Question for those with CB antenna mounted on Tire carrier - Page 2 - Jeep Wrangler Forum

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Old 06-21-2012, 12:33 PM   #31
Kaisoboy's Avatar
Join Date: Oct 2011
Location: Calgary, Alberta
Posts: 38
Yet another CB tuning SWR meter question!!!
Hi Folks

So I bought a Cobra 75 , TF spare tire mount, 4' firestik II, FireStik 18' Fire-Flex coax cable with Fire-Ring from Northridge 4x4. I have installed it. Made sure that all my grounds were solid. Even ran a 12 guage wire from the mount bracket to the sub retaining bolt to ground to the tub. Ran the continuity tests and every thing seems fine. I then bought a Workman SWR-2T CB Radio Test Meter from ebay and proceded to dial in my antenna. I was getting (with the tailgate slightly ajar) ch. 1 = 1.8, ch. 19 = 1.4 and ch. 40 = 1.8. When I close the tailgate fully my SWR jumps up to roughly 2.5 across the board. So I tested for continuity once again and proceded to re-tune my antenna thinking I must have done something wrong but the results were the same.

Frustrated I went to my local CB shop and borrowed a professional cross needle swr/power meter. I measured the wattage power for channels 1, 19, 40 and got forward = 2 W and reflected = 0.5 W with the tailgate closed and cracked open.

So I looked up the calculation for SWR and it states using the formula below that:

Idealy, to trasmit, your reading should be below 2:1.

rho = SQRT(Power Reflected / Power Forward)

SWR Reading = (1 + rho)/(1- rho)

Here is the calculation using my results:

rho = SQRT(0.5 / 2) = .5

SWR Reading = (1 + .5)/(1- .5) = 3

So this means my SWR reading in this case is about 3:1 across the board and I'm about to blow the heck out of my CB. Is my SWR correct when the door is slightly ajar vs when it is fully closed? Like I said my continuity is good. I just seems that as soon as the tailgate is latched the SWR jumps up.

So here is my question. Which is right?
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Old 06-26-2012, 11:43 AM   #32
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Yea that's a good question. I have the same mount location with no problems. But I am also running a 103' whip so I can't tune the antenna. I am getting a 1.2 or so with my setup

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Old 06-26-2012, 10:18 PM   #33
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Originally Posted by Kaisoboy View Post
... I was getting (with the tailgate slightly ajar) ch. 1 = 1.8, ch. 19 = 1.4 and ch. 40 = 1.8. When I close the tailgate fully my SWR jumps up to roughly 2.5 across the board. So I tested for continuity once again and proceded to re-tune my antenna thinking I must have done something wrong but the results were the same.
Not to say any need it, and I am no expert on antenna theory, but:

Antenna 101 - the transmitter sends a signal to the antenna, which is sent to the tip, then radiates out. This radiation of microwaves must then "bounce" off of a large, flat, square, reflective, metallic surface, and from then it propagates into the atmosphere. This piece of metal is a ground plane, or counterpoise, and forms the "2nd half" of your antenna. Without it, the antenna is not matched, and the system is incomplete and will not properly function. Jeeps have little to no ground plane on the rear - there is just nothing there to reflect off of.

From Firestik:
"All "FS" antennas are compatible with autos, trucks, vans, motorcycles and ATV's in a single antenna configuration when metallic ground plane is available. Or, they may be used in dual (co-phased) configuration on any vehicle made from any type of material."

Basically your ground plane is varying when you move the relevant location of the antenna to the body of the vehicle (open/close the gate), which is changing the SWR. Unfortunately, it seems Jeeps get worse with it closed than open.

3:1 sounds like a bad ground, or maybe just a nonexistent counterpoise (too low antenna). Height will help, as others said 1/3 above the roof, especially with hard tops. The fiberglass and real glass are more obstructive than a soft top, leading to lost radiation (the waves can't get through - glass doors on microwave ovens and all).

Having said that, I imagine both measurements are right (variance of door open) with the CB shop meter reading likely having a higher degree of accuracy (you read ~2.5, they read ~3).
“Coming of age in a fascist police state will not be a barrel of fun for anybody, much less for people like me, who are not inclined to suffer Nazis gladly and feel only contempt for the cowardly flag-suckers who would gladly give up their outdated freedom to live for the mess of pottage they have been conned into believing will be freedom from fear.”
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Old 11-07-2012, 02:56 AM   #34
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SWR. Important, but with limits...

I have spent a lot of time, and used professionals to help me, and teach me.
I am not a pro by any means, but I have gained some knowledge over the years playing with CB's.
First of all. It's strange when I hear people talking about SWR under 1:1..
Some are actually mistaken, and all respect for that. It's not common knowledge knowing how to adjust SWR...! But it is just plain impossible to get that result (at least on a car). Period... The fact is, there are other things that are important too, like the shape of your car, weather, cables, watts, ground plane ++. But we tend to get hooked on getting a low SWR. And yes, it's a good thing to keep it low, but don't get crazy...

