Another CB radio SWR issue

[voodooridr]

The tip of my 3ft FireFly on the TeraFlex bracket with a Breedlove quick disconnect sits about 8in above the roofline, is that enough clearance for a top loaded antenna?
I plan on swapping to a 4ft if we're going somewhere but the 3ft fits under the garage door better for day to day living

 

[Dino - KX6D]

To reinforce what @Dino - KX6D is saying, SWR is important, but its not the only consideration.

Its important so you don't burn up your radio. It does not increase antenna performance. The more power you can get into the antenna, the more signal will leave it.

Partially. Antennas like to be a sub-multiple of the wave length. CB is a fairly long wavelength, so larger antennas just work better. The famed 108" 1/4 wave whip is the gold standard of practical CB antennae. Dino can probably explain it better than I can...

You explained it well.

The tip of my 3ft FireFly on the TeraFlex bracket with a Breedlove quick disconnect sits about 8in above the roofline, is that enough clearance for a top loaded antenna?
I plan on swapping to a 4ft if we're going somewhere but the 3ft fits under the garage door better for day to day living

Everything is a compromise. Is it enough? Sure. Can you do better? Sure! Same for 4' and 5'.

I installed a Predator 10K antenna with the 27" shaft so the coil would be above the hard top. That antenna worked great! The Jeeps in front of me said I was too loud and overloading their front end if we were too close. This was with a stock PC-68. I installed a 5' Francis Amazer and I didn't have any more complaints. When that antenna snapped I installed the 5' Firestik. Its equally as weak as the Francis. However, both of those antennas are way more practical than the Predator.

 

[Zorba]

A top loaded antenna like a Firestik moves the current up the antenna. If that part of the antenna is clear of obstacles than more of the signal will get out. So a taller antenna in this case is better. Back in the day when you had a Jeep with a 9' antenna on it they would get out great. I recommend a longer antenna every time over a shorter one. They usually tune in better as well.

Exactly. Which is why I'm perplexed by my 5' FireStik not tuning anywhere near as well as a cheapie 2 footer! Not to be confused with the 2 footer getting out better...

My late father would only run the 108" stainless whips. They sure worked well, but were a PITA to deal with at the same time.

 

[Dino - KX6D]

Exactly. Which is why I'm perplexed by my 5' FireStik not tuning anywhere near as well as a cheapie 2 footer! Not to be confused with the 2 footer getting out better...

My late father would only run the 108" stainless whips. They sure worked well, but were a PITA to deal with at the same time.

Every antenna install is different. What worked on one guys rig may not work on another for multiple reasons. The secret is understanding the differences and making adjustments. Swapping out antennas, changing coax length, checking grounds and bonds, etc. Roll the dice and find out what works. My favorite antenna to date is the 5.5' Francis. It tuned in perfect and was flexible. I'll bet we could get your 5' Firestik to work just fine, but if the 2' works, go with it!

I liked the old videos of those flatties with their big antennas whipping around. But I heard that many organized events actually banned long antennas many years ago because people were getting hurt. It was before my time.

 

[Zorba]

Every antenna install is different. What worked on one guys rig may not work on another for multiple reasons. The secret is understanding the differences and making adjustments. Swapping out antennas, changing coax length, checking grounds and bonds, etc. Roll the dice and find out what works. My favorite antenna to date is the 5.5' Francis. It tuned in perfect and was flexible. I'll bet we could get your 5' Firestik to work just fine, but if the 2' works, go with it!

I liked the old videos of those flatties with their big antennas whipping around. But I heard that many organized events actually banned long antennas many years ago because people were getting hurt. It was before my time.

I'll disagree with you on coax length - yes, you can lower SWR under certain circumstances by adjusting same, but to my understanding its hiding other problems that, frankly, are waaay over my head. I didn't understand the explanation - "velocity factor". But yea, I've always considered RF to be 80% voodoo and 20% black magic! I got the 2' to swap on for trail use, but want a longer antenna for the open road. I'm thinking of trying a 4' or - since you say they make them - a 5' ProComm. Less windage, maybe it won't flop around like the FireStik does. *shrug* I actually had to use a LONGER bolt in the tip of the FireStik to get the best SWR I could. I *do* need to revisit it though. Hell, maybe your Francis would work better - what's its advantage?

