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Old 04-04-2016, 12:59 AM
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CB SWR help Please.

Purchased the CoolTech license plate CB kit for my JK. Mounting stud, Tunable 4' FireStik, CoAxial Cable all included...

Channel 1 tuned in @ 1.5 SWR.
Channel 40 tunes in @ 3.5 SWR.

I have a roof rack in which the antennae mast still reaches over. There are no kinks or coils in the coaxial cable. The mounting stud is grounded (this is shown with the low SWR on Channel 1).

Higher SWR on 40 means shorter antenna right? I screwed the tuning tip down as far as possible. I also used a friends 3' FireStik with and without the quick disconnect and received the same SWR measurements.

Any ideas?

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Old 04-04-2016, 04:10 AM   #2
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Did you try removing the screw?

You should be able to get 2.0 SWR on 1 and 40 with about a 1.5 on channel 20 with that mount and antenna. I needed to remove the screw to get the SWR to that level.

I ran a CB whip there with the same mount for a couple of years, and have just recently changed my setup to have a ham radio HF antenna on that same mount with some modification. I enjoyed using skywaves (or "skip") to talk with folks many hundreds of miles away.

SWR on mobile antennas looks like a parabolic graph, so increasing your SWR on channel 1 will make the lower parts of the SWR plot more in the center of where you want to transmit. Here is an example plot of SWR vs frequency (channel) from google images.



The best I was I was able to get with my CB setup was a 1.5 SWR on 1 and 40 with a 1.2 on 20 when using a 108" stainless steel whip with a heavy duty spring on the same mount.

The 108" whip worked really well. I ran a legal 4 watts on a SSB CB radio and was able to converse with folks in Australia on 38 lower sideband when the band was open. I could bend the whip forward and tuck the end of it under my light bar so that the vehicle would fit into parking garages.

Unfortunately, we have entered the part of the solar cycle where it is likely that we won't see the CB band "open" for the next three or four years. I changed to a ham HF setup to be able to use the other bands that are open in this part of the solar cycle.

CB will still work for short range communications just fine though, so if you are using it for standard trail comms then there is nothing to worry about. I still have a CB installed in my rig for that purpose, but I've got a small antenna now since it does not need to go very far.

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Old 04-04-2016, 07:30 AM   #3
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Check to make sure that all the connections are good and that the plastic washer is mounted correctly. Also, you can go to rightchannelradios.com and check out their resource library. They have instructions and videos on how to trouble shoot CB installation problems.
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Old 04-04-2016, 06:53 PM
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I'll keep you guys posted with the SWR measurement.

The plastic spacer/separator is installed correctly (and was untouched when received from CoolTech).

The only thing for me to other than some license plate bracket modification was sanding some paint off the body for a better ground.

I assume because my SWR is at 1.5 on Channel 1; and over 3 on channel 40 that my ground is decent.

I wonder how big an issue is it, when tuning my SWR, I'm tuning it around buildings 3 stories in height?

I wonder
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Old 04-04-2016, 07:37 PM   #5
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It's clear from your SWR indications that your antenna mount is assembled correctly and that the nylon insulating shoulder washer is installed properly.

First, no need to measure on both 1 and 40. Just measure and tune from the mid-point on the band which is ch 20. When you get the SWR as low as possible on ch 20, it'll be as low as possible on 1 and 40 too.

My thought on the higher than usual SWR is very likely due to inadequate grounding where the antenna mount's mounting bracket is attached to the Jeep, and possibly where the bottom half (SO-239) of the antenna mount touches the bottom of the mounting bracket if it is powder coated.

There's a difference between a ground that is good enough for a light bulb, radio, etc. and the RF ground that an antenna requires. A simple wire can provide an adequate ground for most things but not an antenna system... you need a large surface area ground to make for a good enough RF ground. That means a good amount of bare metal surface to surface contact between the body mount and what it is bolted to... as in grind the paint or powder coating away from where they touch so they have an adequate size bare metal connection.

Sometimes a braided ground strap can help too. Braided ground straps have a bigger surface area for the RF to travel on which will help make for an acceptable RF ground. Some automotive stores carry braided ground straps but you may need to go to a Fry's, Lowe's, Radio Shack, ham radio or CB radio shop, Home Depot, etc. to get it.

So #1 I'd stop messing around on 1 and 40 and see what you can get ch 20 down to. Once it's at its lowest possible SWR, start working on those body connections so the antenna mount will get a good RF ground. An ohmmeter may show a great ground connection but that same ground connection may be and usually will be inadequate as an RF ground connection if it's just a plain piece of wire.

And those buildings aren't close enough to affect your SWR.

Good luck with it.
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Old 04-04-2016, 09:15 PM
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Thanks for the tuning tip removal!

After a couple dozen attempts of back & forth of removing tips, locknut, washer, extending, shortening, etc...

I settled for the removal of the tuning tip locknut and got a SWR reading of 1.7 on both channel 1&40!

HOWEVER now when transmitting I get my voice feedback from my subwoofer???

