5.0 swr!!!

[4x4livn]

So my father and i just installed a cobra radio in my truck. Its behind that cab mounted on the bed. The top foot maybe foot and a half are about the cab. The lowest i can get my swr reading to is 5 on channel 40 and 8 on channel 1. Does this mean that my antenna is just too short? Thanks for any help

[231]

Welcome to the forum.

No, it means one or more things is wrong with the installation usually. More often then not it's at the mount where guys just bolt it on not realizing how important that connection really is. It's what makes the vehicle the "other half of the antenna" system, which is needed to make it work right. The antenna itself is really only have of the antenna, your vehicle is the other half. Failure to make a good counterpoise connection usually makes it impossible to tune the antenna...just like you are seeing. I'm just guessing mind you not knowing all the details of your set up. But that's a good place to start.

There are a couple of other possibilities too like a bad antenna or too much side reflection off of the cab. Heck, it could be a combination of all of them (plus more). How do you know your SWR is so high, are you using an analyzer? If so, where is the antenna resonate at? Keep dialing up/down to find the dip point. That'll tells us for sure. If it doesn't dip anywhere, it's a counterpoise problem for sure.

Hope that helps.

[Nagant]

WOWA! That sounds like its grounded. What type of antenna and what sort of mount? What kind of coax are you using and are the ends installed correctly. Have you tested you coax for shorts? What sort of SWR meter are you using?

If you have it mounted on a mirror mount you might be missing the nylon insulator.

['Doc]

Going only by what you've said so far, it sounds like you haven't tried to do any tuning on that antenna yet, so that would probably be a good place to start. If it's a typical antenna, there should be some means of adjusting length. Give that a shot, but don't do any cutting yet! One simple way of seeing if the thing is too short is to just add an inch or two section of wire at the tip, just wrap it around the whip and extend it upwards. (Hopefully it's not a used antenna? No telling what already been done to it if it is.) It really shouldn't take more than an inch or two, one way or the other.

Something that you are not going to want to hear is that the 'head rail' of a pickup bed is just not a very good place to mount an antenna. Antennas like to be above metal, but not beside metal. Being near things, especially metal things, changes an antenna's characteristics and performance. Tends to make then harder to tune (if they are tunable at all). It doesn't matter how well you like it being there, it's a matter of how well that antenna 'likes' being there.

As already suggested, take a good look at the antenna, make sure it's assembled correctly. Not touching things where it's not supposed to, and making good connections where it's supposed to, that sort of thingy. It's certainly not difficult to get something wrong.
Checking continuity isn't as simple as it sounds sometimes. Some types of antennas will and should appear to be 'shorted' with an ohm meter, that's normal for some of them, but not all of them. What appears to be a short at DC (meters use DC) may not be a short at AC at certain frequencies. All RF is either AC or 'pulsing', none is ever DC.
Checking coax is sort of 'un-simple' too. Disconnect it at both ends to check for shorts. Without some sophisticated equipment, that's the only checking you can do with coax.

None of this is meant to discourage you! There are just somethings that aren't as 'easy/simple' as they appear to be. Antennas are just one of those thingys.
Good luck.
- 'Doc

[North Texas Mudduck]

take a picture or 2 of the antenna setup and where its located and post them up

when you say antenna behind the cab
is it touching something

anyway post some pics
its hard to really diagnose a situation

[4x4livn]

Couple of questions
Is RG6 an acceptable type of coax to use for a cobra 25ltd classic? If not what kind of coax should i be running?

Is there any way to tell how long of an antenna i will need? At my current mounting location, about 3''s behind the cab of my pickup (about 2 feet of the 4 feet are about the cab) my swr is very high around 5-10, and the 1 channel is higher than the 40 channel which from what i have read means i need longer antenna. Im thinking i need a 5 ft antenna.

Also there is a 90 bend in my cable right as it goes into the radio, the radio is mounted really in the only spot it can be, could this be causing my high swr?


thanks all,

Marc

[231]

Couple of questions
Is RG6 an acceptable type of coax to use for a cobra 25ltd classic? If not what kind of coax should i be running?

Is there any way to tell how long of an antenna i will need? At my current mounting location, about 3''s behind the cab of my pickup (about 2 feet of the 4 feet are about the cab) my swr is very high around 5-10, and the 1 channel is higher than the 40 channel which from what i have read means i need longer antenna. Im thinking i need a 5 ft antenna.