 

[Dino - KX6D]

I'll disagree with you on coax length - yes, you can lower SWR under certain circumstances by adjusting same, but to my understanding its hiding other problems that, frankly, are waaay over my head. I didn't understand the explanation - "velocity factor". But yea, I've always considered RF to be 80% voodoo and 20% black magic! I got the 2' to swap on for trail use, but want a longer antenna for the open road. I'm thinking of trying a 4' or - since you say they make them - a 5' ProComm. Less windage, maybe it won't flop around like the FireStik does. *shrug* I actually had to use a LONGER bolt in the tip of the FireStik to get the best SWR I could. I *do* need to revisit it though. Hell, maybe your Francis would work better - what's its advantage?

Its because you don't understand what the coax is actually doing in the circuit. IF...you had a perfect match at the feedpoint of the antenna, coax length wouldn't matter. IF...however you do not, coax becomes a transformer in a tuned circuit and will correct at the radio, the mismatch at the antenna feedpoint. Its not correcting the mismatch at all, its "hiding" it as you said. Setting SWR is more about protecting the radio finals than performance. Yes you want both, but you don't always get it.

A Jeep is a horrible radio platform. No metal roof and a roll bar in close proximity of every place to mount an antenna. So you compromise! You want an antenna to be RESONANT on the frequency of choice. This is when that antenna is most efficient. But sometimes a resonant antenna causes reflections at the feedpoint so we adjust the antenna to fix the feedpoint problem. This adjustment can now make the antenna too long or too short which is no longer resonant on the desired frequency and efficiency goes out the window. You have a good SWR, but the antenna is now inefficient. Another approach is to get the antenna resonant so its most efficient and fix the actual problem at the feedpoint. You can do this by changing coax length. Both are compromises.

Velocity Factor is easy. RF traveling through space travels at the speed of light or 186,000 miles per second. However, RF traveling through an insulated wire is slower. How much slower? It depends on the design of the wire/cable. A velocity factor of 80% means the RF is traveling at 80% the speed of light. the better the cable design, the higher the velocity factor.

If you have power lines in your back yard you will notice that the high voltage lines at the top of the pole are uninsulated. So are the transmission lines running across America. This is for many reasons, one of them being velocity factor. If those lines were insulated, they would be heavier, more expensive, and slow down the flow of electricity causing resistance, heat, and therefore a voltage drop. So its bare wire in open space to be as efficient as possible. This is also why the voltage is so high (voltage increases, current decreases).

I don't know that there is any reasonable advantage to the Francis. I would expect the Firestik with its larger wire to be the better radiator. I like the Francis because it worked without being this big heavy unflexible monster banging into trees and tweaking the mount and tub. My Penetrator worked the best!

 

[Zorba]

Velocity Factor is easy. RF traveling through space travels at the speed of light or 186,000 miles per second. However, RF traveling through an insulated wire is slower. How much slower? It depends on the design of the wire/cable. A velocity factor of 80% means the RF is traveling at 80% the speed of light. the better the cable design, the higher the velocity factor.

Ok, that's understandable. Do you know what this has to do - if anything - with SWR and/or "apparent SWR"? 'Cause I sure don't! I'll go out on a limb and make a SWAG that it has to do with phase angles - voltage vs current - due to reactance...

That's QUITE the monster antenna on the back of that Jeep!

 

[Jerry Bransford]

SWR affects performance for the types of antennas we use on our Jeeps. The further away from a 1:1 SWR for the types of antennas we use, the more the antenna is reflecting the transmitted signal back toward the transmitter instead of outward from the antenna where it can do some good.

A terribly bad SWR on an antenna installed onto a Jeep that cannot be tuned to an acceptably low SWR is often caused by the antenna mount. Either the antenna mount has been misassembled with the insulating shoulder washer either missing or installed on the wrong side of the mount, or the mount does not have a suitable RF ground. An RF ground is far more demanding of grounding quality than a simple ground required for something like a light bulb.

