SWR Trouble

[Scipio Kid]

I've got a service truck I can't get the SWR below 2.5 match on. Originally it had dual antennas on the mirrors (with a great SWR) but they were stolen and the coax cut. So I threw a single firestick above the tool bins and figured it'd work out. Its good enough to work around a jobsite or in a caravan on the highway, and it's just low enough to not worry about burning a radio, but it doesn't get out like the other trucks. I've put lots of antennas on lots of cars & trucks and I can usually get the ratio down to a 1.2 0r 1.3. I hate having a 2.5 and not getting out like I should.

The other day, I took the antenna from another truck, with a near perfect match and swapped it with the Firestick. I also swapped out the radios. The end result was I always get the bad SWR on the service truck regardless of the antenna or the radio. Even with a tune-able antenna, I can never get below a 2.5. I was thinking it was the location of the antenna but, like I said, I've done a lot of installs and it's a good location, in fact, better than most. It's up above the steel tool bins and everything is well insulated and grounded etc. The radio is mounted on the ceiling with the excess coax stowed in the ceiling as well. The old coax for the dual antennas is still up there too but it's not hooked up to anything. Could it be causing some kind of problem lying up there with the coax I'm now using?

Any thoughts or suggestions are more than welcome.

Thanks.

Scipio

[295 antenna]

The coax would be my guess considering a different antenna does the same thing

[Scipio Kid]

Thanks 295.
The coax was new when installed, so I haven't looked at it to be the problem. I re-did the mount and coax connection at the antenna in case that was the problem but I haven't gotten back up into the ceiling to check the coax since I put it in. Maybe it was bad out of the box. The problem has always been there and I always blamed the antenna rather than the coax.

I'll replace it and get rid of the old extra stuff from the duallys and see if that solves the problem.

[MDYoungblood]

I've got a service truck I can't get the SWR below 2.5 match on. Originally it had dual antennas on the mirrors (with a great SWR) but they were stolen and the coax cut. So I threw a single firestick above the tool bins and figured it'd work out. Its good enough to work around a jobsite or in a caravan on the highway, and it's just low enough to not worry about burning a radio, but it doesn't get out like the other trucks. I've put lots of antennas on lots of cars & trucks and I can usually get the ratio down to a 1.2 0r 1.3. I hate having a 2.5 and not getting out like I should.

The other day, I took the antenna from another truck, with a near perfect match and swapped it with the Firestick. I also swapped out the radios. The end result was I always get the bad SWR on the service truck regardless of the antenna or the radio. Even with a tune-able antenna, I can never get below a 2.5. I was thinking it was the location of the antenna but, like I said, I've done a lot of installs and it's a good location, in fact, better than most. It's up above the steel tool bins and everything is well insulated and grounded etc. The radio is mounted on the ceiling with the excess coax stowed in the ceiling as well. The old coax for the dual antennas is still up there too but it's not hooked up to anything. Could it be causing some kind of problem lying up there with the coax I'm now using?

Any thoughts or suggestions are more than welcome.

Thanks.

Scipio

The single antenna wasn't the one left from the duals, it might have been cut too much to match.

3's

Greg

[TNT1450]

Are you still using the dual mounts but with just one antenna? That will make a difference, especially if you're using the same coax. CB's tend to have 50 Ohm impedance at the antenna jack, but with dual whips they are often connected together with 75 Ohm coax. This will make a difference if you just have one antenna now. Just run 50 Ohm coax straight from the radio to the antenna.

[Scipio Kid]

No, the dually's were centerloads on the mirrors with the extra coax stowed above the ceiling (The radio was mounted on the ceiling on a slider bracket.) They had a great match and really got out. The radio and antennas were stolen and we didn't have anything to replace them at the time so we stuffed the extra coax from the mirrors up under the dash and left rest up above the ceiling. We mounted the new radio in the same place, on the ceiling, ran a single coax cable to the new 4' Firestick mounted just behind the cab and stowed the extra coax up above the ceiling along with all the coax left over from the original antennas (so there's a lot of coax up above that ceiling panel). Didn't get a good match off the bat but we needed the truck and figured we'd fiddle with it later. Now, after lots of fiddling, changing antennas, changing radios etc. we still have the bad match.

The truck has steel tool bins and a crane boom that folds down over the cab for travelling. The antenna is mounted right off the inside of the tool bins, just behind the cab with all that steel to use as a ground plane. Thinking maybe the steel boom was giving it too much ground plane and just for kicks, I took the antenna off and set a magnet mount right next to it, running the coax through the window and got a 1.2 match. So I figure the location is good. I took a center load antenna with an adjustable top mast piece that was tuned perfect on another truck and adjusted it from as short as it would go to as long as it would go and found the best match (2.5) was short. So I took a piece of 1/16" stainless steel rod and used it in place of the mast and cut it shorter and shorter in 1/8" increments and never got better than a 2.7.

