Jeep Wrangler Forum banner

Can't Get SWR below 2

11K views 39 replies 7 participants last post by  RedBarron 
#1 ·
Okay, I have a Uniden PC68LTW with a 20 ft coax (all my radioshack had), and a 3 ft firestik. I also have a radioshack heavy duty spring on the antenna. I have checked everything: Polarity, etc. What should I do?

Channel 1=2.0
channel 19= 1.6
channel 40=2.1

Thanks,
-Chase
 
#2 ·
Get a vehicle with a better groundplane?

Jeeps are a beast, and I imagine you have a rear mount antenna. Based of your post, I bet raising the mount or relocating to the cowl or front bumper will show better results.

I had a tailgate mounted whip that was about 2.2 on the extremes, they dropped to 1.6 or so on the front bumper and range/clarity drastically improved.
 
#3 ·
Well, I can't get another vehicle (Don't need one) and I don't want to move the antenna. I have done that 2 times already. What else can I do?
 
#4 ·
Where is it mounted, what kind of mount, what tests have you done and what are the results?
 
#5 ·
It is mounted on the Driver side light. (Was on the passenger side) Mounted with a CB Mount from RadioShack. I have tested everything. No shorts, no damages on cable or antenna. Basically everything in everyother post on here. I even bought a spring to raise it up. I can get about 1.5 on 1 but thats the lowest.
 
#6 ·
Generally speaking, once the high and low end match, the antenna is as tuned as can be... meaning height adjustments of the whip become moot.

Swr is not dc ground. In fact, it is very different from it.

Clap. Seriously, go ahead. You used both hands, right? That's a dipole (two-pole) antenna. Your Jeep has a monopole antenna, so put 1 arm behind your back. Now clap. You used the computer desk, wall, computer, your face... something else, right? Your antenna is one hand clapping, and needs a metal desk to clap with. No 2nd hand (or desk) means bad radiation of sound. That is groundplane, in a nut shell. Antenna height adjusts resonance frequency, and is not quite the same, but must also be matched.
 
#7 ·
So I should get a longer antenna? I have read that the 4' works better on the jeep than the 3', but many have said the 3' has gotten a 1:1.
 
#8 ·
The higher the antenna, the better. So yes a 4' should perform a bit better (I now run a 4' and like it), but if it has no groundplane availability it will still be reflecting power back to the radio (high swr).

Imo, moving it up front would do more than installing a taller whip. We gotta get those hands clapping, right?

Many say many things; I don't think rear mount jeeps can reach 1:1 effeciency. I think 1:3 back there is phenomenal, and youre close. Anything under 2 is OK from an equipment perspective, so do you want it better, are you concerned with system damage, or are you not satisfied with broadcast strength? If you like the mount location, leave it. You are more than fine for trail rides and should have a couple miles of broadcast distance, depending on terrain. If you need the broadcast distance extended, I would mount in the best place possible, which is not the taillight. Cowl, front bumper, and 3rd brake light are all better options from an RF perspective, with the first 2 being far and away better locations.

If ya really want it to get out, you need to track down an antenna analyzer (mfj 269) to find the actual resonance sweet spot of your system. Typically this is overkill for 11M CB, so we use SWR testing to get close enough. Or you can move to a longer range platform, like Ham or gmrs bands.

CBs in Jeeps have to compromise something. Location, distance, swr... something.

Got a hardtop? You can add a groundplane to the underside of it with copper tape and a bonding strap or 2.
 
#9 · (Edited)
I do a lot of antenna problem solving for Jeepers that cures their SWR problems. 99% of those Jeeps have their antennas mounted at the rear, and typically up near the rear tail lights, where I never have any problems getting their SWRs down to levels like 1.1:1 or 1.2:1 which are very good ratios. Many of those installations are 2' Firesticks which I happen to run myself, the rest of them are 3' or 4' antennas of various types. So a good SWR can be obtained with even a 2' antenna if the basics of proper mounting are taken care of.

Typically, a very high SWR is caused by the antenna mount not having been assembled properly. A lower but still bad SWR is very commonly caused by the mount not having an adequate RF ground connection to the tub.