#1 the mount must be well grounded to the tub. If it's not a sky-high SWR can be the result. A simple ground wire is inadequate. If you can't grind enough paint or powder coating off to give the mount an adequate surface area of bare metal to bare metal connection, then use braided ground wire between the mount and a suitable bare metal grounding point. The bigger the surface area of the connections the better.

This mount is my preferred mount. It works well if assembled and installed properly.

This first illustration shows what is grounded (green) and what is signal related (red).

This next one shows two key points.... what must be done if your mounting bracket is powder coated which acts as an insulator. You must grind a bare circle through the powder coating UNDERNEATH the mounting bracket everywhere the "SO-239 Coax Connector" touches it. The mounting bracket must have a solid bare metal connection to the tub or, less desirable, it must be grounded to a bare spot on the tub via a braided ground strap.

Then note the position of the "Insulated Shoulder Washer" below. It must be as shown and it must be centered in its mounting hole. This serves to insulate the antenna from ground and if missing or improperly positioned will cause the antenna to have a dead short to ground. This is super critical and I've probably fixed at least a half-dozen Jeep antenna problems by nothing more than replacing that insulating washer or moving it to the correct position.

Getting a good low SWR is important and worth the effort to try to obtain. It's not black magic but it does require the basics be paid attention to... like the above. If your SWR is under 2.0:1 that is ok.

A time-saver I use is to only check and adjust the SWR at the mid-point of the CB band, channel 20. If it's as low as possible on channel 20, it's as low as it's going to be on Channels 1 and 40 too. Some people check and adjust on Channels 1 and 40 but I consider that an extra step that produces no benefit to just doing it all one time on channel 20.

 

[Dino - KX6D]

SWR does affect performance for the types of antennas we use on our Jeeps. The further away from a 1:1 SWR for the types of antennas we use, the more the antenna is reflecting the transmitted signal back toward the transmitter instead of outward from the antenna where it can do some good.

The antenna isn't the problem, its the feedpoint doing the reflecting, sending power back to the final amplifier and causing the amplifier to fold back on itself reducing the power output. The antenna is adjusted to minimize the losses in the feedpoint. By adjusting the antenna we affect its resonance which may or may not change its efficiency and performance. Basically we're using the antenna to fix a problem elsewhere because its the easiest. At the end of the day I want X=0 on channel 20 and a properly set up coax and mount.

A terribly bad SWR on an antenna installed onto a Jeep that cannot be tuned to an acceptably low SWR is often caused by the antenna mount. Either the antenna mount has been misassembled with the insulating shoulder washer either missing or installed on the wrong side of the mount, or the mount does not have a suitable RF ground. An RF ground is far more demanding of grounding quality than a simple ground required for something like a light bulb.

Ding ding ding!!!

A time-saver I use is to only check and adjust the SWR at the mid-point of the CB band, channel 20. If it's as low as possible on channel 20, it's as low as it's going to be on Channels 1 and 40 too. Some people check and adjust on Channels 1 and 40 but I consider that an extra step that produces no benefit to just doing it all one time on channel 20.

I chucked all the mount info. I don't care for that mount but it works. Breedlove makes great mounts. I'm running a Hi-Q quick disconnect. I use an analyzer to adjust the antennas. It gives me lots of info so I can make an educated decision on what needs to be tweaked. It also allows me to sweep the band to see what's happening on other frequencies as well as "double dips" when something really weird is going on. Lots of ways to skin this cat for sure!

 

[Jerry Bransford]

The antenna isn't the problem, its the feedpoint doing the reflecting, sending power back to the final amplifier and causing the amplifier to fold back on itself reducing the power output. The antenna is adjusted to minimize the losses in the feedpoint. By adjusting the antenna we affect its resonance which may or may not change its efficiency and performance. Basically we're using the antenna to fix a problem elsewhere because its the easiest. At the end of the day I want X=0 on channel 20 and a properly set up coax and mount.

