CB radio help

[RayH]

I just installed a CB in my vehicle. I had a CB in it till last year I had to remove it to put a brake controller in. I also have CBs in other vehicles so Im not brand new to installing them.
Its a basic system, Ive only got $80 or so in it so I dont expect alot. Its just a little Cobra 19 hooked to a bottom loaded steel whip. The radio receives great for what it is. I can clearly pick up signals from mobile radios 5+miles away through the mountains.
Heres the problem. It isnt transmitting past line of site. I checked SWR and made adjustments. My SWR is excellent at less than 1.5 across the freq band. The built in meter is showing full power when transmitting. I did get a radio check from someone on a nice base unit that was about a mile away. He said he showed that I had good power but he could barely make out what I was saying like it was garbled and fuzzy. He suggested that my mic was bad. This is a brand new Cobra, right out of the box. I had another guy tell me I need to increase my modulation.?????? How do I do that without having someone jack up the radio for me? I dont want or need to put alot of money in this but I should be doing better than line of site. Any ideas? I dont have another mic I can try with this radio.

 

[bill177]

From what you describe, it will turn out to be a mic problem and nothing else. All carrier and no modulation or weak audio is just another way of saying the mic or mic amp circuit aren't doing what they are supposed to. Unlikely it is an adjustment, as most of the current batch of CB have very little that is adjustable inside of them.

Try a different mic. It must be from the same make and maybe model or the wiring may be different.

 

[RayH]

I guess I can visit work tomorrow or Monday. They use all Cobras in the trucks. Different models but I think the mics are the same. I'll call Cobra Monday also and see what they say and if I can get them to just replace the mic so I dont have to remove the radio and rewire it.

 

[JoeR]

Do you have access to a watt meter? Does your SWR meter show forward power? You should show 3 watts or so forward power. Although, your problem does sound like the microphone. Does the garbled audio happen when the vehicle is not running? Maybe alternator whine getting into audio circuit, or possible bad ground.

Joe

 

[RayH]

Do you have access to a watt meter? Does your SWR meter show forward power? You should show 3 watts or so forward power. Although, your problem does sound like the microphone. Does the garbled audio happen when the vehicle is not running? Maybe alternator whine getting into audio circuit, or possible bad ground.

Joe

My SWR meter does have a watt meter but I need a special fitting or something for it to work, I forget what it is, Ive never used it for that.
It happens with engine and all vehicle power off, as well as with engine running.
I know the radio has good ground because it has power. I did double check the ant ground and its fine. Wouldnt a bad ant ground show up as a high SWR?

 

[George2615]

Check your voltage (across red and black leads) when the mike is keyed and unkeyed. It should not drop below 11.5 volts with the engine off. Does the radio lights dim down when you key it up? If so you may not have a good ground. The vehicle should be running for optimum voltage supply to the radio. The radio will light up without the black lead even hooked up if the antenna is connected. Unhook the antenna from the radio in receive and see if it stays on. The radio needs a good circuit board ground which is the black lead out of the radio. The antenna ground is through the vehicle chassis. Did you wire it into your fuse box or directly to the battery? Your watt meter should have a switch or two to read forward and reflected power and shouldn't need a special fitting. Are you sure your checking the SWR properly. Another problem may be RF from the antenna getting into the mic.
My guess is going to be a poor ground but a defective new radio isn't out of the question.

 

[RayH]

Check your voltage (across red and black leads) when the mike is keyed and unkeyed. It should not drop below 11.5 volts with the engine off. Does the radio lights dim down when you key it up? If so you may not have a good ground. The vehicle should be running for optimum voltage supply to the radio. The radio will light up without the black lead even hooked up if the antenna is connected. Unhook the antenna from the radio in receive and see if it stays on. The radio needs a good circuit board ground which is the black lead out of the radio. The antenna ground is through the vehicle chassis. Did you wire it into your fuse box or directly to the battery? Your watt meter should have a switch or two to read forward and reflected power and shouldn't need a special fitting. Are you sure your checking the SWR properly. Another problem may be RF from the antenna getting into the mic.
My guess is going to be a poor ground but a defective new radio isn't out of the question.