I will not go into too many details. There are other threads for that, but let me say it like this. If you place your antenna on the rear part of a car, it will normally have a better range forward (if unobstructed), as it uses the shape and thickness of the metal in your car to transmit.
And if the antenna is in the front part, more range backwards. Corner placement has the same functionality.. Opposite maximized range (depending on the frame, line of sight and body shape). A friend connected his radio to a barb wire fence, and it worked too. No antenna! But I guess his SWR was way too high for long term use...

Most cars have a lot of options regarding placement of the antenna.
JK..., not so much.... And if you get an SWR below 2 at your choice of location, you are well on your way, and there is little reason to fight for hours to get it .2 lower... That is unless you have plans other than talking to people that are with you, or at least a few miles away.
If your plan is to make new friends far far away..., and you think this is interesting and fun (like I did), tweak all you want. It might be the last .1 that makes you reach what and where you want. If you want to talk to others close by on the trail/freeway, don't bother.
But make sure your SWR is low enough to NOT cause damage to your CB Radio. You WILL damage your radio if your SWR is too high!
Some say that magic number is 3, some say it's 2.5.
I always stay under 2.1, to make sure I am ok! Other than that, I don't care anymore. My priority is to get the antenna high for a "no obstacle transmit" (Not on a bumber, behind a hardtop and/or other "bad" places)

This is just my opinion, and I am sure some will disagree. That's why we bring these things up in forums. To find solutions for everyone!
And to give and gain knowledge. I am gaining every time I log on a JK forum, and I am smiling every time I find and answer to my question!
As I said, up to you, and your needs.

Have a great evening!
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Old 11-07-2012, 03:23 AM   #35
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Location: Double Wells, Arkansas
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If you are having swr problems with vehicles that have a crappy ground plane configuration then do yourself a favor and order or find a truck stop that sells the Wilson brand silver load antennas. These antennas have a ground wire at the base that can be hooked to ground if you are having issues.

I have worked on and repaired base and mobile CB equipment for over 20 years and I run 5ft silver loads on my Duramax with great performance and anytime I get a Jeep, tube buggy or atv that's giving me fits the silver load has saved the day.

These are fiberglass antennas from 2ft to 5ft and have the adjustable tips. They have the large type that's good for 1000 watts and a smaller diameter "flex" series that's good for 200 watts.

Here some information

They can be ordered from just type silverload in the search. Try a truck stop first shipping from there is pretty high for an antenna.

Hope this helps.
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Old 11-07-2012, 05:45 PM   #36
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Location: Watsonville, California
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Those who have an antenna mounted on the tire carrier and notice the VSWR change when the carrier is opened (lowest VSWR) or closed (highest VSWR) are seeing, first hand, the Near Field Effect. Look it up and choose your source carefully. Scientific papers are the best source, antenna manufacturers are the worst. Wiki isn't very good either.

The short story is that objects surrounding the antenna, up to one wavelength out or 36 feet in the case of CB will affect the way the antenna operates. These objects do not need to be conductive. They distort the electric field and therefore the loading of the antenna. The increase in VSWR thus caused will not respond to antenna tuning. This is because it does not change the resonant frequency of the antenna. As you open and close the tire carrier and monitor the resonant frequency, you will see the depth of the antenna resonance change as the VSWR changes, but the primary dip point will not.

The closer the interfering items are, the more the effect will be. Things like tires, tops, tubs, jacks & other antennas will all have an effect. As mentioned in other posts the higher your antenna, the less these effects will interfere.

Prattle about grounds, ground planes and bonding are another issue and not related to this effect.

As in real estate, the primary concern is location (of the antenna). Some of the best installations with regard to this subject, have the antenna installed on either front fender, a foot or more in front of the windshield. If you can tolerate that position and your wheeling style doesn't kill it, then consider it.

Most of us who understand VSWR and its minor effects (within reason) settle for a rear mount as high as possible and remain happy.

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Old 11-08-2012, 10:02 AM   #37
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Good post, and it's a subject that is rarely, if ever discussed, during CB installs. Without being aware of this effect it can cause all sort of difficulty during a setup.

So when you are trying to tune that antenna, do it the same way you operate. Tailgate closed, doors closed, driver in seat, top in the most used position, and nobody standing too close to the vehicle.
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Old 11-09-2012, 10:10 PM   #38
Join Date: Jul 2010
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Originally Posted by Sbroadus View Post
I have mounted my antenna on tire carrier and SWR is about 1.6 when I open the tailgate and swing it open it goes down to 1.1. I can't seem to get it any lower than 1.6 with the tailgate closed. Any suggestions? It's no closer to the jeep than others that are mounted with tail light mounts.

You are mounted to the tailgate and you are getting 1.6? You are one lucky dude! I would kill to have 1.6.

