Mods, if this is in the wrong forum, please move it to the correct one....

I have a Cobra 18 WX ST II CB mounted in my truck as shown here..
http://www.frontiernet.net/~jcray/04...Install-02.JPG

I have a 3' Firestik II antenna mounted to the driver's side cowl metal as shown here..
http://www.frontiernet.net/~jcray/04...Install-09.JPG

The weather stations come in pretty good, but I seem to have a problem with noise on the CB channels. If someone is talking, I can hear them great (no noise, just them talking) and they can hear me, but between transmissions the noise is bad. I have to turn the squelch almost 3/4 of the way up to get it to go away.

Today, I noticed that if I touch the metal connector for the microphone, the noise goes away. Similarly, if I touch the microphone cable, it goes away.

Could there be some problem with the mic? I haven't tried taking it off, or connecting a new ground wire to it yet. Any ideas on what could be going on here?

Thanks in advance!

PS... I did adjust my SWR and got it down to about 1.2-1.3

 

Touching the chassis of the radio should increase the noise, your body is acting as an antenna. I think what you have is a ground loop. There is a potential difference between your antenna mount and the radio. Finding a better ground for the chassis should help, but you will probably need to ground your ground plane better.

 

two more items that may help. Create 2-3 loops of the coax near the antenna. Also what kind of CoAX are you using. Cheap wire with poor connectors can pick up a lot of interference. I like RG-8U-mini, the wire is a tad smaller than RG-58, but the shielding is much better.

 

First have you had another CB radio before? Are you familiar with the noise on CB? Especially in the city the noise can be high enough to come through the squelch even near maximum. Also the static between conversations can be pretty bad depending on the local conditions like weather or storms, static build up or electrical wires and such. Also have you allowed for engine and vehicle noise? My PSD is really niosey when running! Makes my HAM radio hard to use for weak stations. Just thoughts since you didn't say if you'd done or checked these.

As for touching the mic at places and lowering the noise my first thought would be to do some serious grounding. Ground the antenna and the radio. Not just a wire but the true braided grounding straps, you can put eyes on the ends if needed. This should help quite a bit. You might need to ground the cab to the chassis too. Also check your SWR on channels 1, 20 and 40 and look for spikes, it's always possible for the antenna to be damaged although it doesn't sound like it in this case. Never hurts to check though. Good luck!

 

I agree with most of what has been said but the mic thing sounds like your connection to the radio is loose. I know with mine if the plug is not tight it will do either of two things. One will be no noise or it will have noise but you wont hear me talk. I found this out after I had my radio tweaked at a local shop. He saw me key up on his meter but nothing came over once I tightened the connection I talked like a champ. Try pulling it off and see if any of your connections are corroded or loose.

 

My $.02 is, go direct to the battery with your power cord, use POWERWERX cord. Its for Ham and CB, you can then ground to that also. It can be found on Ham radio sites, then go get your Amateur radio Lisc. and you can learn what keeps smoke inside wires.

I got a 1959 John Deere 530 2 cylinder for my wife. Pretty good trade I think.

 

Thanks a bunch for the replies!

I'll have a look at all that stuff this weekend.

 

I took a look at my CB, antenna, and wiring today and didn't come up with too much worthy of discussing. I think the fuel pump is causing alot of the noise. I've read a few old threads that mention this. I can hear a buzz of noise when I turn the key on, then steady after I start it. I reconnected the ground of the CB to a better spot and this helped a little. I also found that the shield of my coax (RG58 from the Firestik install kit) wasn't connecting in the connector as good as it used to, so I re-hooched that. In the end, its a bit quieter but I think bigjimcruising nailed it... its just the nature of the beast. I can get by with my squelch set to about half way now though.

Given some more time I'm going to try connecting the power & ground directly to the battery as suggested, and also look into the noise filters mentioned in the other threads that talk about fuel pump noise.

Thanks again to everyone for the replies!

 

Wise old man (Papa Pond Scum) once told me "Make your ground connection as close to the radio as you can get it, as solid as you can get it, with nice fat wire. Then make sure the frame and cab, frame and battery, and battery to alternator all have less than 1 ohm through them" - in other words, the closest ground point to the radio in the cab. Then make sure your whole truck is grounded together as near perfect as you can make it.