Ok, that gives me some things to check. Actually the radio does dim ever so slightly when the mic is keyed so you may be onto something. I did check the ant ground and its good. Im positive I checked the SWR correctly, Ive done that a few times before on other radios and I double checked on this one because I am having issues with it. I have it wired through the fuse box. I had a CB installed perviously and removed it this summer, This new one is wired in where the old one was, I just used crimped connectors to splice the new radio in. My last radio was a cheap RadioShack brand and the antenna sucked so it had high SWR but still got out much better than this new one.
How and what would cause the RF from the ant to feed back to the mic? Wouldnt that only happen if the SWR is off? My SWR is actually very good, below 1.5 on all freq. My antenna is mounted on the rear drivers side corner of the hood, so it is in close proximity to the mic and radio with only about 3' of air and a windshield between. (I know, the hood isnt the best place but I carry a canoe and kayaks on the roof, loading and unloading them really limits roof mounting, at least I get some ground plane from the hood). I could probably do alot better with a center or top loaded ant. I should still get out further than line of site with the setup I have.
I'll go back and check the ground and volts tomorrow. I'll also refresh myself on the SWR meter instructions to see what I need to check watts.

 

[RayH]

Check your voltage (across red and black leads) when the mike is keyed and unkeyed. It should not drop below 11.5 volts with the engine off. Does the radio lights dim down when you key it up? If so you may not have a good ground. The vehicle should be running for optimum voltage supply to the radio. The radio will light up without the black lead even hooked up if the antenna is connected. Unhook the antenna from the radio in receive and see if it stays on. The radio needs a good circuit board ground which is the black lead out of the radio. The antenna ground is through the vehicle chassis. Did you wire it into your fuse box or directly to the battery? Your watt meter should have a switch or two to read forward and reflected power and shouldn't need a special fitting. Are you sure your checking the SWR properly. Another problem may be RF from the antenna getting into the mic.
My guess is going to be a poor ground but a defective new radio isn't out of the question.

Ok, I did some more checking today. Heres what Ive got.
Volts across red and black with engine off = 12, volts with engine running = 13.5-14, Heres something I didnt expect, Volts across red and black with mic keyed = 18 ??? Any idea how that can be?
I did check power output (I need an oscilator if I check SSB, thats what I was thinking I needed but I dont) Power output is 4 Watts when Jeep is running and just a hair under that when not running. I disconnected the ant and still have good power to radio, With the Ohm meter I checked all grounds from ant and radio to vehicle chassis and they are good.
The only thing I see thats odd ,(besides 18 V when keyed), is that the radio dims just a little when keyed. I guess that could indicate that Ive got a weak connection, weak ground or need heavier power wire but Ive still got good transmit power (4 W). It should be getting out.

 

[RayH]

Im suspicious of the low SWR I was getting. The needle would barely move on channel 40, and would only go up to about 1.25 on channels 19 and 2. I did some reading and those numbers are not normal, they are too low, for a mobile CB system. I read that an antenna can be so badly matched or adjusted that it will absorb the signal and not transmit it and not feed it back to the radio, showing extremely low SWR but still not transmitting anything.
I switched antennas out for an old 2' fiberglass I used when off roading. My SWR went up to 2 on channel 19 and between 2.5 and 3 on channel 40 and channel 2.
I just had a very clear and strong conversation with someone about 1 miles away through the hills.

 

[George2615]

RF is the signal that is radiated off your antenna when you key the mic. As you see with your meter it reads 18V when you key up. This is an example of RF power being radiated into the 12V power leads to the radio and RF into your voltmeter. 3 ft. between the antenna and mic is kinda close. I had a similar problem with a Ham rig in my truck. When I keyed up I had so much RF getting into the power leads and mic that the radio VFO would change frequency. I had to do three things to correct my problem.

First was to use a top loaded antenna instead of base loaded. (I use the std. 3/8 - 24 thread antenna mount so they are easy to change). This change radiates the majority of signal off the top rather than bottom of the antenna.