Dino we both have solid backgrounds in RF communications, dragging the thread down any deeper into esoteric theory discussions is not needed. I have focused my comments on the antenna itself since that is how the SWR is adjusted on Jeep appropriate antennas if everything else is as it should be. Changing the cable length, as you know, is not how we adjust the SWR either on the type of antennas most commonly used on our TJs either so that leaves tuning the antenna by whatever means is appropriate to its design. Cutting a wire protruding from its tip, turning its adjustment screw, etc.

I try to keep such discussions at levels appropriate to the OP here, he doesn't need to know about feedline theory, etc. when he's just looking for how to fix a high SWR.

 

[D M]

Dino we both have solid backgrounds in RF communications, dragging the thread down any deeper into esoteric theory discussions is not needed. I have focused my comments on the antenna itself since that is how the SWR is adjusted on Jeep appropriate antennas if everything else is as it should be. Changing the cable length, as you know, is not how we adjust the SWR either on the type of antennas most commonly used on our TJs either so that leaves tuning the antenna by whatever means is appropriate to its design. Cutting a wire protruding from its tip, turning its adjustment screw, etc.

I try to keep such discussions at levels appropriate to the OP here, he doesn't need to know about feedline theory, etc. when he's just looking for how to fix a high SWR.

I dont NEED to, but I want to I dont mind the derailing at all, if anything im glad its happening because its answering my future questions. I want to learn more

 

[Dino - KX6D]

Knowledge is power. Lots of misinformation and guessing on the various threads here and I aim to share what I know so others will know too. When it comes to antennas, I'm a myth buster. I've spent decades studying antennas and SWR and reading books like Reflections by Walter Maxwell. He was able to answer questions nobody else could. I've ripped antennas apart just to see how they are made. I've made mistakes and learned from them. I still have many mistakes to make. I'll take the time to explain a difficult subject in a way people can understand so they will learn too. Its amazing to see the lights come on when they actually start to understand how it all works.

 

[Deputy Dodge]

I always thought SWR affects performance though, for example if you have high SWR, doesnt that prevent the unit from transmitting far since the power being reflected could be used to transmit? Or am I mistaken

High swr does affect performance but not exactly through power output. If your radio put out 4 watts and 2 watts is reflected back to the radio, those 2 watts will eventually be transmitted. But first the signal will travel back down the coax to the radio, then go back to the antenna. Now you have the same signal being transmitted after a delay and can have an 'echo' effect, or poor audio. Think of it like making waves in a swimming pool. The wave will bounce off the side of the pool and come back to you. If you start a new wave out of synch with the old one then they crash into each other. Same power, different directions.

 

[Zorba]

High swr does affect performance but not exactly through power output. If your radio put out 4 watts and 2 watts is reflected back to the radio, those 2 watts will eventually be transmitted. But first the signal will travel back down the coax to the radio, then go back to the antenna. Now you have the same signal being transmitted after a delay and can have an 'echo' effect, or poor audio. Think of it like making waves in a swimming pool. The wave will bounce off the side of the pool and come back to you. If you start a new wave out of synch with the old one then they crash into each other. Same power, different directions.

Not necessarily. Some of that energy is dissipated inside the radio - just exactly where you don't want it to be. That's why high SWR can blow the finals.

 

[Jerry Bransford]

Not necessarily. Some of that energy is dissipated inside the radio - just exactly where you don't want it to be. That's why high SWR can blow the finals.

Exactly. That's why during the CB craze of the 70's there were so many blown CBs sent back to the manufacturers for warranty replacements. It was out of control for a while so far as CB manufacturers like Cobra were concerned. Few CB users then (mostly hippies and mullet wearers) knew anything about tuning CB antennas or even installing them properly which usually ended up blowing the CB's final power amplifier circuit... all caused by excessively high SWRs.

That wised the CB radio manufacturers up to begin incorporating safety circuits into the transmitter section to start reducing the power output when a high SWR is detected. The higher the SWR the more it reduces the transmitter's output power. That safety circuit brought their warranty repairs way down to a far more manageable level. Ham radios don't usually have such circuits in their transmitters since the average ham is more astute about how to properly tune and install their antennas.

