i stopped in a local cb shop and was talking about installing a radio in my truck. he asked if id be using the 18ft coax cable.... i replied i guess so although i dont need near that much to get from my antenna to the radio...... his reply, yes you should use the 18ft cable and dont ball it up under the seat.... lay it out back and forth on the floor under the carpet.. thats better for your swr's............ so anyway my question is........is this correct ?? i hate to question his theory but i just want opinions from guys who know about this cb stuff... thanks guys, scott

 

It's my understanding that the length of the coax has something to do with the length of a radio wave. Longer or shorter and it could be impossible to get the SWRs down low enough.

Also, don't just coil the excess and tuck it under the seat. You will have created a "Choke".
Make a figure 8 with the excess.

 

Yep, the guy was right. You need to use 18' in order to get the lowest SWR (standing wave ratio). If you do coil up your coax you will make what we used to call "an FCC choke". It will limit your range.
Tom

 

So thats what I have been doing wrong these years.

 

18 feet is one half wavelength for the general CB band, but this is not the corrrect length to cut your coax. The "experts" often forget the following. Electrons flow slower thru the wire cable than they do thru a vacuum; about 66% to 80% slower. This called the velocity factor of the cable. THis figure is available for each type of coax used. RG-59/U (foam) coax has a fixed velocity factor of .79

Here is how to figure out your true 1/2-wave:

492 x (Velocity Factor) / Frequency (MHz)



So, the true 1/2 wavelength for the CB band is about 14 feet or so depending on the channel. You can add multiples of 14 feet for longer lengths. Trust me on this, cut your coax using this method and your CB will get out.

 

Uhoh.

I didn't cut mine, I think it was 18ft as Firestik recommended, but I do have the excess kinda coiled up next to the fuse box under the hood.

Maybe that's why I can't get my SWR down below 1.8:1.

I did what Firestik recommends, and serpentined it like a skein of yarn, then cable tied it in the middle though.


phox

 

Unless the firestick has a coil to match the impedance, you will be running a 2.1 to 1 mismatch, which is normal for a stick antenna. Differences in the coax length will make a difference to show your current swr.

 

I don't know much about CB's. All I know is I have dual firestick 3' antennas mounted on opposite sides of the bed with an 18' coil just kind of snaked under my carpet. I had it professionally tuned and they guy had no problems with it. My Radio Shack CB picks up great reception. Now, if I could only figure out what the RF gain is supposed to do, I'd be in great shape.

 

The RF gain just cuts down on the strength of the signals if you have a bunch in there real loud, you can cut down on the overload of the receiver. You know how they talk in a truckstop from truck to truck, you can cut that down so that you can here what they say.

If firestick packs the coax with the antenna, thay have matched it. The coil is bad though, creats a choke that stops rf from coming through. Put it any way except a coil. Sorta does what the rf gain does