[sentinal02]

hey guys, I'm busy trying to cut down the background noise on my Cobra 18 when the engine is running. i know some noise is normal, but I also don't think i should have to turn the squelch 3/4 of the way up and cut out the weaker signals to get rid of it. the thing is, when the engine is off, the signal comes in crystal clear, can't get any better even with the squelch all the way down. the minute you start the engine it's like someone running a chain saw into the mic. it's also a constant buzz, ie doesn't vary with engine speed that i can tell. so far I've grounded the tail pipe, cut out the extra coax and run new power lines directly to the batt terminals using coax cable. still no dice. hasn't changed anything. one thing I have noticed though is that when i meter the coax with an ohm meter center to shield, I get a resistance of about 30-40 ohms (assuming my little meter is calibrated correctly). I'm wondering if this is normal, and I'm just reading the electrical length of the antenna, or if I'm looking at a short somewhere. I've checked the coax with the meter without the antenna attached and get nothing center to shield so i know that's good. it's all int he antenna. so is that normal or do i need another antenna? also, anyone got any other tricks to try for getting rid of this stupid buzz? I got spoiled on my old realistic but that suddenly took a crap on me and doesn't put out any sound, either from the speaker or the external speaker jack. I'm wondering if something in the system fried it and is causing me problems with the new radio as well. SWR is great though. I'm reading about 1.2 across all channels. any ideas guys?

[Jerry Bransford]

Sounds like ignition noise. Did you by any chance install those aftermarket gimmick low-resistance ignition (spark plug) wires or non-resistor spark plugs? Both of those are potential BIG sources of ignition noise. For no noise on AM radios like a CB is, you need both resistor-type spark plugs as the factory normally installs, as well as good quality ignition wires that are not low-resistance type. Trust me that the usual resistance in wiring and spark plugs is supposed to be there and in no way reduces engine performance or the spark intensity in a significant manner. Ohm's Law and all that. The extra resistance that is designed into spark plugs and ignition wiring is there to reduce/eliminate ignition noise from radios.

Is your CB's power wired directly to the battery? The battery is by far the most noise-free source of clean power for your CB.

The resistance you measured between the coax's shield and center conductor is normal.

[98JEEPTJ]

Hmmm. I think maybee it's your ground on your battery. Nothing to do with your CB at all check the heavy cable from your battery to the frame or where ever it goes. Does the noise change when you rev your engine? does it get louder with RPM?? I am no expert but I don't think it's on the power side of things....Got me stumped. Wait and see what SirGCal, or TJ N Oregon has to say...Or Jerry


Edit I could be way off.

[ab6cr]

Remove the antenna cable from the radio and listen/look for noise from the engine. If most of the noise goes away then don't worry about the power leads yet as most of the noise is from the antenna input which comes from radiated noise from the engine. Follow Jerry's info to start with.

Don

[sentinal02]

well i can't remember installing any gimmick plugs or wires last time i did a tune up. just asked for the basic stuff, basic Accel 8mm wires (maybe 7, not 100% sure) and champion plugs. CB power is direct from both battery terminals via the same type coax that i ran the antenna with so i know it's not the ground at the battery. the radio's not relying on that cable for anything. after some more hunting I think i've pin-pointed the fuel pump as the major source. I happened to have the radio on when i went to start the jeep and as soon as the pump cycled on to prime the system the radio buzzed. after a second the pump went off and the radio went quiet again. should have done that test first, but i wasn't thinking, lol. so now the question is what do i do about it? I know that there are rf filters that i can use somehow to help this, but I'm not totally sure how they work or what I'm looking to buy. if i can get rid of the pump noise then I can probably live with the engine noise (if any). if not I am due for a tune up anyway. any recommendations on which plugs and wires to go with?

[Raybz]

Normal resistance for an antenna is 50 ohms
If the noise isn't following engine RPMs and only happens while the engine is running ... just as you said FUEL PUMP!
few ways to try to help that

#1 GROUND, GROUND, GROUND
Make Sure that your radip is Grounded...the shorter the ground wire the better!

#2 Route the antenna coax as far away from the pump as you can

#3 get a Noise suppressor from Wal-Mart/RadioShack/whatever

I would do them in order listed

[ab6cr]

Read here:

http://www.stu-offroad.com/cb/cb_install-4.htm

Some people seem to have had success with a .1 mfd ceramic capacitor across the power leads where they exit the fuel tank.

A search on CB Noise in this forum brings up lots of stuff..

Good Luck!