And there is plenty of "groundplane" on a Wrangler, simply providing the mount with an adequate RF ground connection to the Jeep via the tub is all that is needed. And with that adequate RF ground connection, which is more demanding than a simple DC ground connection, is usually enough to get a good solid low SWR.

Quite often the difference between a 2:1 SWR and a 1.1:1 or 1.2:1 SWR is nothing more than a little grinding to remove the paint and powdercoating so the antenna mount has a good bare metal connection to the Jeep. That is required to provide a good RF ground which is essential to a good low SWR and good antenna performance.
 
#12 ·
Hmmm.
Today I went and bought a 10ft coax. It made a huge difference. I can now get about a 1.5 across all channels. I will try making a better ground (taking some paint off) The mount is from raido shack and is assembled how the instructions say. I also made sure that there is no continuity between the parts that don't need it.
What do ya'll think will help now?
 
#13 ·
While someone started this thread im running around 7 i have no idea what the problem is hopefully I'm measuring that right i know nothing about cbs this is my first time but i have the same one as you. I have a 3ft firestick firefly mounted to the factory spare tire carrier. I even tried running a ground wire from the mount to the frame it didnt help any. Any suggestions something has to be wrong?
 
#14 ·
Whoa! 7 isn't even on my meter. Sounds like your antenna mount isn't properly grounded. That would be the first thing I would check. I know you said you have, but your cable could be messed up or you don't have the mounting stud put together properly. Other than that it could be the antenna which rarely happens.
 
#15 ·
That was my bad its a little past 3 I was running on 4 hours of sleep when installing and looked at the wrong part of the gauge. I'm sure it probably its a ground issue the antenna is put together right I'm just not sure what to do to fix it I could swear I have it grounded as well as possible paint is ground off each surface. Got a 14 gauge ground wire running from the mount self tapped to the frame
 
#17 ·
This is how I have mine mounted I have the ground wire self tapped to the rear frame cross member. I'm at a loss I got one of the coax with a removable end so I could get it through the tailgate hole I wonder if that could be my problem. Its harder to test when I have one around me to see if its actually receiving and transmitting
 

Attachments

#18 ·
It all depends on the setup. If your antenna is on the same side of the vehicle as the radio, that is normally a bad thing. Rule of thumb is antenna on the opposite side than radio. Also i have found that an 18 ft coax is best. And dont just wad all the extra coax. Roll it up into 3 ft lengths. I have a galaxy dx939 with a 4ft francis antenna and getting 7-10 miles range
 
#29 ·
I can't let this one go.
It does not matter what side of the vehicle your radio is mounted. I have no idea where you heard that one.

Also 18' is not best. Best length is the one that reaches from radio to antenna with a little slack.
 
#20 ·
I would go with a longer antenna then. Wilson 5000 is an awesome deal. Generally around $40. I am a trucker so i dont spare any expense on a good cb setup. My cb is under the driver seat with an ext speaker mounted on the roll bar above my head and i fabbed a bracket for my antenna thats bolted to the frame on the back pass side. It works great. You can always take your jeep to a local truckstop to see if they have a cb shop. It may cost $5-10 to have them check it out but depends on how much you would use the cb
 
#28 ·
Okay,
They tune the antennas at the factory to make sure they work....basically. No matter what you have to check your SWR. If it is fine, no tuning. If it really sucks, you must tune it. Try getting a shorter Cable. I had an SWR of 1.6-2.1 with a 20ft, then went to a 10ft and it went way down to a 1.2 across the band.
So in conclusion:
They tune the antenna Yes, but it isn't tuned for you. You have to tune it. If that doesn't work GET ANOTHER COAX! Sounds like yours is bad. Try a shorter one if you can.
 
#33 · (Edited)
Have you thoroughly read post #9 yet? The type of problem you have is often from something minor like not having scraped the paint or powder coating away where the antenna's mounting nut is supposed to get a good solid RF ground from. An RF ground is different and more difficult to obtain than a simple DC ground which can be provided by a simple wire. An RF ground needs a larger surface area at its grounding point so make sure all the bracket grounding points are bare metal to bare metal.

Look guys, im not here to argue. I know what ive been told and what my personal experience has figured out.
I can only say I've been doing 2-way radios for 50+ years (commercial, military, ham radio, and CB radio), have taught the subject, have designed & built antennas, studied electrical engineering in college, and can only state again that with the type of antennas our CBs use, coax cable length does not truly affect the SWR of the antenna system. If changing the coax length ever changes the apparent SWR, there was something else going on that affected it... but not the coax cable length. Neither does which side of a vehicle a radio or antenna is mounted affect the SWR.

