no experience with CBs other than some limited reading on the firestik website and here.

The CB antenna was mounted to the roof rack. The roof rack is clamped onto the stock plastic crossbars. There is no ground wire on the antenna bracket. From a bit of reading I figured the reason we couldn't tune it is because the CB antena isn't grounded.
It seemed reasonable that a wire running to the bolt for the crossbar mount would probably be sufficient for a ground.

In addition, the CB is powered by a cigarette lighter adapter. Would this cause any interference by itself?

by **dvanbramer88** » Wed Oct 03, 2012 10:54 am

Lose the cigarette lighter power source, hard wire it.

Is there a ball of extra coax cable tucked up somewhere? That can cause issues too. You want to use as little coax as possible, and dont store extra touching and criss crossing itself tucked up under the dash.

Yes, ground the mount.

by **SaltWaterDrinker** » Wed Oct 03, 2012 10:54 am

Let me get home and get in contact with you and M00ts. I need more specifics and what not and I should be able to help you.

by **bartonmd** » Wed Oct 03, 2012 11:00 am

Yeah, ground is your issue. You can mount an antenna to the roof rack without grounding it, but it has to be a "no ground plane" antenna, that's got a chunk of steel in its base to act like a ground plane, instead of the body.

A wire to the rack bolt would add some ground, but is not ideal, and really, would need to be tuned with the antenna, as it'll also catch some radio waves. You likely won't be able to get your SWR down as low as if you were bolted directly to something with a ground plane.

Cig lighter adaptor is fine. Not ideal from a noise standpoint, but I've used them.

FWIW, extra coax is fine as long as it's in a "pile" and not in a coil, as coils add inductance (which is like resistance to AC circuits, and varies with frequency).

Mike

by **Moots1288** » Wed Oct 03, 2012 12:35 pm

I well ground the mount tonight. And try again. I do not suspect that the extra cable is the culprit but it could be I will take suggestions and thanks Jason for posting this I honestly forgot :wallbash:

by **SaltWaterDrinker** » Wed Oct 03, 2012 12:48 pm

Without reading what anyone else wrote as I'm going by route memory:

Forget about the cig plug. It causes noise and doesn't ground appropriately. Bite the bullet and hard wire it. I'm going to get specific:

If you got the CB that m00ts has, your CB will NOT retain its channel once power is disconnected from the unit (I.e. if you go on channel 36 and then turn the truck off, it will not be on 36 when you power back on). Please keep this in mind as I proceed:

If you don't care about the channel retention, find a fuse in your internal box that runs the windshield wipers or something like that - something that only receives power when the truck is turned on. Put the wire in with the fuse; I just jammed mine in there and pushed down.

If you do care about channel retention and will remember to turn the unit off, drop your wire into the main power point of the internal fuse box.

Now, you can feed both wires under the carpet and down the door frames under the plastic mouldings.

Once you've found a good area for your red line, find an empty spot for the black line and drill/secure it to the metal on the door frame. This will ground the unit.

For the antenna - You have multiple options. Mounting it too high will tear it to pieces; but if you don't care, it don't matter. The antenna should be grounded to the truck in some way - I don't feel like grounding it to a basket or the rack does much for range or anything. I'd suggest you throw it on in the rear area above the tail light - check the threads...everyone has their own variant of it.

As far as tuning goes...

You aren't going to be able to do much with the midland radio. Its a kick ass unit, however, its not great for power pumping or modifying - afterall, its a copy of a pre existing radio from another manufacturer. Does it mean its a bad thing? absolutely not. Schematics for it simply do not match up to what it is in reality - that's all.

Check the fire stick for proper length and what not - you can modify the antenna itself if the swr throws shit readings when you're on 40 and 1. Try 20 too - see how it throws it. You can lengthen it or shorten it by popping the cap off the top and play with it that way.

I use a Laird CB antenna - its a professional whip tuned like a beast - I suggest it highly.

Finally, make sure your power meter on board is throwing out what it should - if not, you could have a coxal problem...or an antenna problem.

Do not hook a power AMP up to it unless you get it to a shop and let them tear it apart - an AMP will only hurt the radio and/or you and your truck.

If you can't get that to work out...google a cb shop near you. There has to be one out of that area at some truck stop - NYC has some of the best skippers in the world(high powered CB users).

Please let me know whats up - have M00ts text me if something comes up. Or, you yourself can - 609-203-9945. I may be delayed as I am doing some school work.

by **SaltWaterDrinker** » Wed Oct 03, 2012 12:48 pm

Moots1288 wrote:I well ground the mount tonight. And try again. I do not suspect that the extra cable is the culprit but it could be I will take suggestions and thanks Jason for posting this I honestly forgot

Extra coax is bad.

by **SaltWaterDrinker** » Wed Oct 03, 2012 12:49 pm

SaltWaterDrinker wrote:
Moots1288 wrote:I well ground the mount tonight. And try again. I do not suspect that the extra cable is the culprit but it could be I will take suggestions and thanks Jason for posting this I honestly forgot

Extra coax is bad.