I mounted to a terraflex spare tire bracket, and I've had nothing but problems.
I scraped and ground away paint and powder coating. I ran a separate 10ga. wire to the bracket from the tub AND the tie down bracket inside the rear tailgate. According to my multimeter, I have .1 not only at the bracket but at the coax coupling. I was still only getting my swr to 2.75 with a 3' firestik firefly.
I called the firestik tech and he told me that I needed to try jumper cables from the frame to the coax. If I didn't see improvement, it wasn't my ground, it was my antenna. I swapped for a 4' firefly. I was able to get the swr down to 2.25ish. And that is the best that I can get from the spare tire mount. I hate having a 4' antenna and it pisses me off even more that I can't get my swr below 2.
I have a windshield light bar so I stuck a flimsy "L" bracket to the light bar and ran my coax to it. I was able to get 1.2 I really don't like the antenna there, but I think it is time to rethink my mounting location.

If I knew how to run a ground strap, I would try that just for shats and gaggles before relocating my antenna. I've only seen photos of one ground strap, and it looked unclean.

Anybody have a good way to run a ground strap? I would like to see how you did it.

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Old 11-10-2012, 05:02 PM   #39
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Location: Double Wells, Arkansas
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Don't confuse ground and ground plane. Get the silver load antenna and your equipment will be happy. If you absolutely want to have your antenna where it is. I have set up hundreds of ATV's that have non existent ground plane surfaces using the silver load and they perform good and check out at below 1.6 with a 42 to 54 ohm load using a MFJ Antenna Analyzer.
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Old 11-10-2012, 10:55 PM   #40
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Any one in SoCal want to buy a new 4' Firestik Firefly?
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Old 11-18-2012, 07:00 AM   #41
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Instead of starting a new thread since this is dealing essentially with the same topic..... I want to install a 103 in whip on a license plate holder ( on top of a spring (6in? to make it a true 1/4) then ground it to the battery to make sure of a solid ground connection. Questions would be what is the best coax to purchase for a CB application. I understand there is great variability with the quality of copper coax, its shielding rating ect. As I understand the above discussion, it really does not matter (for a CB installation) the length of coax. As to position is there truly a better position than another with the 103in whip? And if so, would the left rear corner be an acceptable compromise for said application. I understand that personal preference is a huge factor in what brand of cb to buy, but any suggestions on a small compact unit that has what is really needed in a radio that would fit above the rear-view mirror on a 2013 JK? Comments, suggestions?
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Old 11-18-2012, 09:30 AM   #42
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Coax for a CB shouldn't really matter, so long as it is purchased from a quality source. RG58 is gonna be what most probably use, RG8 is another decent option. I run RG8X (mini 8) for my 144mhz rig.

18' has been said to be the magic number, although I can find no science to support cable length matters at all for tuning purposes unless a NGP antenna is used. 18' is common and usually gives enough for any Jeep install, so it is good for real reasons.

Antennas on Jeeps are comprimises. The recommended place for vehicle use onroad is pass rear bumper, as the typical vehicle will give an RF pattern favoring the drivers front, or where oncoming traffic comes from. Truth of the matter, if you get good swr you should be good to go. Best electrical placement would be the hood, a bit unrealistic with a 102 whip. Front bumper would be better than rear, but still not as good as hood or cowl.

As far as radio, Basic CB radio buyers guide should give some insight.

If you are planning on going trail riding, be careful using a 102 whip. It will beat your Jeep and any friends who dare too close pretty intensely.

Oh, I would ground to the frame and make sure your primary ground strap is good. Connecting things to the neg batt terminal can cause big trouble if the primary strap has a problem. If you want to ground to the battery, either fuse the line or connect at the common ground. Be wary of noisy spots. I ran my mount to frame, then ran a strap from that spot to the batt neg. It is sort of like a safety gate for a ground, but leaves the potential for a loop.
“Coming of age in a fascist police state will not be a barrel of fun for anybody, much less for people like me, who are not inclined to suffer Nazis gladly and feel only contempt for the cowardly flag-suckers who would gladly give up their outdated freedom to live for the mess of pottage they have been conned into believing will be freedom from fear.”
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Old 11-18-2012, 11:11 AM   #43
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Grounding the antenna to the battery won't do much good. The antenna wants an RF ground, not a DC (voltage) ground. The shorter the RF ground connection, the better.

Using a license plate holder for a 108" whip might place too much stress on the mount. It looks like it only has one bolt holding it on. Hopefully, that bolt is designed to go into the sheet metal of the rear fender. I'd be afraid to use that small mount with a 3 or 4 foot CB antenna. The first time you hit a branch or other overhead obstruction, that one bolt might tear loose. I think I'd drill at least 2 more holes in the mount and use some 5/16" or 3/8" sheet metal screws. That will help with both the physical strength of the mount and the RF grounding/bonding.

Whatever mount you use, and location, ensure you have a good, clean, metal to metal connection. On the mount you listed, that should be fine. It is unpainted, the bolt holding it on hopefully goes into the fender sheet metal, which should also give you a good metal to metal connection.

Good luck.

Wrong Way Hal
1972 C-101 / 1984 CJ-7 / 2012 JKR
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