The below is from http://signalengineering.com/ultimate/coax_basics.html

"Why does my coax length affect the SWR of my antenna?

How many of you change the length of your coax to tune your antenna? One of my good friends said to me, "I think changing the length of the coax is the same as moving the gamma rod adjustment on my Moonraker 4". Sorry to say, this is not true. As most people will find, varying the length of coax to the antenna will vary the SWR that the SWR meter is reporting. Actually, SWR should remain relatively constant no matter how long the coax is or where it is placed on the line (if its 5 feet down the coax from the radio or 50 feet down the coax from the radio). In most cases, the cause of inconsistant SWR meter readings is from poor SWR meter design or component aging / failure. For the SWR meter to read consistant SWR readings on the coax, the meter has to have an impedance itself of exactly 50 Ohms. Any deviation of the SWR meter's self impedance (from 50 Ohms) from poor design or component aging / error / failure will cause slightly inconsistant SWR readings when the SWR meters position on the coax or length of the coax is varied. In practice, generally you will find varying the coax length seemingly effects the SWR reading. Most SWR meters (built into radio and external type meters) and impedance "humps" in coax lines and connectors will cause minor variations in SWR as jumpers and coax length are varied. In reality, the mismatch at the antenna's feedpoint / coax junction is unchanged. Therefore - the actual SWR is unchanged.

Another reason SWR could vary is from the situation where the coax is acting as part of the antenna. Not a favorable or normal situation. The signal is traveling back down the outside of the of the coax braid (note power should only be traveling on the inside on the coax braid). Therefore, the coax is part of antenna system and changing the coax length will change the SWR. This situation is more likely to occur in mobile installations. You can try to eliminate this situation (called "Common mode currents") by winding an "RF Choke". Wind about 6ft of RG-213 or RG-8 into a coil (6 to 8 turns). For RG-58 use 4ft with 6 to 8 turns. Wind the coax up, placing each turn right next to one another. Use electrical tape to secure turns together. You should place these as close to the antenna as possible. Right at the antenna coax connection point being optimum. Most times, you can verify that you have common mode currents flowing back down the coax by grabbing hold of the coax while transmitting and moving the coax around. You can watch the SWR waver by moving the coax while transmitting (don't speak into mic!). You have to do this with all the doors closed from inside the vehicle. SWR should waver, if you notice that SWR jumps rapidily between two values, you might have a intermitant (bad) connection in the connectors (PL-259s) on the coax. In most cases of "common mode currents", just grabbing the coax will cause the SWR to change.

The "RF choke" described above stops the signal from traveling back down the outside of the coax. The signal inside the coax is * u n a f f e c t e d * by the choke (contrary to what you may have heard about coiling up excess coax). Common mode current kills antenna efficieny. You could have a decent SWR and not realize half your signal is being broadcast into you car (result very poor antenna performance). If your linear amplifier causes serious problems with your car's computer, lights, etc....you may have common mode currents. If moving the coax around the vehicle results in SWR change, this is a good indicator you have common mode currents flowing back down the coax line.

This doesn't happen often with base station antennas. Most base antennas have some type of device that will decouple the antenna from the feedline (gamma match, balun, etc.). Make sure you run your feedling (coax) straight down from the antenna, taking care not to run close to antenna to prevent "common mode" currents which could still occur if coax is oriented in a way to pick up strong antenna signal.

Coax Length Issues Simplified

Question: What is the "correct" length of coax?
Answer: The shortest length that makes it from the radio to the antenna.


Question: Are there any exceptions to the above rule?

Answer: 75 Ohm harnesses for Co-phasing is the only exception.


Question: Why do most mobile antenna makers recommend 18 feet of coax?
Answer: You got me, they claim you should use 1/2 wavelength multiples of coax. 18 feet isn't even close to being a 1/2 wavelength in any 50 Ohm coax you will find. Check some commonly used coax using the above formulas. RG-58, the most commonly used mobile antenna coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.


