I have been working on my CB installation. Up to now, I have been using a magnet mount antenna with my Cobra 75, but wasn't real happy with performance. I learned several things along the way:
1. These things really do perform differently on different cars. I had used my rig on several rented sedans over several months, and tested Standing Wave Ratio (SWR) on a couple. It always tested OK (<1.7). When I finally got the Rubi, I didn't have time to check, just threw it on and went to a Jamboree where it seemed to work OK. When I did check the SWR (a measure of how much of the transmit energy is reflected back into the transmitter by the antenna system), it was well above 2.8 on every channel (should be <2.0). Not very good!
2. There is some magic to the length of the coax between the radio and the antenna. It has to be 18' to avoid unnecessary reflection of transmission energy. I coiled the excess in a circle, then squashed it into a figure 8 with a cable tie. According to the technicians, a simple circle causes reflections by induction, and, the larger the loops, the better the performance. My looped excess is about two feet long. Also, the High quality coax is only slightly more expensive than the lesser stuff.
3. Based on recommendations on this forum, I thought I wanted a 4' FireStik, but rather than leave it to chance, I went to a local CB shop and took their advice. I came away with a 5' PowerStik, quad wound, top loaded antenna, and went home to install. After driving around for a week, I was not happy because the 5' antenna I ended up with was so long above the Jeep, I knew it would be catching on over-hanging branches. I went back to have them tune it and traded it for one that was a foot shorter. According to the technician who tuned the antenna, the shorter one actually performed better than the original. The final antenna is a PowerStik, dual wound, top-loaded, 4 footer. It is mounted so the entire top-load coil is above the roll cage.
4. As you may all be aware, an antenna's performance is best in a straight line across the center of metallic mass of the vehicle. I mounted mine on theright side just ahead of the seam in the body tub. I hope I am taking advantage of the additional strength of the tub at this point due to the flange at the seam. This location means that my best performance will be ahead and to the left of the vehicle. This is probably the best I could do for highway travel, but that is not my primary use. However, knowing the direction of best performance will allow me to point my car in an appropriate direction when extra performance is needed.
5. You must be careful not to overheat the coax connector when soldering it in place. I tried to solder the first one with a torch, but found the center conductor was shorted to ground. Had to cut it off and try again, this time with a solder gun with more moderate heating capabilities.
6. Had lengthy discussions with two technicians regarding the pros and cons of the Cobra 75. One hated it. The other thought it was about the best for our use and environment. Apparently, the circuit boards are shock mounted withing the hand-piece and epoxy coated to protec the components. Both techs agreed that the use of the squelch control limits performance. They would prefer an RF gain control, instead.
BTW: My install includes a quick-disconnect which allows me to remove the antenna easily to put the car in the garage.
My believe my radio performance is much improved, but I really don't have any quantitative measurements. Having the system tuned professionally ($10) lends an illusion of respectability. I will be at the Red River, NM, Jamboree this weekend where I hope to get further evidence regarding performance.
I am no expert, merely a neophyte, sharing my research and conclusions. Please contribute any additional information you may have.
One thing that I had always been taught all through my electronics training with radios is the longer the wire/antenna, the less power actually got radiated out to broadcast with. So, I don't see how an 18' coax is best for CB usage when you only need 9' from the radio to the antenna. Now, given the shorter wire length your "load" is higher per say thus giving you a higher possible SWR. My antenna is a 5' firestik which I believe is a bottom loaded antenna and I have it mounted on the left rear above the tail light at tub level. The height of the antenna base relative to the vehicle/metal mass's plane greatly affects the amount of reflected energy as well. For all intents and purposes I don't think you can get much better in our usage due to water, trees, and terrain obstructions. I think VHF may be a better way to go, but thats a whole nother money pit.
For all radios you want the lenght of your antenna feed line to be a multipy of the transmit frequency.....That's why the standard CB comes with the 18 foot cable. Jeeps are hard to get good radio performance out of then most vehicles because of the fiberglass roof/soft top. In a standard car you have the roof which acts as a ground plane and greatly boost performance. There are different wave lenght antenna that have some characterists that can be used to great advantage....
1/4 wave antenna have no gain but can be used across a wide bandwidth
1/2 wave antenna does not require a ground plane....I use this anntenna for my VHF radio on my Jeep
5/8 wave antenna has very good gain so you will get increased transmisstion range but it reguires a ground plane (side mounting this anntenna will result in better transmission across the ground plane but with poorer results away from the ground plane.
Full wave anntenna....too long for vehicle use.
I have been told by a few "experts" that the loss of signal due to the combination of a longer cable and having to coil it up (even in a figure 8) is at least as bad as having a shorter cable, so you might as well use what fits your vehicle.
I heard from others that mounting an antenna on the rear above the taillight reduces reception because of the roll bars. I have mine mounted off the third break light and dont get that great of reception on the highway. Also to get the lowest SWR my antenna tip (3' firestik)is tuned all the way down. I will be taking it to a run this weekend to see how well it performs on the trail and let you all know. I do run with the 18' of cable not wound into a circle but rather just run back and forth above the wheelwell. Hopefully I wont have a problem on the trail as this is what I really want it for. I have a cell phone for road emergencies.