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Unread 02-20-2007, 04:01 PM   #1
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CB Noise - stumped

Cobra 25 LTD Classic, peaked and tweaked.

Before I take this to a CB forum, figured I'd post it here.

SWR is sitting at about 1.3:1, I'm quite impressed with that - got a Firefly today, popped it up there and that was the reading. Happy as a clam

Most channels are nice and clear with squelch all the way down, until I get to channels 16 thru 19. Add a horrid buzz to the squelch, and that's what I get. Only on those 4 channels too - I used 20 to dial in my meter, then did my measurements on 1 and 40 like a good little boy.

Only thing I can think of at the moment is perhaps I'm getting a little bit of mount grounding, so might stop by the hardware store tonight to get some nylon washers...couldn't hurt to freshen those up I guess. However, if I'm grounding out, I should have nothing

So, I wonder if anybody has any other ideas on this weird (interference?) issue - BTW, ignition was off, so it's not engine noise. That's more of a whine, so I know what it's like. This is definitely a buzz, squelch does get past it, but it's at 50-60% squelch, 25% or so on a clear channel.

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Unread 02-20-2007, 04:15 PM   #2
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Okay, closer to answering my own question I guess...noise is between 16 and 32 (yikes, worse than I ever thought). Interesting thing is, I put the meter back on just to see if it made a difference -- it did. Noise is gone. Guess the jumper on the meter is making a difference. It's a 9" jumper I think.

And 1.3:1 on the SWR is phenomenal for a side-mount like I've got (Firestik MK-204SS mount at the passenger taillight).
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Unread 02-20-2007, 04:52 PM   #3
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Does this "noise" happen only in your driveway or all the time at other geographical locations? Driveway only= stray elecrical noise from something other than Jeep & CB. Try someone elses CB in your Jeep to eliminate radio or antenna as problem.
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Unread 02-20-2007, 04:54 PM   #4
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Come to think of it, I was in front of an electrical feed here at work - I guess that could have caused some issues. I should try for a more open area.

However, that doesn't explain the noise being gone when I put the meter inline, does it? 9" of coax (the jumper) shouldn't make any difference at all in the reading, or so I understand.

I'll have to try the wife's rig tonight. Same model, just not peaked, tweaked and modded like mine
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Unread 02-20-2007, 05:05 PM   #5
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One of the steps for peaking the '25 is to boost the gain amp, so it's possible that you've increased the sensitivity to a point where it will be susceptible to stray noise.

Another thing that often gets overlooked is a damaged or frayed coax cable. Check for binding around corners and fraying anywhere that it passes through an opening. The combination of a worn out cable and a peaked unit could make it just sensitive enough to pick up stray noise.
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Unread 02-20-2007, 05:49 PM   #6
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I didn't do the peaking - that was done by the shop, around a year ago. I learned my lesson just out of high school after killing a couple Cobra 19 Ultras

Cable looks good along the whole length, no sharp corners at all in my turns, also not coiled up. When I needed to lose some length, I splayed it out so it wouldn't cancel.

Checked the SWR again (meter is still inline as of now) in a more open area, and it was down to 1.2:1 That makes me a super happy guy. I'd love to hit 1:1, but I'm not gonna risk it.

Kinda makes me wish I could have afforded a 148 all those years ago...SWR built in, NOAA, etc

*Edit: Cable is Firestik Fire Flex with the ring, so that end is known good. Radio end is soldered clean, no fraying there, no pinches.
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Unread 02-20-2007, 07:02 PM   #7
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How is the radio being powered and what gauge wire? How is the chassis grounded?
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Unread 02-20-2007, 07:15 PM   #8
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Standard 12-ish gauge wire, using the harness from Cobra - this one has been replaced in the past few months. Good ground at the power distribution center in-cab. Hot is coming from an open always-on behind the kickpanel. Install is in a ZJ.

Eventually, I'll quit being a lazy bum and run the hot straight to the battery. Ground was tested using the "icepick with a lightbulb" -- power source should be ample, and surprisingly is experiencing very little interference from in-cab items.

Power doesn't seem to be an issue here, I'm just confused as to why my signal is clean and pretty with the SWR meter kept in-line.
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Unread 02-20-2007, 07:36 PM   #9
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u check out the sticky on here?

not the best, but it might help....
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Unread 02-20-2007, 08:47 PM   #10
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If it was engine noise or atmospheric noise that weren't solved by putting an SWR meter in the mix, the sticky would be great.

Quote:
Originally Posted by Me
BTW, ignition was off, so it's not engine noise. That's more of a whine, so I know what it's like. This is definitely a buzz, squelch does get past it, but it's at 50-60% squelch, 25% or so on a clear channel.
Not engine noise, goes away when I add the meter. Since I had it running clean on the way home with the meter there, I made sure to turn down the squelch to see if engine noise was an issue - it wasn't.
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Unread 02-21-2007, 04:15 AM   #11
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The SWR is great, so I can't believe that there is an Antenna ground problem.

Since the noise goes away when the added cable is put in line, it appears to be related to a particular range of frequency.

You mentioned that you were by an electrical power box when you got the noise, have you checked anywhere else?
The frequencies at the 16 - 32 level maybe closer to the Electrical Box noise level or a resonant frequency of the Electrical Box and that may be the only reason, but double check.

A chassis ground from a chassis screw on the rear of your CB to a good grounding point may help, usually this is only for transmitted noise but it might, try it out.

How much Squelch do you need to reduce the noise on the noisy channels when you are next to the Electrical Power Box and then when you are not?

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Unread 02-21-2007, 07:16 AM   #12
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I'll have to pull the Heep to an open area today to see if there's a difference in the noise.

In order to drown out the static/noise on 16-32, I had to run the squelch up to about 60-75%. Clean channels (33-40, 1-15), I only had to run it to about 20% of the range.

The shop mentioned making sure to ground the chassis as well. I'll have to try that.
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