Question: Ok, seriously nerd, when I trim my coax it changes my SWR. You can't tell me it's not good to lower my SWR from 1.5 to 1.2 by taking off a few extra feet of coax.
Answer: Hey, I'm not a nerd! Go ahead, change your coax length. If you change coax length and it affects your SWR in minute amounts, everything is working fine. If your SWR was 2.5:1 and putting in a 4 foot jumper brought it down to 1.3:1, this large change indicates you have real problems...i.e. common mode currents (see above). Really, you should be changing the antennas length to alter SWR. NO special length of 50 ohm coax is going to fix or lower your SWR signficantly and/or boost performance. Period.

Question: I notice when I change coax length, my "modulation" needle jumps more / harder / faster when I talk. I get more watts out of the radio (verified by a watt meter) with certain lengths of coax. Is there a certain length that will allow my radio to put out the most power?
Answer: No, there isn't a magic certain length that will do this. Certain lengths of coax will allow your radio to "see" a load that it can couple with better which results in more power out of the radio / amp. Unfortunately, there isn't a good way to determine what length you need to allow the radio to put out the max wattage its capable of. I am only refering to newer solid state radios with transistors. Old tube radios usaually have devices built into them to tune the radios finals impedance to match that of the input end of the coax. If you want to accomplish the same effect with your solid state radio, pick up a device known as a "antenna tuner". The term antenna tuner is misleading because it doesn't actually tune the antenna - or take the place of tuning the antenna - it simply lets the radio couple to the antenna system with better efficiency. Other more appropriate names for the antenna tuner are the transmatch or feedline flattener. If your SWR is low (below 2:1), don't expect to notice a (performance) difference from using an antenna tuner.

Question: Why do so many people recommend using 1/2 wave mutiples of coax? Will it really hurt me if I take the time to measure a 1/2 wavelengh multiple of coax?
Answer: The idea of using 1/2 wavelength multiples of coax comes from the fact that the antennas feedpoint impedance is "mirrored" at the input of the coax when using the said length. Many operator make / made the assumption that was a good thing because it was just like having the antenna hooked right to the radio / SWR meter. If anything other than a 1/2 wavelength mutiple is used, the impedance the radio / SWR meter sees is the antennas feedpoint impedance transformed to some other value of impedance. So, if your antenna has a feedpoint impedance of 25 Ohms and you use a 1/2 wavelength wave length of coax, the radio will "see" 25 Ohms in the input end of the coax. If you were to use some other length, say a 1/4 wavelength of 50 Ohm coax, the radio would "see" an impedance of 100 Ohm. What consequences does this have? None. Whether the impedance is 25 Ohms or 100 Ohms, the SWR is STILL 2:1. No matter what the 50 ohm line length, the resultant SWR is still 2:1...at the antennas feedpoint, at the input end of the coax and at any and every point along the coax line.

Many operators take half the truth of transmission line theory and make up their own rules. If you have been reading my page since its inception, you know I used to be "uneducated" when it came to transmission line theory. Sorry to admit I thought coax length was important. It was drilled into my head by somebody I respect(ed).

This isn't the easiest part of CB to wade through. Hopefully I've covered this with enough detail to set everyone straight. Many beginner amateur radio operators and students have misceptions and make false extrapolations in tranmission line theory. There are many conditions that must be stated when simplifying things. I have made one assumption here. I have been assumming the coax loss is negligible. At CB frequencies this is a pretty safe assumption to make.

Question: Ok then, why is the length of 75 Ohm coax line important? If coax length doesn't matter, why is 75 Ohm coax different?
Answer: I've heard this a question more than once. The fact your radio has a 50 Ohm jack on it is the reason you use 50 Ohm coax. The antenna is designed to have a feedpoint impedance of 50 Ohms. When all the impedances match, maximum power is transfered from the radio, through the coax, out the antenna. Using coax with a (characteristic) impedance of 75 Ohms can potenially transform the antennas feedpoint into another value..another value such that the resultant SWR will vary with line length. This function is handy for matching antennas to the feedline that do not have feedpoint impedances of 50 Ohms. This is beyond the scope of this section, but antenna makers can specify certain lengths (1/4 wave) of 75 Ohm coax to achieve the proper match between radio and antenna. Again, this form of matching is not possible with 50 Ohm coax. "
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top