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post #1 of 15 Old 04-02-2009, 06:42 AM Thread Starter
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CB Radio SWR

I just installed a Cobra 75 with a 4 ft. Wilson Firestick. I have tuned the antenna, but my best SWR reading is: 2:1 on channel 1 and 2:5 on channel 19. This was done after moving my ground...before that, the readings were horrid! Is this reading suitable? As I see on the instructions, these readings should be a max of 2:1. Does the higher reading just shorten the distance of transmission and receiving? What can I expect? Can I do anything else to improve the readings? I have already removed the tip...so I can't get any better with that. Thanks in advance.
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post #2 of 15 Old 04-02-2009, 07:39 AM
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It will work, but the range wont be as good as it could be

What are you using for a mount, and how are you grounding it?

Jason
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post #3 of 15 Old 04-02-2009, 07:47 AM
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SWR, 2 to 1 is adequet, 3 to 1 is OK for RECEIVING ONLY

1 to 1 is perfect (unatainable), but anything over 2 to 1 will add more DC PWR load to the radio circuits when TRANSMITTING

2 to 1 and less (1.9 to 1) SWR will do for talking all day, check your hookup and also the mounting point

Of course the middle of the roof, is the most ideal spot, every thing else will degrade the SWR, of the sys-checkitout

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post #4 of 15 Old 04-02-2009, 09:28 AM Thread Starter
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Quote:
Originally Posted by SDMF View Post
It will work, but the range wont be as good as it could be

What are you using for a mount, and how are you grounding it?
I have installed the antenna on the brake light pedestal in the middle of the tailgate. Keep in mind that this mounting point is cast alluminum, and apparently alluminum is not a good ground. When I realized that the SWR was not delivering what it was suppose to have; I remembered reading that the higher SWR (most of the time) was caused by poor grounding. I them cut a 6 ft. piece of 16 gauge wire, sanded a spot on the frame, and utilized a self tapping screw to improve the ground. The SWR got better, but still isn't within the guidlines suggested in the instructions. I'm hoping for some input that might improve these SWR readings.
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post #5 of 15 Old 04-02-2009, 10:01 AM
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Yea, thats where I put mine, with a gnd strap to the tailgate

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Works fine

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post #6 of 15 Old 04-02-2009, 10:06 AM
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What gets a lot of people is the ground when they do a tailgate mount. No proper way to ground it except grounging the tailgate to the frame in addition.

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post #7 of 15 Old 04-02-2009, 10:35 AM
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I have a similar setup. Best I'm getting is 1.8 on ch 1 and 2.4 on 40.

My group uses ch 4 or 7 mostly, so I haven't messed with it too much. I mounted on the tire carrier and ran a wire from the antenna and grounded to the tub and the roll bar.
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post #8 of 15 Old 04-02-2009, 11:20 AM Thread Starter
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I have a similar setup. Best I'm getting is 1.8 on ch 1 and 2.4 on 40.

My group uses ch 4 or 7 mostly, so I haven't messed with it too much. I mounted on the tire carrier and ran a wire from the antenna and grounded to the tub and the roll bar.
I grounded mine to the tub also, but I just read that the ground should be soldered...I used Sta-con connectors, but will change them out this afternoon, then run another SWR check. My readings are pretty close to what you're getting. What's your range?
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post #9 of 15 Old 04-02-2009, 12:34 PM
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devildog77663

The SWR is nothing more then an indicator (ratio) of reflected RF power from the antenna back into the radio. This ratio from channel 1 to 40 is a bathtub curve..In theory the dip (lowest point) should be 1..However.. Staying below 2 will do no harm to your set up..The short CB antennas are a compromise anyway.. If you want distance then run a 102 inch whip and the SWR will be 1.1 across the 11 meter band.

In your situation check channel 1 and then channel 40 (don't forget to re-calibrate the SWR meter) If the channel 1 reading is lower then the channel 40 then your antenna is too long.. ..You can trim and set the lowest point where you operate the most..

Keep in mind that RF grounding is different then DC ground.. RF ground runs via the outside surface of the ground system.. A 16 gauge wire will not do it.. You need to have braided grounding straps (1 inch wide) like this one



to ground the tailgate to the tub..The contact surfaces need to be bare metal to bare metal..I am convinced that ground strapping the tailgate will be a good start..

Personally I don't use the spare carrier for CB mounts.. this is good enough for 2 meter and up..My CB mount is on the side of the jeep straight into the tub.. (a nice hole of course..) shown here



Good luck..I am sure you will learn alot about antenna tuning..