You'll lose your outbound and inbound signals in that stuff.

by **bartonmd** » Wed Oct 03, 2012 1:18 pm

SaltWaterDrinker wrote:
SaltWaterDrinker wrote:
Moots1288 wrote:I well ground the mount tonight. And try again. I do not suspect that the extra cable is the culprit but it could be I will take suggestions and thanks Jason for posting this I honestly forgot

Extra coax is bad.

You'll lose your outbound and inbound signals in that stuff.

FWIW, I have a big wad of extra coax and an SWR of 1.2 on my setup... Just need to make sure the extra coax isn't in a tight circle. Since the coax is tuned for 50ohms with the radio, cutting coax down can be worse than having a loose wad of extra somewhere.

Mike

by **v7guy** » Wed Oct 03, 2012 4:55 pm

It was Moots truck we were working on. The best SWR we could get looked to be about 2.25, the worst was a bit more than 2.5. There's a significant coil of coax under the dash.
It didn't seem to make a big difference how far out the firestik was adjusted. All our testing was on channel 20. It was never in the red zone, but we were never close to 1.5.

Josh, I'll probably shoot you a text tomorrow.

by **SaltWaterDrinker** » Wed Oct 03, 2012 4:59 pm

Okay Jason sounds good. Just let me know who you are when you shoot me one since the idiot crew (spammers) like to text.

by **v7guy** » Wed Oct 03, 2012 5:58 pm

by **djthumper** » Thu Oct 04, 2012 3:04 am

If it is a small amount of coax you can alternate the coils and it will be fine. If you just coil it up and it is resonant to the band you will actually create a choke. When tuning the antenna you want to hit channel 1, 20 & 40 so that you know whether to lengthen the antenna or shorten it. The rails don't provide a strong enough bond so your ground is floating and not allowing you to get a good SWR.

Your maximum power output from a stock CB is around 4 watts.

by **The Roadie** » Thu Oct 04, 2012 9:30 am

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

A shielded coax doesn't radiate EM waves outside its jacket (the reason for the shield) so it doesn't make any difference how it's coiled. The adjacent parts of the coil can't interact with each other and that's how you form an inductor - with unshielded conductors interacting with each other and a core. My boss and I are in the middle of a project that needs some pulse inductors running 200A, 1-10uS pulses, and we wound some ourselves on concrete forms to get lower parasitic capacitance. Have to package these with ferrite cores for customer use, because the concrete forms are a bit large. :facepalm:

by **bartonmd** » Thu Oct 04, 2012 10:01 am

The Roadie wrote: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

A shielded coax doesn't radiate EM waves outside its jacket (the reason for the shield) so it doesn't make any difference how it's coiled. The adjacent parts of the coil can't interact with each other and that's how you form an inductor - with unshielded conductors interacting with each other and a core. My boss and I are in the middle of a project that needs some pulse inductors running 200A, 1-10uS pulses, and we wound some ourselves on concrete forms to get lower parasitic capacitance. Have to package these with ferrite cores for customer use, because the concrete forms are a bit large.

Hmmm... I hadn't thought it was a big deal with coiling the coax, being shielded/grounded, but the paperwork for one of the radios I installed, as well as one of the coax that I used both said not to neatly coil the extra, because of the inductance.

What I had read about the cable length had more to do with matching resistance than with the wave length. It said that different length cables were fine, as long as they were, I think it was 50-ohms, to match the antenna and tranceiver.

I honestly haven't looked into either, on a scientific level, and trusted what the manufacturer said. This whole industry can't be that fly-by-night... Can they?

Mike

by **fishsticks** » Thu Oct 04, 2012 11:08 am

I'm no expert on radio comms, but I can tell you that 75ohm coaxial cable doesn't care about coiling (as long as it's in proper bend radius). At least in the 25Mhz-1.8Ghz range we used it for. This was all point to point work with no antenna in the middle though.

by **navigator** » Thu Oct 04, 2012 3:08 pm

I would also expect a lot of it also has to do with the quality of the cable as well. I would expect a cheap cable with cheap insulation would pick up more noise/interference than a higher quality cable.
One time we were picking up some strange signal over our cable but only on certain channels.
The cable guy came in and figured out it was due to a cheap splitter we had that split the cable internet signal and the cable TV signal. The splitter was basically serving as an antenna

by **v7guy** » Sun Oct 07, 2012 9:38 pm

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. The closer the tip got to the roof the better the signal. CB is hardwired and the antenna it's grounded to the rack bolt that screws intro the roof.
Any idea?

by **djthumper** » Sun Oct 07, 2012 11:18 pm

Is anyone near the antenna when you are trying to tune it? Or are you under a tree? Or other objects very near by?

by **Moots1288** » Mon Oct 08, 2012 1:36 am

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.

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