Ground Plane/SWR Antenna Tuning

[Porkchop]

I'm actually doing nothing with it. I've just always read that it needed to be higher than the highest point of the car/truck, which mine isn't...exactly. On a Predator 10K or a Wilson Trucker, you can change/adjust the bottom section of the antenna, by buying a longer or shorter section. I'm just asking if this is something that needs to be addressed since I can't get below 2:1 for SWR, actually 2.5.

I'm not touching the coil. The only thing I've changed is stinger length. I make a ground change or whatever and then I check SWR. If nothing changed, I either raise or lower the whip, then check again. So far, I've seen no change in anything I've done. The upside down antenna has no coil...I'm only talking about the right-side up antenna.

 

[prcguy]

How close is the original whip running parallel to any large metal surfaces? If its something like using a bumper mount on a van where the entire antenna is only a few inches from the body then it will never match.

If the two whips are mounted on a mirror that sticks out from the vehicle they will operate somewhat independent from the vehicle and grounding should not be critical. A single whip on a mirror on a fiberglass vehicle will have a lot of RF current on the coax and grounding is critical because the coax is a major part of the antenna in that case.
prcguy

 

[Porkchop]

Well, let me try to better explain things, I'll try to do it where everyone can understand me.

The truck has a fiberglass hood, which includes the fenders on the front of the truck. The doors are both fiberglass and bolt to a metal (steel) cab. The cab has rubber mounts on the front that connect it to the frame (chassis) and I'm almost certain it's insulted by rubber. The rear of the cab is sitting on dual air bags and I know they are insulted as well. The wheelbase of the truck is about 25 or 26 feet, it's a quint-axle dump truck, meaning it has a front axle, tandem rears and 3 drop axles that are powered by air. The dump bed is probably 22 feet long, all steel, and made by Ox Body.

Originally, I put a Wilson 1000 on the truck and got a fair match, SWR. Then I decided to move up to a Wilson 5000, again mag-mount, and why...I don't know cause the difference between the two was none. Due to tree limbs and using this truck to haul asphalt, the plant knocking over my antenna, I decided to move up to a tri-mag mount. I thought this would solve my problem of it being knocked over, it didn't. About this time, I heard of Predator antennas and was told I'd have a much better antenna if I went with one, which I'll agree...the single coil version has been the best antenna I've ever run. The problem...a single coil Predator on a tri-mag setup requires a long stinger (whip) which slaps over-passes and is really a huge problem going under an asphalt plant.

I ended up contacting Kale, the guy who builds/owns...whatever...Predator 10K antennas. He sent me a double coil version...and I took parts from both the single and double coil and built the antenna I have now. It matched to about a 1.7 across the band on the tri-mag setup, which was fine with me. I never had a problem except tree limbs and that plant situation. I got tired of getting hung in the plant and having to fish it out of the bed right before I got a drop of asphalt being about 275 to 325 degrees. Not a good situation to put yourself into a dump bed under an asphalt plant. So I removed it and put my Wilson Trucker 5000 on the mag setup. I got about the same match from it, 1.5 to 1.7, with the lower being on channel one and the higher being on 40. Which tells me I need to shorten the antenna a bit...which I tried...but couldn't ever get an equal reading across the band. So I just left it as it is...has worked perfectly fine...on the tri-mag mount that is.

Fast-forward to the economic mess that started last year...the truck was parked in June of 08. Work was slack and I couldn't find anything to do with it. Work just picked up again for me in the beginning of June 09...so I needed to do something about my antenna system because I was going back to the same asphalt plant that caused me so much trouble. Aside from bending whips and knocking the antenna over, the tri-mag has ruined the finish on the roof of my truck. Keep in mind, I bought this truck brand new...at a cost in the neighborhood of 150 grand. So that stupid mag-mount should have never been put on that truck...

The truck did come with dual fiberglass antennas. But as mentioned eariler, they run into some type of box that lets the stereo use them to receive as well. And even a standard CB radio, out of the box, will feed back into the stereo when it's on (or off) and bleed through the speakers. I did see about a 1.1 or 1.5 match however...but those antennas and that system just doesn't have any receive or transmit. Once you can't see the other person, you lose contact with them...what's the point in that system?