LOOPING the antenna cable is a bad idea - it can cause inductive coupling that changes the transmission line characteristics.

One other note - when I was in Norfolk the overhead powerlines in some places made it near impossible to get any reception at all. Be aware of power lines and towers, they can really ruin your day!

 

Please help me out with the inductive nature. On a proper inductor, the wire is bare, touching and looped. I believe that the low Freq suppression induced is from the current being able to jump from one length of conductor to the next. Good coax is shielded, the shielding sends most all of any radiated EM back to ground. The shielding layer is effectively a Faraday cage, EM from the out side radiates to ground, EM from the inner conductor goes to ground. The tighter the weave the higher the frequency it can keep from radiating in either direction.

So the shielding is doubled when you coil, how much induction will the conductor see? The shielding however can radiate to itself, and then ground it. The closer to the grounding point, the more likely the EMR is being rejected to ground?

 

Induction is the result of magnetic fields building and collapsing around a given conductor.

If a conductor is looped next to itself with an RF or an AC signal the fields created can either aid or impede eachother - resulting in either NO LOSS AT ALL (if they are exactly in phase) or INTERFERENCE - which is much more likely.

With a random lay of loops of wire you are far more likely to introduce unwanted impedence to the total SWR than not.

The stronger the signal - the more pronounced the affect is. Even with shielding.

Keep your cables away from eachother.

*NOTE: There is no such thing as an inductor (think transformer) in which the windings are not insulated. A shorted inductor is no longer an inductor. The wires CANNOT be bare - it would defeat the purpose.

FORGET ABOUT COILING!

You are messing yourself up.

You are right about one thing - you won't radiate any frequency at all if you go down that road.

On a correct inductor there is a coil, which has all of it's coils insulated from eachother. The purpose of an inductor is to resist the instantaneous flow of current so as to delay it through electromagnetic action.

Capacitors act in a way that is directly opposite.

After 20 years of doing electronics I can tell you outright that you have a lot of misconceptions.

 

Then why is it, when buillding the 2m jpole they (They being Oprah Winfrey) reccommend coiling 3 loops of coax just below the attach point on the jipole to prevent unwanted RF in the rear of a transmitter? Why not put ferrite coils around the coax? To prevent unwated interference from electrical pumps or alternators you can use condensers in the hotwire.

In theory there is no difference between theory and practice, in practice there is. Yogi Berra

 

Ok so I am wrong about inductors, but what about the Faraday cage created by the shielding?

 

The Faraday field isnt deep enough on most shielded mobile coax.
A Faraday cage is best understood as an approximation to an ideal hollow conductor. Externally applied electric fields produce forces on the charge carriers (usually electrons) within the conductor, generating a current that rearranges the charges. Once the charges have rearranged so as to cancel the applied field inside, the current stops.
If a charge is placed inside an ungrounded Faraday cage the internal face of the cage will be charged (in the same manner described for an external charge) to prevent the existence of a field inside the body of the cage. However, this charging of the inner face would re-distribute the charges in the body of the cage. This charges the outer face of the cage with a charge equal in sign and magnitude to the one placed inside the cage. Since the internal charge and the inner face cancel each other out, the spread of charges on the outer face is not affected by the position of the internal charge inside the cage. So for all intents and purposes the cage will generate the same electric field it would generate if it was simply charged by the charge placed inside.
If the cage is grounded the excess charges will go to the ground instead of the outer face, so the inner face and the inner charge will cancel each other out and the rest of the cage would remain neutral. A Faraday cage is capable of completely stopping an attack using electromagnetism such as an EMP.<SUP class="noprint Template-Fact">[citation needed]</SUP>
The cage will block external electrical fields even if the cage contains some charges and an electric field in its interior. This is a consequence of the superposition principle and the fact that the Maxwell equations are linear.
A Faraday cage will not shield its contents from static magnetic fields. However, rapidly-changing magnetic fields create electric fields in accordance with Maxwell's equations. The conductors cancel the electric fields and therefore the changing magnetic fields as well. The wall materials' thickness and skin depth set the frequency at which the cage suppresses electromagnetic fields. Static or slowly-changing magnetic fields penetrate the cage; rapidly-changing ones do not.

 

If 1/4 wave is 108", full wave length is 432", and the shielding is at least as thick (wire gauge size) as the conductor, is this enough to shield the interior from external EMR?