 

[SSTJ]

SWR affects performance for the types of antennas we use on our Jeeps. The further away from a 1:1 SWR for the types of antennas we use, the more the antenna is reflecting the transmitted signal back toward the transmitter instead of outward from the antenna where it can do some good.

A terribly bad SWR on an antenna installed onto a Jeep that cannot be tuned to an acceptably low SWR is often caused by the antenna mount. Either the antenna mount has been misassembled with the insulating shoulder washer either missing or installed on the wrong side of the mount, or the mount does not have a suitable RF ground. An RF ground is far more demanding of grounding quality than a simple ground required for something like a light bulb.

#1 the mount must be well grounded to the tub. If it's not a sky-high SWR can be the result. A simple ground wire is inadequate. If you can't grind enough paint or powder coating off to give the mount an adequate surface area of bare metal to bare metal connection, then use braided ground wire between the mount and a suitable bare metal grounding point. The bigger the surface area of the connections the better.

This mount is my preferred mount. It works well if assembled and installed properly.

This first illustration shows what is grounded (green) and what is signal related (red).
View attachment 245112
This next one shows two key points.... what must be done if your mounting bracket is powder coated which acts as an insulator. You must grind a bare circle through the powder coating UNDERNEATH the mounting bracket everywhere the "SO-239 Coax Connector" touches it. The mounting bracket must have a solid bare metal connection to the tub or, less desirable, it must be grounded to a bare spot on the tub via a braided ground strap.

Then note the position of the "Insulated Shoulder Washer" below. It must be as shown and it must be centered in its mounting hole. This serves to insulate the antenna from ground and if missing or improperly positioned will cause the antenna to have a dead short to ground. This is super critical and I've probably fixed at least a half-dozen Jeep antenna problems by nothing more than replacing that insulating washer or moving it to the correct position.

View attachment 245113
Getting a good low SWR is important and worth the effort to try to obtain. It's not black magic but it does require the basics be paid attention to... like the above. If your SWR is under 2.0:1 that is ok.

A time-saver I use is to only check and adjust the SWR at the mid-point of the CB band, channel 20. If it's as low as possible on channel 20, it's as low as it's going to be on Channels 1 and 40 too. Some people check and adjust on Channels 1 and 40 but I consider that an extra step that produces no benefit to just doing it all one time on channel 20.

@Jerry Bransford , I've seen you explain this in a few threads now, and this one with both pictures is especially helpful. Can we make this a 'resource' sticky, since it seems to come up so often? It was very helpful to me.

 

[SSTJ]

Hey everyone, trying to get my SWR dialed for the CB radio, Uniden Pro 520xl, 10ft male to male coax cable, 2' firestik on the teraflex taillight mount. Cannot get my SWR below 3, its around 3.7 ish no matter what I do. I used a dremel to get a clean ground for where the extra mounting bolt goes, but that solved nothing. There are no kinks in my cable, and my uniden is brand new. Could my SWR be high due to it being so close to my GMRS antenna? Heres a pic
View attachment 243793

@DylanM345 , I'm eager to hear if you get better results with a longer antenna. I have the same setup, and posted about similar SWR levels here. My next step is to try a longer antenna and compare results.

 

[Jerry Bransford]

@DylanM345 , I'm eager to hear if you get better results with a longer antenna. I have the same setup, and posted about similar SWR levels here. My next step is to try a longer antenna and compare results.

My 2' Firestik FS-2 gives a nice 1.2:1 SWR at mid-range ch 20 where I tuned it.

 

[SSTJ]

My 2' Firestik FS-2 gives a nice 1.2:1 SWR at mid-range ch 20 where I tuned it.

Aha. Well, that's encouraging in some ways, because it means it's possible. I was kind of hoping that I just needed a longer antenna.

Will keep tinkering. My next idea is that my tub might not be well-grounded to my chassis.

 

[Jerry Bransford]

Aha. Well, that's encouraging in some ways, because it means it's possible. I was kind of hoping that I just needed a longer antenna.

Will keep tinkering. My next idea is that my tub might not be well-grounded to my chassis.

The tub should be very well grounded to the chassis. How is your antenna mount attached to your tub? Posting a couple clear well-lit (use a flash) photos would be very helpful.