View Full Version : CB Antenna Wire Question
Okay so I know that for whatever reason, 18' of cable is common and have read it's the "best" length.
The problem is - it's not long enough. I have to go from the left side of my bumper where the antenna mount is to the right side because the hinge is there. I went in through the rubber boot behind the taillight. Then I have to go back to the drivers side under the plastic molding, and along the driver side since apparently the passenger side could cause interference problems due to fuel pump.
I run out of cable just behind the driver seat, so I need to add about 10' - what's the drawbacks, if any to this?
Even if I mounted an antenna tab on the passenger side of the bumper, I still wouldn't have enough to reach. The only way to realistically reach would be to do a tab mount on the drivers side taillight area, and go in through there.
ChefTyler
03-13-2009, 11:29 AM
The length of the antenna cable has to do with the wavelength that CBs transmit on, I've always read go in increments of 18' and every connection you make (radio to cable, cable to cable, cable to antenna) creates the possibility of introducing noise to the system so I wouldn't add a 10' piece, I would have one made that's the proper length.
Hmm...yeah I thought about adding another 18' piece. I'd have to coil some up, but oh well.
I just want to get this FINISHED!! Ugh.
I see...I read somewhere about the wavelength being 108"....
18 x 12 = 216 / 2 = 108
Makes sense.
And IIRC someone here mentioned that 108" was wound onto the antenna as well.
MisterTinkle
03-13-2009, 11:59 AM
CB frequencies are about 27MHz. So, the wavelength is about 11M, which works out to about 18' 1/2 wavelength. Antennas lengths are typically 1/4, 1/2, 3/4, or full wavelengths of the transmitted signal. Same thing for the cable run. So, 108" is equal to a 1/4 wavelength. Any longer antenna (1/2, 3/4, or full wavelength) would just be really long on a vehicle. But it would definitely attract attention!
CurleyMan
03-13-2009, 11:59 AM
That's just the whip Odin. I know you want to do it yourself but get your ass over to the CB shop.
That's just the whip Odin. I know you want to do it yourself but get your ass over to the CB shop.
I'm going on Saturday - I just want to know what I need to do - if an 18' additional cable would work, then I need to buy that from that guy, and since I'll be there anyway I'll have him tune it.
I really just wanted to get the cable routed and done so I could have it ready to be tuned for the guy.
Chainsaw
03-13-2009, 04:44 PM
Yeah get one cable that is long enough with out joints. Dont "coil" the wire you will get way to much noise, either run it in a manner that will take up the slack or if your really lazy make large figure eights.
By coil I didn't mean in a circle.
JeffW
03-13-2009, 06:23 PM
The wavelength is actually closer to 36'. You antenna system is a dipole with 18' of cable and a fixed electrical length of the antenna. Both are 50 Ohms. Adding cable in 18' increments will help maintain a low SWR.
A figure 8 is considered better if you have excess.
I busted my antenna off last time on the trails. I think I'll get a new one at the truck stop.
Eric P
03-13-2009, 06:56 PM
The problem is - it's not long enough. I have to go from the left side of my bumper where the antenna mount is to the right side because the hinge is there. I went in through the rubber boot behind the taillight. Then I have to go back to the drivers side under the plastic molding, and along the driver side since apparently the passenger side could cause interference problems due to fuel pump.
Run the cable straight from the tail light through the hatch floor under the carpet to the transmission tunnel. Then follow that to your CB. I did that and had about 2' extra of an 18' cable. But my CB is in the middle of the swing arm & the CB box is located on the passenger side of the tunnel near the firewall.
There was noise from the fuel pump, but wrapping the section near the fuel pump with aluminum foil made most of it go away.
The CB you have (and I have) is not very good at filtering out noise & having it near your stereo doesn't help.
theking
03-14-2009, 03:28 AM
I far as I understand, matching the coax length to wavelength is a myth. You just need to make sure your coax impedance matches that of your antenna. i.e. 50 ohm rg8 coax to a 50ohm antenna. Make the cable as short as possible to minimize loss, make sure the antenna has a good ground to the chassis, and position the antenna for an adequate ground plane.
Yup talked to the guy at the CB shop - said length didn't make any real difference as long as there is a good ground.
johngraves2
03-14-2009, 02:58 PM
What CB shop did you go to? Yellow pages hasn't been very helpful..
Alpine Spirit
03-14-2009, 03:57 PM
Radio Service Center
Google Link (http://maps.google.com/maps?f=q&source=s_q&gl=us&q=CB+Store+Sheridan+Denver,+CO&ie=UTF8&hl=en&sll=39.759337,-105.034584&sspn=0.248840,0.232086&ei=0Rm8SbiuIYmsNvPC6cYO&cd=2&cid=39699849,-105053213,1548104362602852845&li=lmd&z=14&t=m)
918 S Sheridan Blvd
Denver, CO 80226
(303) 922-8107
MisterTinkle
03-15-2009, 09:50 PM
I far as I understand, matching the coax length to wavelength is a myth. You just need to make sure your coax impedance matches that of your antenna. i.e. 50 ohm rg8 coax to a 50ohm antenna. Make the cable as short as possible to minimize loss, make sure the antenna has a good ground to the chassis, and position the antenna for an adequate ground plane.
Yup, that is right. It is really a matter of getting the antenna tuned properly because that is where the energy is emitted. Kevin's has good information for a good CB shop.
johngraves2
03-16-2009, 03:16 PM
Odin did you get it all figured out?
Yes and no. Apparently the ground wire I was using isn't good enough for AC power. So I gotta figure something out. Sand off the paint on the bumper on the tab mount so it will get a good ground.
Geeman256
04-08-2009, 09:15 PM
http://www.cbradiomagazine.com
Here's a real good place for info on CBs. How tos on getting the most out of your rig. How to Mod the radio. All kinds of good Info.
johngraves2
04-08-2009, 10:50 PM
Any update on your cb. is it all hooked up and working now? I saw your antenna is mounted in your new tire pic on the other post.
It's hooked up, but only semi-working. Ground plane issue. I'll be working on it. I need to sand off some paint from the bumper so it gets a better ground. Apparently my idea of a ground wire was not so wonderful after all and doesn't work so well with A/C power. I'm used to working with D/C power.
So I'll sand that off soon and see how it does - if that doesn't work I may have to do a body mount by the tail lights.
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