So...I put several different types of mounts on the driver mirror mount (mirror frame). It sticks off the truck the same distance as any other mirror mount system on a big truck, consist of tublar aluminum (I assume) and is not grounded in any way to the cab or frame. That is, until I grounded it myself with braided strap that is about 1 or 1.5 inches wide. The antenna, that is right-side up, is about 1 foot away from the cab of the truck, same for the one I put upside down but it's closer to the fiberglass part of the truck than it is to the actually cab. It lacked reaching the ground by about 2 feet or better.

 

[Porkchop]

Also...

I was researching what you are saying about cutting the coax... I believe in matching the length by the velocity factor, etc....but this guy seems to believe somewhat different.

8) Trimming the length of my coax will lower my SWR.

Wrong. Trimming the length of your coax will trick your SWR meter and you in to believing your SWR is lower. SWR is the ratio of the impedance of your coax to the impedance of the antenna. Standard CB coax is rated at 50 ohms. It has a 50 ohm impedance regardless of whether it is 3 feet long or 1000 feet long. To get a Standing Wave Ration of "1 : 1" both the antenna and the coax must have an impedance of 50 ohms. If the antenna has an impedance of 100 ohms and the coax has an impedance of 50 ohms then your SWR is 2:1, or put another way the impedance of the antenna is twice the impedance of the coax. Likewise an antenna impedance of 25 ohms and a coax impedance of 50 ohms is STILL a 2:1 SWR because the coax impedance is twice the impedance of the antenna.

Changing the length of the coax will neither change the impedance of the coax, nor will it change the impedance of the antenna. It will trick your SWR meter in to a false reading.. In reality the impedance mismatch between the antenna and the coax will remain, and so will the accompanying power loss. It does not matter whether you have a Bird SWR meter or a Radio Shack SWR meter. SWR meters are not perfect, and they can be fooled.

If you can change the SWR reading on your SWR meter by changing the length of the coax then you can be sure of only one thing. You do not have a 1:1 impedance match between your antenna and your coax.

Signal Engineering has a more detailed explanation of coax and various CB myths on-line at The Ultimate Guide to 11 Meter CB Antennas

 

[prcguy]

Your setup sounds like a typical Kenworth, Freightliner or other big rig except I don't remember the fiberglass doors 30yrs ago. If your were local to me I could and would take it on as a project just to see what it will take to get the antenna working.

The coax length affecting the match is a two part thing. If there is substantial RF current on the feedline due to (bad) design or lack of a counterpoise, then coax length can certainly affect the antennas resonant frequency and impedance. Some antennas use a critical length of coax other than 50ohm as an impedance transformer and varying that length will cause all sorts of changes and problems.

In your case the coax will probably have lots of RF current on the coax due to insufficient counterpoise or ground plane and changing the length will affect the match. I'm not saying change the length but just be aware of it.

Another thing to try is a simple counterpoise wire or two to see if the main antenna will match. Try a full 1/4 wave wire heading out away from the truck (only a test!) or a pair of 1/4 wavelength wires heading out opposite directions and see if the antenna will tune. If it tunes ok and you can get it to tune with just one wire you may be able to snake the wire somewhere in or along the truck and retain the good match.
prcguy

 

[Porkchop]

I wish you were closer too...I wish anyone was with one of those MFJ analyzers right now. I see what you're saying now...and I agree with you...smart. But...I'm a bit dumb...about waves anyway...so what length would be 1/4 wave?

You're saying to put this wire, I assume it doesn't actually matter what type of wire either, on the mount like a ground wire and run it out and away from the truck. I'll try this and if one doesn't work, you're saying to add another. So at two wires...if it doesn't work...you're saying I should burn the truck and see how long it takes that little lizard to show up at my house? And if it does work, then I should attempt to run it along the truck and down the frame...try to hide it? I can do that as well...and I will if this works.

Thanks!

 

[prcguy]

Try just one wire attached to the ground side of the mount near the base of the antenna. A 1/4 wave at 27MHz is about 8 1/2ft, try a 9ft wire then try tuning the antenna. You can then try tuning the counterpoise to see if anything improves and the antenna will interact a little with the counterpoise and may need touching up.

