CB interference
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From: Midland, MI
A couple thoughts.
Is the fan noise isolated to when the A/C is running? Or if you turn the A/C off and just run the fan do you still get the noise?
Secondly - the squelch. If you look at the signal meter (I am assuming it is an analog needle) what the squelch does is it cuts the audio of the receiver below a certain level so you don't get all that white noise (the SHHHHHHHHHHHHHHHHHH you hear with the squelch open). If you have a buddy key up his radio at a certain distance, say a couple miles away, so your radio's needle is a steady peak, you can see where the squelch cuts off the receiver. Now move in to a mile apart and do the same thing. Your buddy's radio signal will be stronger and therefore will be above the "bar" you set with your squelch. So yes, you are entirely correct - if you crank the squelch up it IS going to quiet down the receiver by cutting the audio. However, what you did in the same process is made all the weaker signals (other CB users) you want to hear disappear - their signal levels may very well be below the level that the interference is and therefore the cranked up squelch closes them out. It would be like setting your squelch for your buddy at 1 mile and then try to go back to 2 miles - he's not going to get through. Your best bet is still to cut the noise out from the source.
Is the fan noise isolated to when the A/C is running? Or if you turn the A/C off and just run the fan do you still get the noise?
Secondly - the squelch. If you look at the signal meter (I am assuming it is an analog needle) what the squelch does is it cuts the audio of the receiver below a certain level so you don't get all that white noise (the SHHHHHHHHHHHHHHHHHH you hear with the squelch open). If you have a buddy key up his radio at a certain distance, say a couple miles away, so your radio's needle is a steady peak, you can see where the squelch cuts off the receiver. Now move in to a mile apart and do the same thing. Your buddy's radio signal will be stronger and therefore will be above the "bar" you set with your squelch. So yes, you are entirely correct - if you crank the squelch up it IS going to quiet down the receiver by cutting the audio. However, what you did in the same process is made all the weaker signals (other CB users) you want to hear disappear - their signal levels may very well be below the level that the interference is and therefore the cranked up squelch closes them out. It would be like setting your squelch for your buddy at 1 mile and then try to go back to 2 miles - he's not going to get through. Your best bet is still to cut the noise out from the source.
Later today I'll try running the fan with A/C on and off and see what results I get.
Thanks for explaining the squelch. I have a basic understanding of it, so you confirmed what I already knew and also taught me some new things about it.
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Posting Guru
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From: Midland, MI
So I've been continuing to play around with this and it's getting better.... I've added more ground straps and have messed around more with the settings on the CB. Using the squelch, and other noise cancelling features the CB has, I can have the A/C on and be driving down the freeway with no interference.... and I can receive loud and clear. Not sure what my range is yet, but I'm still working at it. I do want to get my SWR down more.... last time I checked it was 2.0 on channels 1, 19, and 40. I'll probably take a ride to my local CB shop too, and have them checked it out. Thanks for all the help guys I'll continue to update this thread as I find out more.
I'll continue to update this thread as I find out more.
Cargo Master
Joined: Jul 2008
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Thanks for the update. Glad to hear it is getting better.
Regarding the antenna SWR - if you can get a good direct ground under it you'll be better off there too. Make sure any cables are SEALED WELL. If they are wet, let them dry out. Magnet mount antennas - I had a couple 3x 5" magnet mounts on my Silverado years ago. I took a 14 gauge stranded wire (THHN hook up wire from Lowes) and put a self-tapping screw behind the cab roof brake light. There were two screws holding it to the body so I pulled those out and drilled a hole in the back plate underneath, screwed the ground wire on, and put the light back in. That was one of the best additions I made to that antenna set up - I was using a 10m hamstick whip for my CB antenna at the time.
The magnet mount antennas work with a couple different theories. Some antennas are "ground independent" and work well on a magnet mount, regardless of whether or not it has a lot of metal under it, or any metal at all. This is because the feed point impedance is good as it is. I don't know that I have ever heard of a CB antenna that is exactly in this category because the frequency is so much lower than other mobile systems - typically VHF and UHF. This is where capacitive coupling comes in to play. If you know what a capacitor is you will note that, on a very basic level, a capacitor is essentially two plates spaced apart and there is an electrical charge that is stored between them. They pass AC but block DC. So what does this have to do with a magnetic mount antenna??? The feed point of the antenna is at the center of the magnet(s). The center conductor of the coax cable goes to the antenna (whip sticking above the mount, and insulated from the rest of the mount) and the shied is connected to the magnet base - usually a disk. Most of the time the only insulation between the magnet mount and the metal you are sticking it to is the paint on the vehicle. Since this is thin and there are, for all intents and purposes, two "plates" resting next to each other they make a quasi-capacitor and "couple" the return currents from the antenna through the metal the antenna is stuck to (vehicle roof, etc). RF is AC so it is free to pass, provided there is enough capacitance. The less capacitance the less coupling the less of an affect the "grounding" ends up being.
So if you've ever wanted to put a layer of tape under your magnet base, or a sheet of rubber/plastic, this is bad for 2 reasons - the further away the magnet is the less holding power it has AND you are reducing the capacitive coupling.
Direct ground the magnet base = direct ground no matter what the capacitance is.
