new antenna install having an issue ive never seen

[firefighter_21701]

new to this site not very computer savvy. i just installed a dual whip antenna system on a 2006 freightliner Columbia. antenna's were easy to install and didn't have to tune them once installed. started driving today after installing them last night and did a moving check of the swr on the swr meter on my cobra 29 lx lcd. now not for sure cause its hard to concentrate on the road but it is between 2 and 3 guessing 2.4:1 rough estimate. that is while moving my next break i checked the swr's again and it was below 1:1 maybe 1.5:1. still trying to get used to reading that display. the only reason i checked it while moving was i was getting distortion while moving but a crystal clear sound while sitting in the truck stop. the cb itself is cobra and the antenna is the mix and match crap you can get at just about any truck stop. antennas are fiberglass for now looking at getting something different down the road also they are angled forward so as to keep them off the same plane as my xm radio. mounts are specifically for a frieghtliner/mack mirror mount and wiring consists of 2 9' pieces of i believe rg58 and a t with a double male and a 3' pc of rg58 going into the back of radio. since i cannot check the ground going down the road and it is kinda hard to check directly to the negative terminal from the mirror since my batteries are under my cat walk i tried directly to a bare metal spot i could find and my ground plane seems fine when it comes to a ground plane while not moving. could the vibration of my semi cause a bad ground plane while driving down the road and if so should i mount a ground bracket to the mounts? any advice would be helpfull.

 

[mmckenna]

It could be ground, but I doubt it. You should be able to check that by wiggling things (driver side mirror) while parked. Make sure everything is tight. If things are vibrating so much while driving that it is interrupting the ground path, you likely have bigger issues.

I'd expect more issues from antenna flexing while moving. I've experienced issues with the fiberglass antennas flexing enough that they break the thin wire that's wrapped around the outside. This could be checked by swapping antennas with something new, or a metal antenna. Or, try shaking the antenna while parked and see what happens.

Other thing to try would be to disconnect the antennas at the "T" connector and connect only one directly to the radio. See if one of them is bad and the other is good.

 

[nonperson]

If you want to simplify this use only one of the antennas and point it straight up. You can certainly leave the second antenna up for looks if you want to. And if I understood your post correctly.... if you connected a ground wire to the base of the antenna mount to another part of the truck remove it and check the SWR. Every time I tried doing anything in regards to grounding like that it made things worse. I now know better and why it made it worse. BTW the internal meter are OK in a pinch but are usually unreliable. I suspect it's a lot worse than what it is showing.

I never ran dual antennas on my vehicles but from what I read about them they are a lot of work to get them to function correctly. So if you want to run dual antenna you'll need one antenna connected to the RG 58 (50 Ohms) and the other one connected to a 75 Ohm piece of coax. From their good luck tuning them. Like I said I never tired it myself.

As for the change in the SWR readings, I've heard that is expected a little (moving vs. standing) but I don't think it should be that much if properly setup. I never tried testing it using an external meter while moving.

Try that then get back and let us know?
One more thing there is a lot of incorrect information out there on CB radios and how to set them up. Beware!

 

[sdeeter19555]

Other thing to try would be to disconnect the antennas at the "T" connector and connect only one directly to the radio. See if one of them is bad and the other is good.

Keep in mind the coax for duals (RG-59) is different than that used for a single lead...you can't just disconnect one side. If you were to isolate on antenna at a time, you will need to buy a piece of mini-8 just for that purpose.

I do agree, it sounds like the SWR is changing because of antenna flex...if the antenna is flexing back towards the truck body, that will amplify the issue.

 

[KC4RAF]

+1 with sdeeter19555

post about the coax. It is different than just a simple single antenna hookup.
Also, the SWR WILL change during driving your vehicle. All because the antenna will flex in the wind and causes different distance to the metal of your truck. Usually it isn't of concern. You need to get it set to a good SWR reading while stationary, and then you're good to go.

 

[nonperson]

It could be ground, but I doubt it. You should be able to check that by wiggling things (driver side mirror) while parked. Make sure everything is tight. If things are vibrating so much while driving that it is interrupting the ground path, you likely have bigger issues.

I'd expect more issues from antenna flexing while moving. I've experienced issues with the fiberglass antennas flexing enough that they break the thin wire that's wrapped around the outside. This could be checked by swapping antennas with something new, or a metal antenna. Or, try shaking the antenna while parked and see what happens.