If the 9ft wire gives a good match just try running it somewhere on or around the truck so it heads away from the antenna but will still allow you to drive. I think its best to run the wire down away from the antenna first and then run sideways. A single wire counterpoise will radiate and running it down first may help keep the radiation a bit more vertical from the wire and will also run the wire away from the coax which I assume runs into the cab near the mirror bracket and door hinge.

If the wire runs near lots of metal it will probably benefit from some trimming. Hope this works.
prcguy

I wish you were closer too...I wish anyone was with one of those MFJ analyzers right now. I see what you're saying now...and I agree with you...smart. But...I'm a bit dumb...about waves anyway...so what length would be 1/4 wave?

You're saying to put this wire, I assume it doesn't actually matter what type of wire either, on the mount like a ground wire and run it out and away from the truck. I'll try this and if one doesn't work, you're saying to add another. So at two wires...if it doesn't work...you're saying I should burn the truck and see how long it takes that little lizard to show up at my house? And if it does work, then I should attempt to run it along the truck and down the frame...try to hide it? I can do that as well...and I will if this works.

Thanks!

 

[Porkchop]

Well, I'll try that today and I'll let you know what I find. I'll also try to get some pictures taken of what I've got going on and get them posted as well. I appreciate all the time and effort, thank you.

 

[jim202]

Why don't you just do things the normal way and drill a hole in the roof. Put in a normal roof
mount base and screw the antenna onto the roof mount. Plug the coax into the radio and go
down the road.

You have already spent more time screwing around with different solutions that it's worth.

Jim

This is going to be complicated, I know, but try to hang in here with me on this one.

Ok. I've got a dump truck with fiberglass doors but a metal cab. I've been running a tri-mag setup on the roof with a Wilson 5000 Trucker antenna but it's got to leave here. It's ruining the paint on my truck, which I bought brand new in 04 and it's about to drive me crazy looking at those scratches.

So, you can't just plop a mirror mount on a Sterling. You'll get a high SWR cause there is no ground plane or counterpoise, whichever one it is. So...following the advice from Wilson and FireStik's sites, I added ground cables. My cab to frame ground is a huge braided strap, about 2 inches wide. All connections are soldered, nothing crimped or nothing like that.

The best SWR that I can get is a 2.5 across the band. I've tried my Wilson 5000 Trucker and I have a Predator 10K double coil antenna, both give the same results. I ground the antenna bracket itself to the lowest bolt on the mirror bracket, ran a ground inside the door, grounded that to the cab. Then ran a ground from the cab to the frame. No ground is over about 8 inches or so...so I know it's not a problem of the signal trying to radiate off the ground line.

Anybody have any idea what's going on or how to get the ground proper? I can reach out the window and grab the antenna and the mount and the SWR will fall to around 1.1 or so.

 

[Porkchop]

Well...one would think that huh? But, as I mentioned...this is an expensive truck. It's not your regular run-of-the-mill dump truck. This is a freaking nice truck dude and I don't want to put a hole in the roof. I'm worried about rust and water leaking into my cab. Plus...the factory messed up one of the welded "nuts" behind where the overhead console bolts into the roof. So I can't take down the overhead console to remove the headliner to put a roof mount antenna in even if I wanted to drill a hole in the roof. When you try to take out the last bolt...it just spins. No matter what you do, you can't get the bolt out of that nut that is welded into the frame of the cab's roof.

We're talking about a really nice truck here...not the average dump wagon you see running down the road. If this thing had a toilet in it, then I'd only be missing the kitchen sink to be able to say it's fully loaded. It's more like a show vehicle than it is a work vehicle...an 04 with 180,000 miles and looks like it just rolled off the showroom floor...

But, I might have to end up doing just that but I want to exhaust every effort before I do. Height of the antenna plays right back into that though...an asphalt plant and a tall antenna would just cause too much trouble again. Putting it out on the mirror lowers it enough to clear the plant's braces without it hanging in the silo doors. I think it would just be lovely to put it on the roof and then go under that plant and it rip it out the roof, don't you? A big massive hole ripped in the top of my truck...I'd just love that. So that's why I'm wasting all this time and running up and down a ladder like a moron to attempt to get a better match.