Second I had to coil up and tape the excess coax to act as an RF choke which stops th RF from following the coax back into the radio. You also should have at least 19 - 21 ft of coax between your antenna and radio or some multiple of 1/4 wave length for the frquencies you use. Don't make the coax as short as possible.

Third I did the same to the power leads coiled them up and taped. I also bought some choke suppressors at Radio Shack that clamp over the coils of coax and power wire to reduce RF getting into the radio.

Just read your last post and see you have a 2' antenna. If you can do it get a 4 or 5' top load antenna with a tunable tip like the truckers use and you'll see a big difference in signal both received and what you send out.

 

[RayH]

So is the 18 V a problem? There shouldnt be any RF radiated back to the power source right? Can I use that as a gauge of how much RF Im loosing back to the radio?
Ive got 18' coax. Ive always heard not to coil the coax?
I think youre right about the RF and the antenna being too close to the radio. Ive got the antenna mounted on the hood and the radio is mounted between the visors so the radio is actually higher than the antenna.
I did a little more just to see what happens, The antenna I was using was a bottom loaded antenna. First I took that off and put the little short fiberglass antenna on. It worked much, much better than the bottom loaded steel whip. Then I removed the coil off the bottom of the whip (I had a basic ant mount from another vehicle to use) and it works marginally better than the fiberglass antenna. It seems the coil at the bottom was the problem. I think probably the best antenna for this setup would be a tall top loaded ant or at least a center loaded ant.

 

[RayH]

I just checked the volts with mic keyed and the new non loaded whip. I now have 20+ volts between the red and black. Im confused now because this antenna works better than the other but apparently is feeding more RF back into the power wire. Does the 20v represent how much RF Im pumping out of the antenna or how much im loosing back to the radio?

 

[George2615]

No it doesn't. The 18 or 20V you see is a false reading. Unless you have a semi truck running a 24V system your car alternator should show 14 - 14.5V when running. RF is radiated energy sent through the air. It can be measured with a field strength meter but readings vary based on how the signal is reflected off the vehicle. I still think you need an RF choke as I described before and yes a top loaded antenna will work better for local talking while a base loaded will work better for skip or DX talking due to its higher angle of radiation. The 18-20V reading you see from RF is affecting the voltage the radio gets from your battery. That is not good for the radio since it's designed to work from 10.5 - 15V. If you ever had the cover off the radio you'd see that most parts (especially capacitors) are rated for 16V max. RF energy getting inside a radio through the coax or the power leads can cause oscillator voltages and parts values to change creating audio distortion or frequency deviation.

 

[bill177]

Hey RayH - did you ever get it working? I'd sure like to know what the problem was.

 

[RayH]

Hey RayH - did you ever get it working? I'd sure like to know what the problem was.

Nothing new to report. I got busy and put it on the back burner for a couple days. I did go buy a longer whip thinking that if I could get the signal above the roofline it would be better. I didnt see a noticable change. I think my problem is the location of the antenna in relation to the radio. Not sure what Im going to do about it though.

 

[Paul2000]

It sounds like a bad mic. You should only have a few basic elements to check on the radio. Receive and Transmit. You say you receive the guy with the base fine at a distance so the receive side sounds good.

The audio quality of the transmit portion can be tested by getting someone with a radio closer to you verifying it is clean, or tune a scanner to the frequency if you don’t have a spare CB to have someone listen on. Here are the frequencies the channels actually transmit on. CB & FRS Radios FAQ

If they are parked a short distance from you and it is garbled then RF power out is not a factor. You said you measured the power out and the SWR. It sounds fine. 4 watts out is the max legal limit on a CB and the lower the swr the better. There is no such thing as too low SWR. I use to tune them until I could not see the SWR needle move at all when I keyed it. The reason it changed when you changed channels is it is a direct indicator that the antenna is matched to your frequency. If the antenna is tuned to channel 20 and you move to 40 you have moved away from the frequency it was tuned to. An antenna will support a deviation from it’s center frequency and work fine, but on a CB it will only be tuned for one frequency.

If the audio sounds poor at a close distance then you probably have a bad mic. At that point you need to try a different one on your radio, or try yours on a different radio to see if it makes the other radio sound bad. Good luck. Paul