Other thing to try would be to disconnect the antennas at the "T" connector and connect only one directly to the radio. See if one of them is bad and the other is good.

FYI "shaking the antenna," touching the antenna, open doors, hood and the surrounding trucks can be enough to change the SWR reading.

 

[nonperson]

Keep in mind the coax for duals (RG-59) is different than that used for a single lead...you can't just disconnect one side. If you were to isolate on antenna at a time, you will need to buy a piece of mini-8 just for that purpose.

I do agree, it sounds like the SWR is changing because of antenna flex...if the antenna is flexing back towards the truck body, that will amplify the issue.

He stated he was running two pieces of RG 58 coax.

 

[jim202]

Something else to consider is where is the sweet frequency of the antenna? What I ment by this, is check the antenna SWR at the lowest channel and then at the highest channel.

Most antennas are not broad enough to have a very low SWR over the entire band. You end up tuning the antennas to the channel your using the most. In the case of a fiberglass antenna, there isn't much you can do if it has a high SWR on the low channels and much lower on the high channels. That indicates the antenna is too short. If it was the other way around and the low SWR was on the low channels, it would indicate the antenna might be on the long side.

Antenna placement is very SWR dependent. While stopped, the antenna is pointed up. While moving, the antenna will lean back due to the wind bending it. You could try leaning the antenna forward some to try and counter the effect of the antenna bending and changing the SWR.

 

[KC4RAF]

RG-58 isn't the correct coax for duals.

That cable is only 50-52 ohms where dual need 72 ohms, (RG-59/U or RG-59 A/U). Here's a good pro-con site for other information concerning CBs:

Things Every CB'er Should Know -- ©1997 FIRESTIK® ANTENNA

But back to the coax, it HAS to be a 72 ohm harness to operate properly, other wise sooner or later you'll find the finals no longer put out. Yeah, it'll work for a while if you're using RG-58, but later you're gonna pay for it.
Just my two cents worth of knowledge.

 

[nonperson]

That cable is only 50-52 ohms where dual need 72 ohms, (RG-59/U or RG-59 A/U). Here's a good pro-con site for other information concerning CBs:

Things Every CB'er Should Know -- ©1997 FIRESTIK® ANTENNA

But back to the coax, it HAS to be a 72 ohm harness to operate properly, other wise sooner or later you'll find the finals no longer put out. Yeah, it'll work for a while if you're using RG-58, but later you're gonna pay for it.
Just my two cents worth of knowledge.

I read that info on that link before and not all of it is sound advise. For the part about coax lengths for example. If everything is mounted correctly, in good location and tuned than the length of coax doesn't matter. But yes if the antenna system (coax and antenna) are not done correctly then longer lengths of coax can mask the problem. However the radio will still not operate at it's full potential.

Here's is some excellent info on antennas: Antenna Mounts Granted it's geared towards HAM radio but the same applies for all antennas.

 

[sdeeter19555]

He stated he was running two pieces of RG 58 coax.

I missed that in his post, that is causing SWR issues right there...but the rest of it is antenna movement, bad antenna mount/ground, or bad antenna.

He needs to get a co-phased harness to run duals...or replace his coax with RG59.

 

[LtDoc]

There's more to impedance matching than is readily apparent. The part about using 75 ohm coax is correct. You also have to factor in velocity factor (right, that nasty word) for the coax used. The end lengths aren't right unless you do that. This is very common knowledge, nothing extraordinary about it.
- 'Doc

 

[nonperson]

There's more to impedance matching than is readily apparent. The part about using 75 ohm coax is correct. You also have to factor in velocity factor (right, that nasty word) for the coax used. The end lengths aren't right unless you do that. This is very common knowledge, nothing extraordinary about it.
- 'Doc

I figured I would have forgot something. That's why I stopped where I did and said good luck.