Before I finish ruining the roof of my truck...I'll leave it at 2.5 and just burn out transistors in radios. I'd rather spend a few hours once a month replacing transistors in my radio(s) until I can find a viable solution. But thanks for the input!!

 

[DX949]

Wow ! I'm impress,keep up the good work,You have good work ethics,your a rare breed.

 

[N_Jay]

Have you looked at any CB antennas designed for marine use?

 

[Porkchop]

Wow ! I'm impress,keep up the good work,You have good work ethics,your a rare breed.

At first glance, I thought this was sarcastic but then I realized you were being serious, thanks.

I'm not trying to be stubborn about this or wear everyone else down with me. You have to appreciate my efforts because my Dad bought this truck for me, or laid out the way for me to get it actually. I've worked it and tried to keep it paid for. If I just went on a hole-popping spree and didn't take care of it, what kind of appreciation would that convey for his efforts and sacrifices that he's made for me to have this? The man has shed a lot of blood, tears and sweat other this truck and I feel it's only minimal for me to just take care of it to the best of my ability.

I haven't tried a marine style antenna but that's a good idea. I'm wanting something that will radiate the best possible signal and have the best possible receive that I can get. I'm not trying to broadcast...I'm trying to communicate...and in this area...it's hard to do most of the time. I've dealt with AC fan noise...cutting down my receive to the point to where I couldn't hear anyone, so I felt like this would be a move in the right direction. I've always had a tone of static and white noise to deal with on a mag mount...probably because of RF ground problems.

My 2 meter radio...I wrapped several loops of coax out on the mirror to get a somewhat descent match...and my output signal seemed uneffected by it, as did the receive. I've been thinking that maybe I should do the same on the CB antenna. I just don't know if this actually cures a problem or just creates others that are unseen. I figured if I could whip the CB antenna issue then I could do the same to the other mirror bracket on the passenger side and cure the problem of my 2 meter radio also. Also...hoping to get away from any and all stray noises that I have had to deal with in the past.

 

[DX949]

Your right, I am being serious,I wish i could help.
Question = What type of Radio are you hooking up to your CB antenna .

 

[Porkchop]

A Galaxy 959, the one with the freq counter on it and SSB. You thought I was one of those bandit truckers didn't you...running a massive radio and big foot warmer huh? Nope. Those days are behind me...since I got my ticket I got my mess straight and operate the right way now. More reason why I'm trying to do this the right way and not a patch job.

 

[DX949]

A Galaxy 959, the one with the freq counter on it and SSB. You thought I was one of those bandit truckers didn't you...running a massive radio and big foot warmer huh? Nope. Those days are behind me...since I got my ticket I got my mess straight and operate the right way now. More reason why I'm trying to do this the right way and not a patch job.

LOL...No i did not think that,I am also a ham and like my Galaxy 949 CB radio,and like you i fly straight.
You never know, i might catch you on a skip one day, Galaxy to Galaxy.
I have to put my tower up this summer, then my A99 antenna I just got .73 good Buddy

 

[Porkchop]

I've got an A99 that I want to put up too...but I am thinking of not "flying straight" on that one and putting it up in a pine tree here. I rent and I don't think they'll let me put up a tower, and with all these trees and stuff...I think I need to put it up high enough so as to not cause problems to anyone.

You can usually find me on channel 23 during the day if I'm working and I go by Porkchop, everyone calls me that. And maybe if I ever get my stinking antenna problems worked out here, I'll try some SSB cause I've never messed with it much.

Well...let's go outside and see if this wire thing is going to help my problem or not. Gotta find that camera too and see if I can't get some pictures.

 

[Chipster]

Have a look at this. Granted, it is no gain, but it is more a base antenna made for fiberglass boats.
Shakespeare Marine Antennas Specifications: Shakespeare Little Giant™ 4051 CB Marine

Don't feel bad about not wanting punch a hole in the roof. I been their, done that. A new roof paint job is cheaper than a ripped out hole.

I am thinking something like a small base station antenna that is made for a no-ground application.
Try a Firestick DS14-FG http://www.claysradioshop.com/citizensband_firestick_ds14fg.htm
or a Firestick FG-648. http://www.claysradioshop.com/citizensband_firestick_FG-648.html

(It took me awhile but I finally found them on the Firestick websight. On the left side, look for NO GROUND RV/MOTORCYCLE ANTENNAS.)