Also there is a 90 bend in my cable right as it goes into the radio, the radio is mounted really in the only spot it can be, could this be causing my high swr?


thanks all,

Marc

To simplify things you should be running 50 ohm coax (i.e. RG-8x, RG-58U, RG-8U, RG-213, etc.). RG6 is TV cable and is 75ohm. It can work, but certainly complicates things in your set up.

It's possible you have the antenna too close to the cab, and yes a 5' antenna might help. But if you don't have the mount properly grounded it won't matter what antenna you use, your standing wave will be off the chart.

The 90º bend at the radio may or may not be a problem...it's hard to say. The radio's mount location isn't likely the problem for a high SWR. Based on all the information you've posted I'd say it's a mount problem. Close up images would be quite helpful for us all as previously stated.

[4x4livn]

Welcome to the forum.

No, it means one or more things is wrong with the installation usually. More often then not it's at the mount where guys just bolt it on not realizing how important that connection really is. It's what makes the vehicle the "other half of the antenna" system, which is needed to make it work right. The antenna itself is really only have of the antenna, your vehicle is the other half. Failure to make a good counterpoise connection usually makes it impossible to tune the antenna...just like you are seeing. I'm just guessing mind you not knowing all the details of your set up. But that's a good place to start.

There are a couple of other possibilities too like a bad antenna or too much side reflection off of the cab. Heck, it could be a combination of all of them (plus more). How do you know your SWR is so high, are you using an analyzer? If so, where is the antenna resonate at? Keep dialing up/down to find the dip point. That'll tells us for sure. If it doesn't dip anywhere, it's a counterpoise problem for sure.

Hope that helps.

Im using an SWR meter, not sure about the antenna's resonate, how do i find that?

[4x4livn]

WOWA! That sounds like its grounded. What type of antenna and what sort of mount? What kind of coax are you using and are the ends installed correctly. Have you tested you coax for shorts? What sort of SWR meter are you using?

If you have it mounted on a mirror mount you might be missing the nylon insulator.

Its mounted on an aluminum mount and its mounted to the bed of my truck using self tapping screws with a hex nut tip. My father and i figured that would ground out the mount because the metal on metal contact between the metal screws and the metal of the truck bed. Were we wrong in this assumption?

The antenna being used is a Francis 4ft 300 watts tunable cap antenna. There are nylon insulators on both sides of the mount.
SWR meter is a cheapo $12 unit called a Its a vanco swr-3


The coax is screwed into the back of the cb then a ran it through the firewall down the frame and up through the bedside stake holes. Its completely covered in Plastic shield stuff that is used on wiring harnesses. I didnt want to take any chances with it getting cut over time. We sniped off the coax connector on the antenna end and used a connector to attach it to the bolt that goes through the nylon bushings on the mount.

[4x4livn]

Going only by what you've said so far, it sounds like you haven't tried to do any tuning on that antenna yet, so that would probably be a good place to start. If it's a typical antenna, there should be some means of adjusting length. Give that a shot, but don't do any cutting yet! One simple way of seeing if the thing is too short is to just add an inch or two section of wire at the tip, just wrap it around the whip and extend it upwards. (Hopefully it's not a used antenna? No telling what already been done to it if it is.) It really shouldn't take more than an inch or two, one way or the other.

Something that you are not going to want to hear is that the 'head rail' of a pickup bed is just not a very good place to mount an antenna. Antennas like to be above metal, but not beside metal. Being near things, especially metal things, changes an antenna's characteristics and performance. Tends to make then harder to tune (if they are tunable at all). It doesn't matter how well you like it being there, it's a matter of how well that antenna 'likes' being there.

As already suggested, take a good look at the antenna, make sure it's assembled correctly. Not touching things where it's not supposed to, and making good connections where it's supposed to, that sort of thingy. It's certainly not difficult to get something wrong.
Checking continuity isn't as simple as it sounds sometimes. Some types of antennas will and should appear to be 'shorted' with an ohm meter, that's normal for some of them, but not all of them. What appears to be a short at DC (meters use DC) may not be a short at AC at certain frequencies. All RF is either AC or 'pulsing', none is ever DC.
Checking coax is sort of 'un-simple' too. Disconnect it at both ends to check for shorts. Without some sophisticated equipment, that's the only checking you can do with coax.