[Jerry Bransford]

Originally Posted by Raybz

Normal resistance for an antenna is 50 ohms

You will never measure 50 Ohms of resistance with an Ohmmeter even if the antenna's impedance is 50 Ohms. Its impedance is what is being referred to when describing an antenna as being 50, 52, 75, 300 Ohms etc. Ohmmeters use a DC voltage to measure resistance so it will never be able to measure impedance which is frequency dependent. The 30 Ohms of resistance that was measured with the Ohmmeter is fine but the antenna is still considered a 50 Ohm (impedance) antenna.

Z= Impedance
R= Resistance in Ohms
XL= Inductive Reactance (directly proportional to the circuit frequency)

Z= square root of (R squared + XL squared)

[sentinal02]

Originally Posted by Jerry Bransford

You will never measure 50 Ohms of resistance with an Ohmmeter even if the antenna's impedance is 50 Ohms. Its impedance is what is being referred to when describing an antenna as being 50, 52, 75, 300 Ohms etc. Ohmmeters use a DC voltage to measure resistance so it will never be able to measure impedance which is frequency dependent. The 30 Ohms of resistance that was measured with the Ohmmeter is fine but the antenna is still considered a 50 Ohm (impedance) antenna.

Z= Impedance
R= Resistance in Ohms
XL= Inductive Reactance (directly proportional to the circuit frequency)

Z= square root of (R squared + XL squared)

now you sound like the digital design course i took as an elective (i knew i should have went EE instead of ME, lol). i do seem to remember that impedance is for AC systems as resistance if to DC. makes sense to me.

Ray, 1 and 2 are done. can't get a much better ground on the radio than to the batt (only about 4.5' of cable to get it there and it's gotta be better than grounding to the dash which grounds to the tub which grounds to the....

and as for 2, the antenna is at the back corner of the jeep and the coax routes in through the tail light and then up the roll bar. since the pump is centered in the rear i can't get any further way.

now, on the pump problem, all i need to do with the cap is cross the hot and ground leads with the cap terminals as close to the pump as i can get it? sounds simple enough.

edit: interesting. read through the link there and they mention that a direct ground to the battery is a bad thing. hmmm, guess i'll have to reroute that and see if it helps. i have a feeling that the pump is still gonna be a problem though.

[Jerry Bransford]

Originally Posted by sentinal02

now, on the pump problem, all i need to do with the cap is cross the hot and ground leads with the cap terminals as close to the pump as i can get it? sounds simple enough.

Yep, and the bigger the cap value the better. And go with an electrolytic (polarized, has a + and - lead) capacitor, they work better at this kind of stuff.

[sentinal02]

Originally Posted by Jerry Bransford

Yep, and the bigger the cap value the better. And go with an electrolytic (polarized, has a + and - lead) capacitor, they work better at this kind of stuff.

what would be considered a "bigger" value? .1uF is pretty tiny by any other standard

[Jerry Bransford]

Originally Posted by sentinal02

what would be considered a "bigger" value? .1uF is pretty tiny by any other standard

Um, the biggest reasonably sized cap he can find that will fit that doesn't have to be bolted or welded to the chassis?

[sentinal02]

Originally Posted by Jerry Bransford

Um, the biggest reasonably sized cap he can find that will fit that doesn't have to be bolted or welded to the chassis?

ok, so no 1 farad stereo grade caps, lol. just curious, but doesn't a larger cap filter out lower frequencies? i seem to remember something on that note from college, but we never really got that deep in the courses i took (I was an ME so i never went past design 1 in those classes). then again, i guess when you're working with digital logic controlers and IC's that work in the 100's of MHtz if not GHtz range, the 26-27 Mhtz band that the CB is operating on would probably be considered a low frequency huh?

[Jerry Bransford]

Originally Posted by sentinal02

ok, so no 1 farad stereo grade caps, lol. just curious, but doesn't a larger cap filter out lower frequencies? i seem to remember something on that note from college, but we never really got that deep in the courses i took (I was an ME so i never went past design 1 in those classes). then again, i guess when you're working with digital logic controlers and IC's that work in the 100's of MHtz if not GHtz range, the 26-27 Mhtz band that the CB is operating on would probably be considered a low frequency huh?

A capacitor's resistance (inductive reactance) is inversely proportional to the frequency. He is filtering out an audible low frequncy noise so a bigger capacitor does a better job at that. Capacitive Inductance (resistance for a capacitor) becomes higher with lower frequencies so you need a BIG capacitor at low frequencies for it to be able to help filter out anything at all. A small capacitor at that low audible noise frequency would have so much resistance that it wouldn't be very effective at passing the low frequency noise component to ground.

[Milt]

You might get some help finding the source of your problem here: http://www.cobra.com/index.php?page=corporate/faqdetail&id=1&faq_id=39

Milt