Both are mobile NO GROUND plane antennas. Hate to spend more money. I am assuming you just tried the standard Firesticks? I tried to find info about them on the Firestick website, but could not find any.

I am really surprised the upside down antenna didn't work though. Is the end of the bottom one within a few inches of the fuel tank? Just thinking aluminum tank and lots of fuel would be reflecting back.

 

[Porkchop]

Well, yeah it's probably a foot or less from the fuel tank. What I need is for you guys to just come out here to NC and help me out on this one, haha.

I tried a fiberglass Firestik before...with the tunable tip...I'm not really a fan of fiberglass antennas. I'm probably giving them a bad rap cause of the factory antennas but I don't know. I guess basically it's the same thing except it's wound around a fiberglass rod...

I was a paint and bodyman before I got into the dump truck business. I can easily paint the roof of my truck, which I'm going to have to do now, thanks tri-mag mount. I could repair the hole...I could repair a rip in the roof...but why would I want to do that if I can avoid the situation completely?

IF...if I have to go with a roof mount antenna, I'll back that thing real good...you can believe that. I've heard of people putting 7 1/4" saw blades for backing plates...whatever works. I really think I went to far with the whole ripping out the roof thing...I think it would break the whip first. But in that rare instance that the actual antenna was hit...that could easily ruin the roof of my truck. If this were a pickup truck...I'd have no problem doing that cause it would be so low to the ground in the first place. I pretty well know that a roof mount is the best possible way to go, that's why most business band antennas are roof mount and not mirror mounts. It does make for a better appearance and all that...I agree.

If I can't resolve this issue easily and I've got to spend more money on another antenna...it'll be a roof mount. There IS a solution to this problem and it CAN be made to work...I just have to figure out what the "secret formula" is to it.

 

[Porkchop]

Well, I'm happy to report that I'm still at square one.

I removed the regular style mount that coax just screws onto. I had the lug style mount on my tri-mag, made by Predator, so I put that onto my mirror mount. Believing that LMR400uf is better coax, since it's double shielded, I decided to go with it. I went through all the trouble of routing it through the dash and out the inner jamb, into the door and out to the mirror mount...what a job.

I changed the regular wire I had on the mirror mount over to braided, thinking that maybe that would help the problem some. Since that wire wasn't long enough to start with to actually reach where my coax shield was grounded, I added another piece of short braid to reach between the two spots, about 3 inches. So now, basically all my grounds are broken up...so it's not one continous run.

I got to thinking about what was being said about shortening up the coax, so my original coax was 18ft long. The LMR was 14. When I tested with the LMR, I got about a 2 for an SWR. So I figured...why not try shortening the coax for craps and giggles...so I did. I cut 2 feet off the coax, which gave me 12 feet of LMR400. The lowest I'm seeing the SWR now is about 1.7 or so. But I can clearly tell something is still wrong because now I don't see the needle swinging on the radio like it was doing before. I know that a swinging needle isn't an indication as to whether or not things are right...but I know it should move more than what it is.

I just don't get this. The only better ground you could get on the antenna would be to lay it on the ground...it gets no more grounded than what it is. Cutting the coax has fooled the meter...I don't exactly understand how or why...but it's showing a lower SWR for sure...but I feel it's not really that low. Maybe I need more grounding on the cab? I just finally gave up...I'm give out..it's HOT outside...real HOT.

Tomorrow, I'll try the Wilson 5000 and see what it's doing on there. I'm going to call the guy who built the antenna to start with and see what he thinks and suggest. I've tried everything that has been suggested here and I'm still no closer to things really being right... I even grounded the coax at the back of the radio to the cab, just to make sure. I appreciate the help everyone has offered...but guys...I just don't see this one working out. Any other suggestions?

It's beginning to look like that hole in the roof is my only option. I guess I'll have to break my overhead console around that bolt that is messed up. I sure don't want to...and I sure don't want to put a hole in my cab. But it's looking like that is my last hope.

Maybe a bracket that comes off the frame...try a 102" whip and see what it does...or try that same bracket with my current setup and see what I get. Those are the only ideas I have left...