Like I said before, I've never suspected the coax because it was new when installed. It's about the only thing I haven't fiddled with because it's a big job to take the ceiling apart, sort of the same reason I never took the old coax out of the ceiling. I think before I replace it, I'll just run a new piece of coax through the window and see what I get. If it's not any better, the coax is probably not the issue and maybe I'd better think about moving the antenna (or getting another truck!).

Thanks for the ideas

[TNT1450]

OK, it's likely due to all that extra coax being rolled up in the ceiling. Cut it just the length you need to get from the radio to the antenna and then put a new connector on it. If it's rolled up it could be acting as an RF choke. I'm sure all that extra length is likely what's causing your high SWR.

[Scipio Kid]

O.K. New question, or old one I've been hearing since 1971, when I bought my first little 23 channel midland for 2 months pay ... If I cut the coax to the length I need, I'll only have 5' since I drilled a hole through the back of the cab (sealed with a rubber grommet) for a direct route to the mounting point. Now, since that first radio, I've been told over and over "not" to shorten the coax and not to "coil" the extra but to stow it in big "S" curves, which is what I have up there above the ceiling. I'm no techy, just a dumb guy with a lot of radios and I hope it's not a dumb question, really, I think it's a good question (from a dumb guy maybe, but a good question) ... should I or should I not cut the coax?

[TNT1450]

Cut it. There's no "right" length a coax cable has to be, it's an old urban myth. It only has to be just as long as it needs to be to get from the radio to the antenna. I know that old "coax has to be multlples of 3 ft. length" belief is still out there, but it's bunk. 5' of coax is fine, it's far better than a bunch of extra length coiled around the ceiling of your truck.

The only time coax length is critical is when you're using phased antennas, like the two antennas on each side of the cab. They usually come with a phasing harness of 75 Ohm cable, and the length tends to be critical as I think they're often 1/4 wavelength long. But for a single antenna to a radio, there's no "right" length unless it's too short.

[Scipio Kid]

Thanks TNT. That's great news!

Now let's see how many get on here and disagree with you.

From an electrical standpoint, I've never understood why the coax needed to be "longer" than needed. Sure, if you had PL-259's soldered on both ends, it was easier to use the whole thing, rather than trying to splice it with pliers and duct tape (a method I no longer recommend, by the way) but otherwise, why keep the excess?

When I bought my first radio, the guy who sold it to me (out of his basement in a shady neighborhood) told me I had to use all the coax, or it'd wreck the radio. My shop Foreman at work told me the exact same thing and he was, in my view, ‘the’ great CB expert. He had two mobiles and a base station at his house and said things like “10-4 Good buddy” and “neagatory” all the time! He told me the antenna and coax were tuned to each other and made to work with all (23) CB frequencies. About a month after I got my radio going, I convinced my dad, who owned the company, we needed CB's in the trucks and a big base station at the shop. My Foreman (the CB expert) was thrilled! Two days later, he and I went to work installing our Navaho 23C base station (which I still have, in good working order) and a huge Radio Shack .64 wave antenna high atop a 2” mast attached to our air conditioning unit on the roof of our shop. We also had (2) 50’ lengths of RG-8/U coax to get from the foreman’s office to the antenna on the roof. I asked him how it worked since the 12’ length in the car was so crucial and he said the bigger stuff was made for base stations. I didn’t argue. I thought the base station was 100 times more powerful than my little Midland so the big cable made perfect sense. The base station worked perfectly so I forgot about it but, I’ve still wondered about coax length and size the last 4½ decades, so it’s nice to finally have an answer.

Also, not joking about those who disagree. If you do, please chime in and let us know the science behind it. I've heard there is a resistance factor and something about proper RF reflection or something. I'm a good electrician (meaning I've never electrocuted myself severely enough to be dead, or at least on disability) but I don't claim to understand the intricacies of how radio transmissions work.

There are a lot of really smart guys (and yet they allow me to be here too???) and yes gals (... sheeesh, I hate political correctness) on this forum and hearing from you is always enlightening.

Also, what do you mean by "too short"? would my 5' run be too short?

[TNT1450]

We also had (2) 50’ lengths of RG-8/U coax to get from the foreman’s office to the antenna on the roof. I asked him how it worked since the 12’ length in the car was so crucial and he said the bigger stuff was made for base stations.

LOL, yeah, it's bigger in diameter, but it's still just 50 Ohm coax, and the length doesn't matter. The fact that 50' works OK for a base station installation ought to be a clue that actual length is not critical. No need for 12' of coax in a vehicle when 5' will do.

Also, what do you mean by "too short"? would my 5' run be too short?

By "too short" I mean too short to reach the antenna from where the radio is mounted.