My amp hidden behind the glovebox, approximately 6" away from the Cobra 75 WX.

Both share the same terminal for power direct from the battery.

Any tips on fixing this?
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Old 04-04-2016, 09:15 PM
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Old 04-04-2016, 09:40 PM   #8
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It's so hard to cure problems like that that I just turn my audio system down or off when that happens. It's curable but it's not usually easily curable.
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Old 04-04-2016, 09:46 PM
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Quote:
Originally Posted by Jerry Bransford View Post
It's so hard to cure problems like that that I just turn my audio system down or off when that happens. It's curable but it's not usually easily curable.
Jerry, my OCD just went off the SWR charts!!! lol

If anybody has a similar setup and wants to chime in please feel free.

Here is some more details:


-SWR is at 1.7 on Channel 1&40.
-CB powered directly through the battery.
-Cobra CB module is behind the glovebox.
-Amplifier for the subwoofer also shares the same battery terminal as the CB.
-Amplifier is also located behind the glovebox (approx 6" away from the Cobra CB module).
-CB coaxial is run along the Driver side.
-Amplifier Subwoofer wires run along the Passenger side.
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Old 04-04-2016, 09:53 PM
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Some extra details:

-The feedback through the subwoofer (which the amp powers) will occur through transmission even if the radio is turned off. Without ignition, if there is power through the system - feedback will occur through the subwoofer.

However if my entire vehicle is shut-off... The CB is great?!
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Old 04-04-2016, 09:57 PM   #11
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Here's something that is free to try and can occasionally fix a problem like that. Twist the black and red power wires of both the CB and audio system about 3X per inch. Make sure the amp is solidly grounded. Twisting the power cables can cancel out some forms of induced noise.

By the way, only check the antenna's SWR with the vinyl tip cap on. The vinyl tip cap has a dramatic effect on the SWR. The trick is to mistune the antenna off to the correct side so replacing the cap brings the SWR down to it's lowest point. Yes that procedure is trial and error. The SWR meter might indicate 1.5 with the cap off but installing the cap can increase it significantly like to even 2.5. The goal is to mistune it to a higher than desired SWR and bring it down to where you want it by installing the tip cap.
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Old 04-04-2016, 10:15 PM   #12
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Issues like the RFI penetration into your subwoofer are typically due to common mode interference.

Wrap the subwoofer cabling around some type 43 ferrites and/or clip some over the coax. The bigger/thicker the ferrites the better.

The right ferrite type is critical. But they're not cheap. But using some random ferrite will not apply the needed RFI chocking force.
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Old 04-04-2016, 10:33 PM   #13
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Quote:
Originally Posted by MRT2 View Post
Some extra details: -The feedback through the subwoofer (which the amp powers) will occur through transmission even if the radio is turned off. Without ignition, if there is power through the system - feedback will occur through the subwoofer. However if my entire vehicle is shut-off... The CB is great?!
How is your subwoofer control wired? The subwoofer control could be wired in such a way that the signal to power the amp is not controlled by the radio but rather the ignition. If that was the case, that would explain why the interference stops only when the sub is off.

The 12v control wire is a simple binary power = on, no power = off signal.

If you're activating the sub amp with a 12v control wire (typically light blue) then you could install a simple switch inline with the wire and switch off the sub function when using the radio.
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Old 04-04-2016, 11:44 PM
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Quote:
Originally Posted by arjeeper View Post
How is your subwoofer control wired? The subwoofer control could be wired in such a way that the signal to power the amp is not controlled by the radio but rather the ignition. If that was the case, that would explain why the interference stops only when the sub is off. The 12v control wire is a simple binary power = on, no power = off signal. If you're activating the sub amp with a 12v control wire (typically light blue) then you could install a simple switch inline with the wire and switch off the sub function when using the radio.
Thanks AR Jeeper!

Can you provide a link for these ferrites I can purchase?

You're correct, the amplifier powering the sub is "auto-on" and controlled by the ignition and not the radio.

If those is the case, can the ferrite used be done without installing this switch?
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Old 04-05-2016, 12:01 AM   #15
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If your head unit is aftermarket, it should have the ability to control the amp power so it's not controlled by the ignition switch.
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Old 04-05-2016, 12:05 AM
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Thanks Jerry,

My system is the stock 430N on the JK. I was looking at the amp and I don't have a blue wire, however these hi/lo inputs I have I believe are "auto-on".

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Old 04-05-2016, 12:11 AM   #17
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Yes. Looks like you have an auto-on function driven by speaker level inputs so my idea on the switch wouldn't work.

Ferrites can cost a lot so I'd frankly exhaust your troubleshooting options deforestation buying.

Again, you want mix 43 for the CB frequencies.

http://www.kf7p.com/KF7P/Ferrite_chokes.html
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Old 04-05-2016, 09:05 AM   #18
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Some antennas are more broad banded than others. Some antennas are not adjustable. The normal recommendation for a 40 channel CB radio is to tune the antenna for the best SWR's on channel 20. Any steel or aluminum object that is fairly close to the radiating element of an antenna will have an affect on SWR's.

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