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CB and SWR tech...


james750
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I have been trying to tune my CB antenna all afternoon. When I started, the SWR for both channel 1 and 40 were right around 3. I wanted to lower that a little. First I checked all of my grounds, they were good. Then I double checked that everything was tight. it was. I made sure that the coax was not touching any grounded out pieces that it shouldn't be. then I went and checked my SWR again. and it was WAY higher. It is almost pegged all the way at the set side when I flip the switch to REF. Any suggestions. I am really confused right now.

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I have a Cobra 19 ultra III with a firefly 3' tunable tip antenna. I have a hood mount that I got from Quadratec. I am just not sure how the measurements are about 3 one minute and pegged to the left after tightening everything down.

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When tuning your antenna, park in the middle of a large deserted parking lot. Any structure nearby, like building and even cars will affect it. Despite what dealers tell you (they want to see more to make more $$$), length of cable has no effect on SWR. At least not measurably with my equipment. A good ground plane is a must, an even ground plane is better. Best spot on a Comanche is middle of the bed, second best is middle of the hood, third best is middle of the roof. A 5/8 wave whip (about 10 feet long) will almost universally give you a low SWR. These antennas can't be tuned and don;t need tuned. Most people opt for a shorter antenna, often fiberglass. These must be tuned. The longer the antenna, the better usually, but quality also plays a large factor. My 3' Firestick is better than my 4' Radio $#!&. And you want as much of the antenna to be higher than anything surrounding it. For this, the roof is the best spot.

 

Also, the little plastic cap on top of the antenna makes a difference. Remove the cap, move the adjustment screw, REPLACE cap, test SWR. My SRW with the cap is currently 2.6 at channel 12. Best I can get with the antenna in the stock radio antenna location on the fender (going to be moved to roof at some point in time, tough). Remove the cap and it jumps up to 3.3.

 

A 10 foot whip gives me an SWR of 1.2, but looks just too damned long. Bending the tip down with fishing line makes it shoot up well into the 2s.

 

Some of this is theory, most of it I have personally tested or is from personal experience.

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Thanks for the tips Mvusse. I have had the tip on my antenna for tuning. I am thinking that I may have a bad SWR meter. This is my first use with it so I don't really know how to tell. But I don't think that the SWR should be reading all the way pegged to the right. I will try too reassemble my old magnet mount antenna that doesn't need to be tuned and see if the SWR meter gets the same reading on that. At least that should give me a good idea of if my meter is bad or not.

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One other thing about cable. If you have a lot of excess cable, it's my understanding that you should NOT neatly coil it up under the seat, because the coils create impedence (or feedback, or interference). I don't know the right term, but I remember reading that when I was putting a CB in my XJ a number of years ago. Ideally, don't use an antenna cable longer than you need. Not because of loss strictly due to length, but due to the interference generated when you try to stow the excess.

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One other thing about cable. If you have a lot of excess cable, it's my understanding that you should NOT neatly coil it up under the seat, because the coils create impedence (or feedback, or interference). I don't know the right term, but I remember reading that when I was putting a CB in my XJ a number of years ago. Ideally, don't use an antenna cable longer than you need. Not because of loss strictly due to length, but due to the interference generated when you try to stow the excess.

 

 

You are correct... If you have an abundance of coax you do not want to coil it. If it is coiled it will create a choke and drive the SWR up and lower the effective power levels. When I worked for a CB dealer we saw more people that would go to the truck stop, buy 18-30 foot premade coax rolls, have all this extra coax neatly coiled up and bundled. And then wonder why their SWR was terribly high. Getting the correct amount even if that means having a custom length made generally ends up being cheaper and better in the end.

 

 

 

To the OP.. how well grounded is your new antenna mount? That should always be the first place to start chasing a wonky SWR reading. Many times running a body ground to the base of the mount will clear up a problem.

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When tuning your antenna, park in the middle of a large deserted parking lot. Any structure nearby, like building and even cars will affect it. Despite what dealers tell you (they want to see more to make more $$$), length of cable has no effect on SWR. At least not measurably with my equipment. A good ground plane is a must, an even ground plane is better. Best spot on a Comanche is middle of the bed, second best is middle of the hood, third best is middle of the roof. A 5/8 wave whip (about 10 feet long) will almost universally give you a low SWR. These antennas can't be tuned and don;t need tuned. Most people opt for a shorter antenna, often fiberglass. These must be tuned. The longer the antenna, the better usually, but quality also plays a large factor. My 3' Firestick is better than my 4' Radio poo. And you want as much of the antenna to be higher than anything surrounding it. For this, the roof is the best spot.

 

Also, the little plastic cap on top of the antenna makes a difference. Remove the cap, move the adjustment screw, REPLACE cap, test SWR. My SRW with the cap is currently 2.6 at channel 12. Best I can get with the antenna in the stock radio antenna location on the fender (going to be moved to roof at some point in time, tough). Remove the cap and it jumps up to 3.3.

 

A 10 foot whip gives me an SWR of 1.2, but looks just too damned long. Bending the tip down with fishing line makes it shoot up well into the 2s.

 

Some of this is theory, most of it I have personally tested or is from personal experience.

 

Yes, 1/2 wave or 5/8's wave antennas are great. Techinically speaking a full-wave antenna would be best. But the size would be stupid long. The length of a full-wave antenna for Channel 1 would be 36.467 feet or 18.225 foot for a 1/2 wave antenna. So the 108" antennas that are seen would be a 1/4 wave antenna. Another issue you are going to run into is that organized rides are now requiring these to be tied to down to prevent their flopping about. Which pretty much negates any extra usefulness from them.

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I need to uncoil my coax. I will do that tonight. I am not sure what to do with it though? Any ideas? Where do I get a custom lengh coax? I need a standard connection on the radio side and a loop end on the antenna side. I suppose I could move the antenna to the other side of the hood to take up some of the length of my 18' coax.

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I need to uncoil my coax. I will do that tonight. I am not sure what to do with it though? Any ideas? Where do I get a custom length coax? I need a standard connection on the radio side and a loop end on the antenna side. I suppose I could move the antenna to the other side of the hood to take up some of the length of my 18' coax.

 

Hit the local Radio Shack and get yourself a cable end. Depending on your cable its likely RG58, but if you have big stuff, RG8. cut, install end and you should be good!!

 

CW

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I found an open circuit in the hot wire of the coax. That explains why the tuning didn't help at all and why channel 1 and 40 had the same readings. I emailed quadratec to see if they will replace the cable since it is fairly new.

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