[Scipio Kid]

Well sure, it was at first but I pulled real hard on it and made it work.

[Scipio Kid]

Got a little time to try to fix this thing. Did some research and found some stuff on coax length I thought was interesting and wondered what the experts here think of it;

"The length of the coax is determined by the length of the radio wave and the frequency of the radio signal. A full cb radio wave length is 36 feet, so ½ a wavelength is 18 foot. The cb radio signal is positive for ½ of the wavelength and negative for the other ½. At the ½ wavelength there is what is called a zero-voltage point. This is where the signal is changing from a positive to negative or negative to a positive. The zero-voltage point is at the half wavelength on every signal sent and received, this doesn’t change. The zero-voltage point is where you want your coax to hook up the antenna. This is how the cb shops came up with the 18-foot length of coax. A lot of cb radio shops will tell you that you need to run an 18-foot long coax. Well I’m not a cb technician, I’m a driver, but I can tell you that they are not telling you the whole truth.

How do you correctly determine the coax length?
Multiply the wave length by the velocity factor of the type of coax that you are using. This will give you the proper coax length where any excess standing waves (SWRs) are visible to the SWR meter. This way you can have your antenna properly adjusted, and your cb working like it should.

What is a velocity factor?
The velocity factor is a term for how fast your signal moves through your coax. Your radio signal moves through the air at somewhere around the speed of light, but it doesn't go through your coax at that speed. The velocity factor causes the signal to move through the coax much slower, which puts the length of coax needed to equal a ½ wavelength less than than 18 feet. The velocity factor needs to be accounted for when determining the length of your coax.

How do I find out what my velocity factor is?
The manufacturers will tell you the velocity factor. The velocity factor is known as the "correction factor". The perfect velocity factor for a coax is a 1.0, but there is no perfect coax. The velocity factors of Belden coax cables used by most cb shops are:

RG-58 velocity factor of .66 RG-58/U (foam) velocity factor of .79

RG-8A/U velocity factor of .66 RG-8/U (foam) velocity factor of .80

RG-213/U velocity factor of .66 9913 velocity factor of .84

By using this information you can find out what length of coax you really need. If you are using a mini 8 coax with the velocity factor of .66 you would need a coax of 11.88 feet long (142 and ½ inches). This is how I got that length: 1/2 a wavelength (18 feet) x the velocity factor (.66) 18 x .66 = 11.88.

So when you have a cb shop telling you to use an 18 foot coax, ask them WHY. Now you know the truth on coax lengths. You can get this and other information from the ARRL Antenna Handbook"

With that in mind, and whether it was right or wrong, I took a brand new, 18' length of coax and ran it directly to the antenna (figuring I'd trim it back to the 11.88' as part of the tuning process if needed). I didn't run it up in the ceiling in case that was part of my original problem. It didn't change a thing, still flirting with a 3 on the SWR meter. The antenna was mounted up high above the ground plane so I wondered if I mounted it lower, it'd get a better reflection off the ground plane. So I mounted it about 15" lower, putting the load coils at the top of the antenna closer to the ground plane, and it was worse. Started to wonder if the antenna was bad or the mounting bracket or insulators or something, even though I'd checked them several times. Just for the heck of it, I took the antenna mounting bracket & hardware etc. and bolted them to the driver's mirror, about 3' away and another foot lower (also, about 18" farther from the truck body). The coax was still just strung around the cab and through the window. What do you think I got? A perfect match, the needle doesn't even swing! No movement whatsoever. I won't argue with the result, but sheeesh, what was so different? This is the weirdest thing I've ever seen. Like they say; "location, location, location". I'd sure like to know why though.

[TNT1450]

I still say it doesn't matter if you use 18 feet of coax or 8 feet. Length doesn't matter. I use a multiband antenna on the HF bands and only one coax cable to feed it. It's got a perfect match on each band.

[Scipio Kid]

I've got a box full of coax cables I've picked up here and there. Just for kicks, I took them all out and measured them. All of the dual antenna setups (they're all factory built) have 18' per side. So do the single antenna coaxes (that haven't been cut). I've got a perfect match in the service truck now so I'm leaving it at 18' and stuffing the excess up above the ceiling panel. I figure, If it works, don't mess with it. (I'm finishing it up this morning. If the SWR's go up with the coax in the ceiling, I'll just leave it wrapped around the steering wheel and shifter, like it is now.)

About that length thing I quoted; the theory sounds good but anyone can make up a good sounding theory. If he's right, and it's proven science, I'd figure techies and manufacturers would have figured it out long ago and most packaged coax cables would come at that 142.5 inches, not 18'. The fact that most seem to come at 18' suggests to me a lot of folks are simply buying into an old wives-tale. (However, 18' is sufficient for almost any mobile installation so it's a good length to sell with a package. Maybe that's the long and the short of it.)