Off topic, but I have worked on a lot of farm machinery in the past few years with GPS systems. One problem that keeps coming up every year is someone's 2-way radio killing the GPS signal every time they key up. I don't have a clue why the manufacturer can't get it through their heads that the factory antenna mounts on the machines have to be grounded. They're NMO style mounts and are screwed on to a fiberglass-type material roof. Most of the time we have people pop the roof panels off and get some wire attached to the base. Some other creative fixes I've seen have to do with 3M foil tape and aluminum foil. Whatever works... its all metal.
Regarding the antenna SWR - if you can get a good direct ground under it you'll be better off there too. Make sure any cables are SEALED WELL. If they are wet, let them dry out. Magnet mount antennas - I had a couple 3x 5" magnet mounts on my Silverado years ago. I took a 14 gauge stranded wire (THHN hook up wire from Lowes) and put a self-tapping screw behind the cab roof brake light. There were two screws holding it to the body so I pulled those out and drilled a hole in the back plate underneath, screwed the ground wire on, and put the light back in. That was one of the best additions I made to that antenna set up - I was using a 10m hamstick whip for my CB antenna at the time.
The magnet mount antennas work with a couple different theories. Some antennas are "ground independent" and work well on a magnet mount, regardless of whether or not it has a lot of metal under it, or any metal at all. This is because the feed point impedance is good as it is. I don't know that I have ever heard of a CB antenna that is exactly in this category because the frequency is so much lower than other mobile systems - typically VHF and UHF. This is where capacitive coupling comes in to play. If you know what a capacitor is you will note that, on a very basic level, a capacitor is essentially two plates spaced apart and there is an electrical charge that is stored between them. They pass AC but block DC. So what does this have to do with a magnetic mount antenna??? The feed point of the antenna is at the center of the magnet(s). The center conductor of the coax cable goes to the antenna (whip sticking above the mount, and insulated from the rest of the mount) and the shied is connected to the magnet base - usually a disk. Most of the time the only insulation between the magnet mount and the metal you are sticking it to is the paint on the vehicle. Since this is thin and there are, for all intents and purposes, two "plates" resting next to each other they make a quasi-capacitor and "couple" the return currents from the antenna through the metal the antenna is stuck to (vehicle roof, etc). RF is AC so it is free to pass, provided there is enough capacitance. The less capacitance the less coupling the less of an affect the "grounding" ends up being.
So if you've ever wanted to put a layer of tape under your magnet base, or a sheet of rubber/plastic, this is bad for 2 reasons - the further away the magnet is the less holding power it has AND you are reducing the capacitive coupling.
Direct ground the magnet base = direct ground no matter what the capacitance is.
Off topic, but I have worked on a lot of farm machinery in the past few years with GPS systems. One problem that keeps coming up every year is someone's 2-way radio killing the GPS signal every time they key up. I don't have a clue why the manufacturer can't get it through their heads that the factory antenna mounts on the machines have to be grounded. They're NMO style mounts and are screwed on to a fiberglass-type material roof. Most of the time we have people pop the roof panels off and get some wire attached to the base. Some other creative fixes I've seen have to do with 3M foil tape and aluminum foil. Whatever works... its all metal.
Cargo Master
Joined: Jul 2008
Posts: 2,563
Likes: 53
I had a conversation about SWR a couple years ago with a friend of mine that has worked in the wireless industry for most of his working life. What stood out to me about the conversation is he factored in losses in to SWR. I believe we were discussing sweep testing cable runs and checking antennas as I was working on some 200-300ft installs. You shouldn't have to worry about this in a mobile install since the cable runs are so short, but it is interesting to think about and a mind twister.
Most radios need less than 2:1 SWR to match the transmitter well enough.
If you have a long cable run and you come up with a 1.5:1 SWR at the radio end you're in great shape, right?
What happens if that cable run ends up having 6dB loss? Either small cable/long run, or bad cable, either one.
Every 3dB is essentially a power factor of 2. So 6dB is a power factor of 4. That means 1/4 of the power out of the radio is making it up the coax. But wait - SWR is measuring reflected power. So that is 1/4 power UP the cable as well as 1/4 power back DOWN. See where I'm going with it? That is a power factor of 8.
Take your SWR at 1.5:1. Multiply 1.5 by 8 and you get 12. A 12:1 SWR is not real good.
The moral of it was to check SWR at the antenna, or much closer to (take the hardline out first) - bypasses losses, then they can be brought back in.
Most radios need less than 2:1 SWR to match the transmitter well enough.
If you have a long cable run and you come up with a 1.5:1 SWR at the radio end you're in great shape, right?
What happens if that cable run ends up having 6dB loss? Either small cable/long run, or bad cable, either one.
Every 3dB is essentially a power factor of 2. So 6dB is a power factor of 4. That means 1/4 of the power out of the radio is making it up the coax. But wait - SWR is measuring reflected power. So that is 1/4 power UP the cable as well as 1/4 power back DOWN. See where I'm going with it? That is a power factor of 8.
Take your SWR at 1.5:1. Multiply 1.5 by 8 and you get 12. A 12:1 SWR is not real good.
The moral of it was to check SWR at the antenna, or much closer to (take the hardline out first) - bypasses losses, then they can be brought back in.
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