Rubicon Owners Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
186 Posts
Discussion Starter #1
I have a Firestik 3' antenna and one of the hood mounts, that allow mounting under the front corner of the hood. I can mount it on either side. Any thoughts on which I should use? I thought passenger side so that both antennas are near each other so its easier when avoiding branches etc. Then I thought that maybe they might interfere with each other so maybe the drivers side would be better. Or should I just mount it to the spare tire mount? I also read that 18' of coax is best. With the hood mount I would have lots of excess. Is 18 really necessary? Thanks
Stevo
 

·
Premium Member
Joined
·
2,772 Posts
A hood type mount gives the antenna a more omni-direction pattern for transmit and receive, and IMHO, is better than a tire mount.

If you mount the antenna too close to your AM/FM receiver, you will definately feed some of the CB's 27 MHz transmit into it's antenna. Though, as long as it is not too close, and you haven't increased the CB's output power, it should be fine. I would mount the antenna on the drivers side.

When you tune the antenna, tune it on Channel 19 (27.185 Mhz) since this is closest to the geometric center of the 40 channel frequency band.

As for the coax length, I would leave it at 18' for best antenna to tranceiver matching - it helps with tuning when poor ground planes are used. If you have a good ground plane, it can be cut shorter, but it will probably change your VSWR.

NOTE: Theoretically, the coax can be cut to any length and still be 50 ohms, however, the analysis calculations assume an infinite length and when cut too short, the measured results do not match the theorertical.

Have fun

Ken
 

·
Registered
Joined
·
1,464 Posts
You can use a premade 9 foot cable. You can actually cause more problems with coiling up excess cable trying to keep the mythical 18 foot length. Buy your radio, mount your antenna and use whatever length cable you need and then get an SWR meter and tune the antenna. You'll do just fine.


And for more info, read up on CB's over on Stu's site here:

http://www.stu-offroad.com/
 

·
Premium Member
Joined
·
2,772 Posts
I agree with coiling the coax, too tight of a coil (or kinking) can cause damage to the impedance characteristics of the coax. This will create a high VSWR due to an impedance mis-match between the transceivers output impedance and the antennas feed-point impedance.

You can run a shorter length of coax if you have a good ground system that makes the 1/4 wavelength antennas feed-point appear very close to 50 ohms, however, if the ground plane is minimal, the extra length of coax typically gives a better tuning range for the antenna than too short a coax length will.

If you are using a 1/2 wave antenna system, and the coax is part of the tuned system so that a ground plane isn't needed, then the coax should not be trimmed - it is acting as part of the dipole.

Maximum power transfer requires an almost perfect matching of antenna feed-point impedance, transceiver output impedance, and coaxial cable impedance.

Ken
 

·
Registered
Joined
·
186 Posts
Discussion Starter #5
Appreciate all the help. Will go drivers side, and make sure I get the antenna tuned properly.
Stevo
 
G

·
Guest
Joined
·
0 Posts
If you haven't already installed the hood mount, make sure you mount it close to a part of your fender that is braced well. The closer to the firewall, the less the amount of support it seems the fender has. I installed this type of mount (by firestik) on my old TJ and removed it shortly after. I have been very happy with the Turbo City mount that I installed instead.
 
1 - 6 of 6 Posts
Top