Also, if it's true, adding power amps, power meters, SWR meters, jumpers and chokes etc, should technically screw up the whole installation.

[MDYoungblood]

All of the dual antenna setups (they're all factory built) have 18' per side. So do the single antenna coaxes (that haven't been cut). I

With dual antennas the coax is co-phased, if you look it is also 75ohm coax. Cut to 18ft, seen some as short as 9ft, plus or minus an inch it works out to be 50ohm +/- . Single coax is 50ohm and it is just the way the manufacturer made them, length on a 50ohm coax doesn't have a set, look at a Sirio magnet mount, it has only 12ft. I would watch out using old coax, defects ( kinks, water penetration, etc.) are more likely to be in used stuff over new.

3's

Greg

[Scipio Kid]

Excellent advice on the old coax. Saving old, worn or kinked stuff is just asking for trouble. The stuff I've saved is in good shape and the piece I used was new. But the real problem on this truck was the location. I've got no problem leaving it on the mirror so ... problem solved.

And thanks for the advice on the dually setups. I've actually got 3 sets of dually coax and all the antennas I need we're planning to put on my partner's tractors in a few weeks. He's expanded to 4 and decided to put 2 bases up in the 5 acre yard and radios in all the trucks. I didn't know about the 75 ohms on the coax and was going to use regular (50 ohm) cables with a tee fitting behind the radio to get to the antennas on the fourth and any additional trucks. Sounds like it'd be better to buy more dually coax instead.

Lastly, you mentioned the 12 foot magnet mount coax and antenna. I've got 4 very good mag mount antennas that perform extremely well for temporary use. We use them a lot to throw in a car or truck following a rig or just for the family going on an outing where some don't have permanent radios. The worst thing is the potential damage you do to the coax, running it through a door. With some you don't even notice but with others, you can really screw up the coax. We've got 2 Honda Civics we throw radios into from time to time. They (the Civics) are the worst. One or two uses and you might wreck your coax for good. Now leaving the window cracked just a little to run the coax in works great ... until someone forgets and rolls the window up to stop the road noise. Then it's worse than the door-pinch. Great concept but unless you can find a 3/4" hole somewhere to run the PL-259 through, you might end up with a lot of problems. And since the coax is usually attached permanently to the mag mount, it's not an easy fix.

[arcone]

I like to run a half wavelength in the car, taking in the velocity factor. On Ch35 that would be 3620 mm or 11ft 10 1/2 inches using MIL spec RG58. If you want to run an analyzer and make impedance measurements, I believe this is good practice for accuracy. It will ignore the impedance of the coax, what ever it be, hell I've even run RG6 Quad shield 75ohm coax in the car before with no effect on SWR, however this will be frequency sensitive. Only other time I pay attention to length is when phasing antennas, like the others said.

[Scipio Kid]

Thanks,
I wish I understood half of what you said.

I'm learning.

[Scipio Kid]

Hello everyone,
I haven't been on here in a long time. Life gets busy, especially when you need far more time to rest, recuperate, sleep and binge-watch NCIS (the drawback of which is; you end up hating Gibbs). But that has little to do with CB's. I started this post with questions about bad SWR's on an old service truck and when the problem was solved far better than I expected, I stopped posting (whining) about it. Not that it's important, but the truck is a 1978 Ford F-350 and they're getting kind of rare and kind of cool to collectors and young guys who've never seen one in real life. The Radio and antenna worked flawlessly and I used a dozen or so different radios in it, often just to try them out before putting them in another truck or testing them because I knew this antenna setup was simply great!

Last year, heading to a jobsite I entered an intersection as the light turned yellow. Some poor girl, with no insurance, and no driving skill or sense, panicked, turned in front of me, and I slammed into her at 45 MPH, totaling her new Jeep and doing a lot of damage to my old truck. She got arrested and I got my rig towed to a junkyard. I only had liability on it so I had a decision to make. The following morning, as I stood in the scrapyard, sadly looking at my smashed up 40 year old friend (I bought it new in 1978), I decided I needed to move on. But my buddy who was more emotionally involved than I, with a tear in his eye, spoke these solemn words of wisdom; "we can rebuild it. We have the technology!" It was so stupid I punched him. But I guess I liked his thinking because that's exactly what we did, and the result was awesome. A month later, we were back on the road with what looks like a brand new, 42-year-old service truck. We repaired / replaced the damage and repainted the whole truck, basically fixing everything, except the CB radio and the antenna, they didn't need fixing. It's been a lot of fun ever since. I have guys honk and give a thumbs up as they pass and lots of long-haul truckers give me compliments on the CB when I pass them at 85 on I-80 (although some cuss me out).