 

[mattl3320]

ok first thing to do is to stop transmiting on the radio. why u ask because a mismatch antenna can blow the final in your cb. Then take the coax apart and the t and get rid of them go out to a cb shop and by phase coax it is one cable with to ends for the antenas and one for the radio. next buy a good swr meter and test the radio by holding down on the mic key for few second and check your match u should try for a 1 to 1 or no more them 1 to 3 when transmiting on the radio. if u blow ur cb then u have to buy a new one or get it repair and it could coast alot of money. When i first got into cb i did what ur doing and i burned them both up one was a cobra and the other was a radioshack base. and if u dont think u can do it go to the nearest two-way shop most will install for around 50- 100 dollars. been doing the two way scanner ect for close to 17 years now. ps dont trust the swr meter in the radios. always use a external meter.

 

[LtDoc]

There are a huge number of misconceptions about 'dual antennas', impedance matching, and what those 'dual antennas' can do. All of those misconceptions come about because of a lack of knowledge about antennas in general.
The first mistake with 'tuning' any antenna is not using the right instrument to tell you what's happening. That means that the instrument used has to measure the pertinent characteristics which determine if an antenna is 'tuned' correctly or not. If you can't measure it, you can't tell what's happening. (Sort of like what you are not supposed to do on superman's cape.)
The most common instrument used to 'tune' an antenna is an SWR meter, which is probably the one most useless things you can use since it doesn't know squat about those 'characteristics' that determine if an antenna is tuned or not. They have no 'smarts', the user has to supply that, and very few people have the 'smarts' to know what it's telling them (and what it isn't telling them!).
It ain't simple, too many variables...
- 'Doc

(An I some kind of expert? NO. But i know enough to know that I don't know it all, and learning by making mistakes isn't very cheap or easy. I'm really good about making mistakes!)

 

[prcguy]

Its a big misconception that you will damage the finals in a CB from a mismatched antenna. For the last 30+ years the devices have been rated high enough to withstand this and its very rare to find a bad PA transistor. When you do its usually a manufacturing defect or improper device mounting, etc from the factory.

In my early career I had my hands on literally thousands of CBs on the bench and installed about 1000 in vehicles and big rigs. On many occasions I purposely tried to burn them up by disconnecting the antenna and taping the mic down for the weekend or for an entire week and they did not care a bit.

So, strive to get your antenna working the best you can but in many cases a 2:1 or 3:1 match will have no bearing on how the antenna radiates and if you take precise measurements of radiated signal level from a 2:1 matched antenna and perfect 1:1 match there may be do difference at all. This is somewhat dependent on the antenna type however.

Question for the OP, is the cab on your Freghtliner metal or fiberglass? The fiberglass cabs leave the mirrors and coax as the only counterpoise or ground plane which is not adequate and tuning is difficult and very subject to change with the antenna surroundings like flexing.

Grounding the antennas as in a single wire connected to something metal is not going to do much, the antennas need some substantial metal underneath and it doesn't really matter if that is connected to battery ground, it just needs to be connected to the antenna feed point and spread out.

One trick we used 35yrs ago was a pair of Firesticks with factory 75ohm phasing harness mounted to the mirrors and a second short pair (3ft was the shortest at that time but 2ft would work) inverted and grounded mounted to the same bar of the mirror.

This makes each antenna a dipole, improves matching and relieves some of the problems of inadequate counterpoise or ground plane on a fiberglass cab.
prcguy

That cable is only 50-52 ohms where dual need 72 ohms, (RG-59/U or RG-59 A/U). Here's a good pro-con site for other information concerning CBs:

Things Every CB'er Should Know -- ©1997 FIRESTIK® ANTENNA

But back to the coax, it HAS to be a 72 ohm harness to operate properly, other wise sooner or later you'll find the finals no longer put out. Yeah, it'll work for a while if you're using RG-58, but later you're gonna pay for it.
Just my two cents worth of knowledge.

 

[firefighter_21701]

thank you

i appreciate everyone's replies. everything is being taken into consideration. PRCGUY this is a fiberglass body on this freight shaker. the meter on the cb is the only one i have for know i will be purchasing a good meter this Friday. i was not aware of the ohms needed to run dual whips. when i get home i will be having my local CB shop guy to install new wiring and some metal whips with it granted i don't run into anymore financial issues out here or when i get home. thank you everyone for your responses. very informative and will recommend this site to fellow radio enthusiasts. and anyone who may have questions. im only using this cheap setup cause the truck my company gave me didn't have any antenna's on it and the mounts were missing from inside the mirror. i really needed a CB for shippers and deliveries as i run a curtain side flatbed. again i appreciate everyone's responses.