None of this is meant to discourage you! There are just somethings that aren't as 'easy/simple' as they appear to be. Antennas are just one of those thingys.
Good luck.
- 'Doc

My father is a 32 year man at at&t and he is very very competent when it comes to electrical, we have never really mounted a cb before though. He has checked the continuity and just check and made sure it was grounded.

The antenna has not been cut it is brand new out of the box, i also bolted on my old fiberglass one that i used on my jeep and the swr was still very high


He does want to know if the outer shield of the coax is supposed to be grounded because it is not???

[4x4livn]

Couple of questions
Is RG6 an acceptable type of coax to use for a cobra 25ltd classic? If not what kind of coax should i be running?

Is there any way to tell how long of an antenna i will need? At my current mounting location, about 3''s behind the cab of my pickup (about 2 feet of the 4 feet are about the cab) my swr is very high around 5-10, and the 1 channel is higher than the 40 channel which from what i have read means i need longer antenna. Im thinking i need a 5 ft antenna.

Also there is a 90 bend in my cable right as it goes into the radio, the radio is mounted really in the only spot it can be, could this be causing my high swr?


thanks all,

Marc

To simplify things you should be running 50 ohm coax (i.e. RG-8x, RG-58U, RG-8U, RG-213, etc.). RG6 is TV cable and is 75ohm. It can work, but certainly complicates things in your set up.

It's possible you have the antenna too close to the cab, and yes a 5' antenna might help. But if you don't have the mount properly grounded it won't matter what antenna you use, your standing wave will be off the chart.

The 90º bend at the radio may or may not be a problem...it's hard to say. The radio's mount location isn't likely the problem for a high SWR. Based on all the information you've posted I'd say it's a mount problem. Close up images would be quite helpful for us all as previously stated.

Im not sure what kind of coax im running, its whatever they had in the cb section at frys electronics.

Ill take pictures and get them up here in a minute

[4x4livn]

here are the pictures

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[Red Warrior]

The shield of the coax should show a direct short to ground. If you remove the coax from the back of the radio and measure between the center of the connector and the shield of the connector, you should read an open circuit.

[4x4livn]

Sorry guys i cant figure out how to edit a post on the forum, but the mount is on bed rail closest to the cab of the truck, its a 2008 dodge crew cab long bed. the mount is on the driver side. The window you can see in the picture, it has a sticker on it, is rear window glass.

[BushHog]

you get the option to edit when you get 100 post.

[Sporty Mike]

One problem I see is that spray in bedliner, if your bolted to that, then you ARE NOT bolted to bare metal and therefore there's no way you can have a good ground to the body of the truck. And whats up with how that coax is connected to the antennae?? I've never seen a connection like that before!


Mike

[4x4livn]

My father tested it with his meters and it has a good ground, there is metal underneath the liner and the metal to metal contact between the bed and the 4 screws appears to be enough.

[4x4livn]

One problem I see is that spray in bedliner, if your bolted to that, then you ARE NOT bolted to bare metal and therefore there's no way you can have a good ground to the body of the truck. And whats up with how that coax is connected to the antennae?? I've never seen a connection like that before!


Mike

Do you have any pictures as to how coax is supposed to be connected to the antenna? Thats the only way we could figure to do it if you have another method im all ears.

[231]

I see 2 problems. First off, if you are using self tapping screws, good! You were not wrong in assuming that's what needed to happen. However, take an ohm meter between the mount and bed of the truck and make sure you have continuity. What Mikey says is true, that the bed liner might be part of the problem and acting as an insulator some...especially if the self tappers aren't making good metal to metal contact. The ring connector on the coax is no problem. That's all I use and never have a problem. Some guys get stuck on using UHF (PL-259's) and I rarely use them on my antenna connection points. In my opinion that's okay.

Now, the two problems I see;
1.) The shielding of the coax needs to connect to the mount. It would appear to me that the shielding isn't hooked up at all in the images? This is likely the biggest problem if that's the case. With the coax the center lead goes to the antenna (which is insulated from ground) and the shielding goes to ground.

[ external image ]

2.) The Francis antenna is NOT top loaded, but linear loaded. What that means is yes, without a question you have it too close to the cab. It must be above the metal and well away from any side reflection whatsoever.