Anyway, I'm sure you've all stopped reading or fallen asleep (it's OK, I did too) wondering if I had a point germane to this forum. I do; a couple of weeks ago, I pulled the rig into a lower clearance garage and caught the antenna between the frame and garage door, having to bend it some to get it out. I took it off for a week or so while we were moving the truck in and out of that garage and noticed the outer covering (the antenna is wire-wound fiberglass with shrink-tube covering) was cracked and the rubber top cap torn. When we finished the job, I put the antenna back and tested it and it was still perfect (near 1:1 match on all channels). I didn't want to leave it exposed, so I peeled off the shrink tubing, got a new piece and installed it and put a new top cap on it as well. So now it's basically brand new but when I tried it out, the SWR alarm lit up! I metered it, thinking I’d broken the coil wire but no, I’ve got about a 1.5:1 ratio on channel 1, but it goes up with every channel to a 4:1 ratio on 40. So, yeah, the antenna is long. I’ve trimmed coils before to get a good match, but this antenna was perfect before I put the new shrink tube and cap on it. I remember years ago, I had a center load antenna and the outer plastic shell on the coil cracked. I wrapped it with electrical tape and coated it with Scotch-Coat to keep it dry and the SWR’s went up. It had an adjustable mast above the coil and as I remember, I cut off ¼” and got it back to perfect and forgot about it. I also lost a cap on another fiberglass antenna and wrapped it with tape and Scotch-Coat and it went nuts too. Don’t remember what I did with it. I think I chucked it.

So, with all this lengthy history my question is; why does the SWR match change when the insulation changes? I’m working on it today and I’ll cut some coils off the top and see what happens, but your advice, wisdom, knowledge, and opinions are welcome and, I’m sure, might help others with antenna tuning issues. (I also realize some of you don’t like my long-winded diatribes and figure you may be willing to send gobs of cash to get me to shut up. That works for me too.)

Thanks. It’s good to be back.

---

Post Merge Complete

Added 9 hours 6 minutes 13 seconds after previous.
Well, here's the deal. I took a close look at the antenna, mounting bracket and coax setup I'd done 4 years ago. I wrapped the coax around the old style mirror mount (some say you shouldn't do that) about 5 twists and installed a typical mounting flange off the mirror. The insulators looked good, so did the coax connections. The end of the coax isn't a PL-259, but the shield is grounded with a stainless screw to the mounting flange and the center lead is attached to the bottom mounting bolt of the antenna hardware, below the insulators. I've used this method numerous times and found if you cover the screws and leads with Liquid Tape, Scotch-Coat or RTV, they'll last forever. I'd also installed a 1" extender between the antenna mast and the mounting lug with 3 stainless washers extending the antenna another 3/16ths of an inch. So, apparently, I did some fine tuning on that antenna to get the near perfect match I had before I wrecked the outer insulation.

Since the readings showed the antenna was "long", I took out the spacer and the washers, essentially shortening the thing by 1&3/16". Checked it again and, while it wasn't perfect, it was a lot better and no warning light on the radio. I grabbed another, physically shorter antenna (a 3', the original was a 4') just to see how it looked and it was good enough to use. I noticed I'd modified it at one time or another and had put new shrink tubing on it with a new rubber cap too. But I'd trimmed a lot of coil off the top load area so the end of the fiberglass mast extended 3/4" above the final coil. This made me wonder if the cap I put on the antenna in question was fouling things up. It's thick and 1.5" long and covering 1.5" of coil. With it off, I was still a little high on 40 but perfect on channel 1. Still not perfect, like before, but usable. All I had to do was seal the top instead of using the rubber cap. But since I was experimenting, I took another 4' antenna, and tried it out. It's a RAMI with a lower set of load coils, another load in the center and most up at the top. All the others have all the load coils at the top. The RAMI also has a rubber cap about 1.5" long, covering some of the coils, just like mine and it had a far worse match than any of the others. I was experimenting not only to find a good match, but to get a better understanding of how and why SWR matches work but, there was no rhyme or reason to what I was finding, going longer and shorter changed things, but without any kind of pattern. I'm still confused. I noticed I had a new, in-the-package, 3' firestick on the shelf when I was looking for other antennas to test, so I pulled it down and put it on without the 1" extender or the washers, and Voila!!! perfect match! Almost perfect on 1 and 40 and perfect from 10 to 30. Anyway, I'll be using the shorter firestick.

I know a lot of you experts out there don't like short antennas, but it'll work fine for me. We mostly use our radios on jobsites or on the road where the trucks are within a mile of each other. Another perk is the shorter antenna won't hit the garage door frame next time I pull in there and I won't have to go through this again and everyone else on the forum won't have to read my extra long posts. So see? Everyone is happy now!