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post #10 of 15 Old 04-02-2009, 12:54 PM Thread Starter
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devildog77663

The SWR is nothing more then an indicator (ratio) of reflected RF power from the antenna back into the radio. This ratio from channel 1 to 40 is a bathtub curve..In theory the dip (lowest point) should be 1..However.. Staying below 2 will do no harm to your set up..The short CB antennas are a compromise anyway.. If you want distance then run a 102 inch whip and the SWR will be 1.1 across the 11 meter band.

In your situation check channel 1 and then channel 40 (don't forget to re-calibrate the SWR meter) If the channel 1 reading is lower then the channel 40 then your antenna is too long.. ..You can trim and set the lowest point where you operate the most..

Keep in mind that RF grounding is different then DC ground.. RF ground runs via the outside surface of the ground system.. A 16 gauge wire will not do it.. You need to have braided grounding straps (1 inch wide) like this one



to ground the tailgate to the tub..The contact surfaces need to be bare metal to bare metal..I am convinced that ground strapping the tailgate will be a good start..

Personally I don't use the spare carrier for CB mounts.. this is good enough for 2 meter and up..My CB mount is on the side of the jeep straight into the tub.. (a nice hole of course..) shown here



Good luck..I am sure you will learn alot about antenna tuning..

Thanks for all of that GREAT information! I'm headed to Radio Shack after work for a braided ground cable. I bet that solves my higher SWR readings.
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post #11 of 15 Old 04-02-2009, 01:09 PM
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Originally Posted by devildog77663 View Post
Thanks for all of that GREAT information! I'm headed to Radio Shack after work for a braided ground cable. I bet that solves my higher SWR readings.
I hope you can find it there...Radio Shack is not what it used to be.. (The days they almost had everything are long gone..) You might call ahead or try some auto parts stores..

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post #12 of 15 Old 04-02-2009, 01:36 PM
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Quote:
Originally Posted by USAntigoon View Post
I hope you can find it there...Radio Shack is not what it used to be.. (The days they almost had everything are long gone..) You might call ahead or try some auto parts stores..
Good advice. You may also want to try Fry's if you have one near you. They usually carry tons of electrical stuff. Heck, even Home Depot may have it.

This has been a very informative thread. I know very, very little when it comes to electronics. I can rebuild a 700R4 trans with my eyes closed but when it comes to properly setting up a CB radio system, I'm clueless. I never knew I should be using a braided ground strap. No wonder my SWR readings suck.
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post #13 of 15 Old 04-02-2009, 05:26 PM
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These guys are a good online source for that kind of stuff.
http://www.walcottcb.com/index.php
I have a Firestick Firefly mounted on my RockHard spare tire rack.

Channel 1 is 1.9:1
Channel 20 is 1.5:1
Channel 40 is 2.1:1

A high SWR (2:1 and above) will shorten the life of the final RF amplifiers.
It will limit transmission distance.
It may cause distortion in the audio caused by spurious reflected RF in the microphone circuit.
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post #14 of 15 Old 04-03-2009, 08:24 PM Thread Starter
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No Ground Plane Antennas

Does anyone use a "No Ground Plane Antenna"? Because I have my antenna mounted on the cast aluminum brake light pedestal; I seem to be having problems with the ground. I checked on buying a ground strap as suggested, but can only get one 2 ft. in length. I need about 6 ft so I can hide it inconspicuously. Okay...I'm real anal about this stuff! Anyway, if anyone can give me some information about No Ground Plane Antenna's, it would be greatly appreciated.
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post #15 of 15 Old 04-04-2009, 01:19 AM
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Quote:
Originally Posted by devildog77663 View Post
Does anyone use a "No Ground Plane Antenna"? Because I have my antenna mounted on the cast aluminum brake light pedestal; I seem to be having problems with the ground. I checked on buying a ground strap as suggested, but can only get one 2 ft. in length. I need about 6 ft so I can hide it inconspicuously. Okay...I'm real anal about this stuff! Anyway, if anyone can give me some information about No Ground Plane Antenna's, it would be greatly appreciated.
Stay away from these "no ground plane" antennas.. Might be helpful for RV's.
Check this link (They have almost everything) and type in the search grounding braid and page 910 should show up, scroll half way down..

The third brake light pedestal can be used, but be sure the contact surfaces are bare material.(The Alu pedestal has some kind of an epoxy coating)
Also, I experienced quite some issues with the "mass" of the spare as this will influence the capacitance of the antenna..If you decide on using the pedestal, try to first use a good braided strap to ground the tailgate to the tub and check the results..

However, if you are as "anal" as I am you might end up with another location...

USAntigoon

Last edited by USAntigoon; 04-04-2009 at 01:22 AM.
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