So, even if you get the shielding squared away, that antenna will never tune in the location it's at, and there is no way to tune that antenna. So your choices are simple if you want to use that antenna;
a.) Relocate the antenna to a much more open spot like the hood or further back on the bed
b.) Get a different antenna

You guys did a very nice installation job I might add. Everything looks very clean and well done. Just a couple of details is all.

Hope that helps.

['Doc]

The braid of the coax should be connected to the metal body of the vehicle at the mount. The center conductor of the coax is connected to the whip as it appears to be in the pictures. Don't worry about that spray-in bed liner if the screws are going through the metal of the vehicle. That spray-in bed liner isn't going to affect anything, it's 'invisible' to RF. I've had that done to every pickup I've owned in the last 10 years except for one. Doesn't make any difference to the radio or antenna.
Resonance means getting rid of, or neutralizing any reactances in the antenna. You can't check for resonance with an SWR meter, they will only tell you about the differences in impedance. (If your dad is with AT&T, then he probably deals with 600 ohm impedances fairly regularly. Radio typically uses a 50 ohm impedance antenna system (feed line and antenna input impedance). It's the same ball-game, just a different 'park', so to speak.)
That RG-6 isn't doing you any good, 50 ohm coax works much better, meaning it's easier to deal with. I would suggest changing to RG-58, RG-8, RG-8X, or any other 50 ohm coax you have available. I'd also suggest sticking with a 'name brand' of coax cable, Belden, International, Times Microwave, etc. Might be a tad more expensive, but well worth it.
An antenna 3" or so from the cab of the truck isn't the best place to put one, but it can be made to work. Sometimes that's the only place you have to put one, so make the best of it.
Bending coax isn't a big deal until you bend it too much. If you have to really 'force' it to make a bend, it's probably too sharp of a bend, maybe. If it deforms the coax it changes its impedance, not good.
Good luck.
- 'Doc

[4x4livn]

[ external image ]

2.) The Francis antenna is NOT top loaded, but linear loaded. What that means is yes, without a question you have it too close to the cab. It must be above the metal and well away from any side reflection whatsoever.


a.) Relocate the antenna to a much more open spot like the hood or further back on the bed
b.) Get a different antenna

You guys did a very nice installation job I might add. Everything looks very clean and well done. Just a couple of details is all.

Hope that helps.

For now im trying to keep the antenna where it is, what kind of antenna should i get? I think i should get a 5ft one just to make even more of it stick above the cab.

thanks for the response!

Marc

[muddy_udders]

I would start by getting 50 ohm coax and the shield of the coax grounded to the mount I believe you will see a dramatic improvement in swr then go from there.

I have played the buy a more expensive antenna thing and I have a few that are no better than my cheap Firestik, this is due to the location I have mine mounted at.

[SweetPee]

I agree, fix the coax. If you are still in the market for an antenna then... There are more than enough choices. And if I may, I recommend ANY of the Everhardt antennas. Which you can find at CB World a sponsor of this site. They are FABULOUS! If you buy a new one, and get one of those, you WILL NOT be sorry. It won't solve any installation problems, like a ungrounded coax shield, but once everything else is fixed it will give you all the performance you can ask for.

[4x4livn]

I would start by getting 50 ohm coax and the shield of the coax grounded to the mount I believe you will see a dramatic improvement in swr then go from there.

I have played the buy a more expensive antenna thing and I have a few that are no better than my cheap Firestik, this is due to the location I have mine mounted at.

i dont have rg 6 coax, i was asking about it because i had some laying around, im not sure what coax i used it just the stuff that frys had on their shelf.

[4x4livn]

I agree, fix the coax. If you are still in the market for an antenna then... There are more than enough choices. And if I may, I recommend ANY of the Everhardt antennas. Which you can find at CB World a sponsor of this site. They are FABULOUS! If you buy a new one, and get one of those, you WILL NOT be sorry. It won't solve any installation problems, like a ungrounded coax shield, but once everything else is fixed it will give you all the performance you can ask for.

Cool i will look into them.

Can you look on my pictures to make sure my outer coax is grounded well enough. I have a little clamp with a bolt going through it which essentially connects the outer coax to the mount, the mount is grounded to the truck.