One last note about short antennas though. I bought my first radio in the early 70's. It came with a little base load antenna with a skinny 3' stainless rod antenna and it SWR matched good and worked great on my little VW Baja Bug. I took a trip to the Grand Canyon in that little bug and broke down on my way to the North Rim. I got on channel 9 and asked for help but got no reply, so I tried 19 and got a trucker up in Kanab, almost 60 miles away. Now this probably wouldn't happen today, but he stopped at the auto parts store, got what I needed and brought it to me! The point is, that little antenna got out good, 50-60 miles, no skip. Later, someone told me I'd get out better if I mounted a 102" whip in the middle of the roof. That seemed drastic but I made up a custom mount just below the rear window and put that whip there. It worked great but smacked anything and everything I drove around or under. After taking out the fluorescent lights in the drive-through at McDonalds and my bank, I thought I'd go for something shorter and a buddy gave me one of those little 2' fiberglass stick antennas. I installed it and hooked up the SWR meter to see if it'd work. I had a perfect SWR match and while testing I heard some sideband noise down on channel 8. (I'd just installed a SSB Roadtalker from Sears and tuned in on the guy). He was in Fort Lauderdale and was amazed when I told him I was near Park City Utah. When I told him I was not only on a mobile, but only had a 2' whip for an antenna, he simply couldn't believe it. I didn't mention I was on top of a mountain at 8500 feet, which probably helped some, but the point is, even little antennas can get out! And you don't break lights in the drive- troughs showing them off.

So much for another hopelessly long rant. I hope this info might help someone struggling with similar issues.

[The DB]

Likely the shrink tubing you used had a difference amount of RF transparency than the stock shrink tubing. This will have more of an effect on tuning than performance. When it is so close to the antenna to become a part of it, it doesn't take much of a difference to be noticed on a meter.

Another thing that stood out to me is the different cap you mentioned. The part of the antenna that is most sensitive to changes like that is the tip of the antenna. But again, it comes down to the RF Transparency of the item in question compared to the one you replaced.

When it comes to DX, this simply isn't a good measure of how well an antenna works. There is a saying old timers used to say that if conditions are good enough you can shoot skip with a wet noodle, and some of the skip I've worked people would be dumbfounded with the antenna setup I used and where it was.

If you want the best performance possible from your antenna setup, you mount it center of roof then go as long as you can from there. But if your setup meets your needs I won't tell you to change it, even if your only running a 2 foot antenna. There is no point in getting anything better if it already meets your needs.


The DB

[MDYoungblood]

Thank you DB and glad to see you back.

DB always has good comments and offers plenty of knowledge. I’ve never really paid attention to the shrink tube or end caps but have seen a couple (like FireStik) brands do have these in their accessories list.

I usually say use an antenna over 4ft in length or more only because they will give you a little more useable bandwidth and with op’s going above channel 40 or below channel 1 to talk, it gives them more of an SWR curve. If you need to use a shorter antenna, than do so but keep in mine it goes out of tune quickly toward the ends of the band.

3’s

Greg

[Scipio Kid]

Thanks DB,
Your RF transparency reference is the question I inferred, but never asked. In fact, several questions I failed to ask, come to mind; Do the coatings, coverings or skins on antennas tend to interfere with antenna performance? Are the heat shrunk coatings on factory antennas different, and therefore better than typical electronics-store heat-shrink tubing? Do the rubber caps on fiberglass antennas alter the performance of the antenna? Will repairing a fiberglass antenna with additional heat shrink tubing affect its performance?

All will admit and some proclaim that antenna performance is crucial to proper radio function. Many experts claim the antenna is more important than the radio one might choose. I’ve done little with my CB equipment the past few years other than maintain the mobile radios I use in my business. Antenna problems are common and usually solved by attempting to trim or tune them. But if that doesn’t bring quick results, I generally move the antenna, or replace it with another and never really figure out why I had trouble in the first place. My recent experience with a perfectly matched antenna, not working after replacing the heat shrink tubing is a perfect example. The repaired antenna was like-new, installed on the same truck, with the same mounting bracket, the same coax, and the same radio but had a bad SWR reading once it was reinstalled. The final solution was to use a different antenna and when it had a perfect SWR match, the problem was solved. But my more intense involvement in the issues brought me back to this forum and to researching two-way radios, antennas and such, so now I’m getting nutty about it again, as some of you might understand.

So, with that nuttiness, I set out to get a good base station antenna set up high on the roof of my shop, (still a work-in-progress). But also, the nagging question of why slight changes in an antenna, or slight differences in their manufacture make sometimes huge differences in performance. I have a wide variety of antennas and swapping them from vehicle to vehicle has been the easiest way to get excellent results. While that works well for my needs, it also bugs me that I can’t figure out why. So, I decided to set up something where I can quickly and easily exchange antennas and see the differences with all factors being equal, (other than the antennas). I took the antenna that didn’t work on my service truck and installed it on my every-day driver pickup, and it worked flawlessly. I then ran a random length of coax (about 45’) from my soon-to-be radio room, out to the truck to see if there was a difference with the long run of cable from a radio not mounted in, or powered by the truck. There was no noticeable difference, it worked great. Then I began testing the various antennas to see how they performed. The problem was, the truck was parked close to the shop and I had to move it after only testing a few and had to routinely leave it on jobsites. So, while it was a good idea, it wasn’t reliable. It did lead me to the conclusion that, as TNT strongly suggested multiple times, earlier in this thread, coax length has little or nothing to do with excellent performance.

I have an all-steel trailer parked farther from the shop and not in regular use, so I ran another 25’ of coax, installed a short mast on the trailer, with a ground plane antenna mount (something I picked up second-hand years ago) figuring it could be the mini-base station setup I mentioned above where I could easily exchange and test all the antennas I’ve got sitting around. I was pretty excited about it but the setup was a complete failure. Although the base used a PL-259 connector with a 3/8” 24 threaded coupler for the antennas to screw into (standard fare for CB’s), the radials were only 18” long, which led me to conclude it was originally made for higher frequencies. But it also spurred another question I’ve always had; How much ground plane do you need for a good to perfect SWR match? All vehicles are different as are all antenna installations, that should make every installation different and unique, with very few attaining that perfect 1:1 match. Yet, most mobile setups come close, very close in my experience. So, I’ve come to the following conclusion: You can have too little ground plane and that will cause problems, but you can never have too much ground plane … once you get to a certain (as yet undetermined) size, you’ll get a good match and if you go bigger, you’ll still get a good match.

My setup with the 18” radials failed, so I figured the ground plane was too small. To prove or disprove my theory, I grabbed an old magnet-mount base I once used on an air conditioning condenser outside my basement (which worked pretty well) and set it on the trailer right next to the first setup, took the antenna off the first and installed it on the second and got a perfect match on all 40 channels. Now, this brings me back to another nagging question; how do magnet-mounts work with a rubber boot, paint and primer completely blocking any electrical connection to the vehicle? Some say the antenna is grounded through the coax shield. Not so in my case, the antenna is mounted on a trailer with a 75’ run of coax between the radio and trailer with no electrical connection whatsoever. Heck, the radio and trailer don’t even like each other!, but I’ve got a perfect match. I said this was a nagging question. I always get a good match with magnet-mount antennas, always. And I wonder why, when, as mentioned, the gap between the base and the car frame is clearly non-conductive. It’s always bothered me. I simply figured the mount must have some kind of resistor in there that always gives enough resistance to ensure you don’t fry your finals. That makes sense but would also result in greatly diminished performance, which most folks claim is the case with magnet-mounts. Personally, I’ve always gotten good performance from them.

So, I wondered if the same performance and match could be attained using the trailer as the ground plane without the magnet mount. I dug up an old stainless-steel mount with the same antenna hardware as described above and bolted it firmly to the trailer, making sure there was a clean and perfect ground between the mount and the trailer. This led to another, very unexpected problem or issue. I was using a nice Galaxy 979 for all these experiments. It’s been tweaked somewhat and, when dialed to full power, was putting out about 15 watts. I was talking to a fellow in Florida on the magnet mount antenna at full power, so when I keyed up to test the direct-mount, I flipped it over to SWR and got a perfect match on channel 1 and on channel 40. I picked up some skip down on 38 and asked for a radio check and while identifying myself, blew the overload in my power supply! After checking everything, I reset the power supply and checked my output. I was pegging my meter at 25 watts! I’ve seen this kind of power reading with a bad antenna, meaning I’d fry my radio if I kept it up, but I had a perfect SWR match, which should mean the finals were working perfectly, as was my antenna. So why did I get a 10-watt + jump in power? I turned it down to minimum and was still transmitting at 10 watts, whereas before, I believe It’d drop all the way down to 2.

That was all I had time for today. So, I have a lot of questions. Anyone out there with some answers? (I also accept random opinion, mind-boggling reverse-covalent theory, playful banter, and really good jokes.)

[The DB]

I'm not going to do a direct quote here, but to answer the questions in your first paragraph:

Technically yes, but effectively no. They are both considered RF transparent, its just that one is slightly more RF transparent than the other. If it wasn't for being so close to the antenna, even an inch away instead of right up next to it, you would not even notice the difference. Basically, the antenna is infinitely more sensitive to things that are that close to it. Also, the difference in performance is so small that you won't notice the difference so I wouldn't worry about that.

I would agree that the antenna is the most important part of the radio, but antennas are an infinitely complex device to use. Sometimes people are surprised at how well one works or how poorly another works, but the thing is, there is always a reason for said apparent unusually good or bad results, even if you don't know what that reason is.

When it comes to coax and its effects on antenna performance... That is actually a more complex topic than I think lot of people realize. Can it have an effect on your system, yes, will it and how much? Well, that depends as well. The first thing you have to realize is coax is not a perfect conductor, and this, in and of itself, has consequences. Say you have 20 feet of RG-58 between your antenna and your SWR meter/radio. With 100 watts of power put out from the radio, 91 watts of power will reach the antenna. Increase that length to, say 45 feet, and now 81 watts of power will make it to the antenna. But lets say you are running better quality coax than RG-58 for that 45 foot run, with RG-8x you will get 85 watts out, or with RG-213 you will get 89 watts out. All of these cases are assuming a perfect match at the antenna as well. If you don't have a perfect match at the antenna's end all of these power out numbers go down.

And it gets more complex than that. That loss affects your SWR readings. Depending on how much losses are in said coax, your SWR meter may read an SWR of 1.2, but at the antenna the SWR will be higher, perhaps even above 2. What this means is losses in a coax makes high SWR readings look better than they are.

An engineer called M. Walter Maxwell, who used to work for NASA no less, wrote a series of articles that were later compiled into a freely available pdf file called "Another look at Reflections", and even more recently included in a series of books called "Reflections" had two chapters of note for this. Chapter 5, "Low SWR for the wrong reasons" and Chapter 6 "Low SWR for the right reasons".

This is before getting into common mode currents and their effects on SWR and performance, and more. The rabbit hole here just gets deeper and deeper.

When it comes to base antennas, don't get to caught up on radial lengths, just know that certain antennas are designed to use a specific set of radials. To those who think those short radials are completely useless, try taking them off and watch what happens to the antennas SWR match. It won't be pretty. There are some that will tell you that if you have radials they have to be so long, typically about 1/4 wavelength, which for CB are between 8 and 9 feet long. While radials of said length have certain benefits that other size radials do not have, that doesn't mean those are the only useful radials out there. When it comes to antenna with matching circuits, any radial length, up to a point, can effectively be designed around and tuned out. The main thing to remember is the radials themselves only have a minimal effect on the antenna's performance, and on antennas like the a99, which you can get radial kits for, if adding said radials makes the antenna work noticeably better (or worse) then you had a problem with the antenna to begin with.

Yep, radials are another of those rabbit holes that runs deep.

When it comes to the radio that was putting out 15 watts, and the meter was reading 25 watts, that is actually something that happens. One of the things that can cause that is high SWR, and conveniently, a high SWR can blow your radio like that to. Did you check the SWR of that coax and antenna after the radio blew? Sometimes with antennas, things change, and those changes affect the SWR. Also, as noted above, there are wrong reasons for a low SWR and some of them will come back and bite you. Something you might also try doing after tuning an antenna for low SWR is add a three foot jumper between the SWR meter and antenna. If SWR changes noticeably after adding said jumper, you have a problem that may one day, as I said above, bite you.

I didn't get to every point as my time today is limited... But I hope this helps.


The DB

[Scipio Kid]

Thanks DB for sharing your knowledge and experience.
Everything I did was based on SWR readings. Back in the day, I could rely on any one of a hundred contacts or a smaller number of close friends to take a minute or two to help with a thorough radio check. Today, all you typically get is "it's working" or "I've got mine". The last time I tested several antennas and radios, I had a friend about 15 miles away work with me for a couple of hours giving me feedback on signal strength, modulation, clarity etc. I'd do the radio and antenna exchanging and tuning at my end and log the results he was getting at the other end. I can't do that today. So all my results were based on SWR readings at the radio, with 75 feet of RG 58 between it and the antenna. I was getting out on skip a bit, but once I tripped the overload on my power supply, I was about done for the day.

I mentioned the antenna base with the 18" radials. When I said it was a failure, I meant I was pegging my SWR meter to the far right, didn't even keep the mic keyed. But with the same antenna on the magnet mount (using the same coax), it seemed I had a near perfect setup, no movement on the SWR meter, 15 watts output and talking skip easily. With the same setup to the direct mount to the trailer, again the SWR reading was perfect and talking skip was easy (for a few seconds) until I tripped the overload. Once I reset it, I noticed the power jump to 25 watts (it may have been higher, I was pegging my meter at 25). The only thing I tried after seeing that was turning the power as low as it would go and getting down to 10 watts. I still had a perfect SWR match and could skip but only did a very quick radio check and then shut it down for the night.

The 75 foot run of RG 58 was out of convenience. I have a good amount of RG 8U I was planning to experiment with as well.

From what you said, should I be taking power and SWR readings at the antenna as well as the transmitter?

Seems I have a lot to learn. Thanks for explaining so much.