There are a variety of factors in the range calculations, so I had to make some assumptions--path loss, antenna height, antenna gain, etc. You can't compare your eight mile experience with the numbers I used. Therefore, unless you ran actual range tests and have before and after data, all you may have proved is you increased your range for your particular circumstances from perhaps 7.2 to 8.3 miles. You really don't know (and neither do I, except to the extent that the range calculations reveal--and they are pretty accurate. I'm familiar with this because every military radio installation I helped engineer and purchase used these equations to determine expected radio range).

 

This may be a dumb question, but what is the purpose of the dual antenna setup? I always assumed it was for longer range. What would I gain if I added another antenna to my Jeep?

 

Antennas do not radiate the same level of RF in all directions. If you look at an antenna radiation pattern from overhead (and it's a three dimensional pattern, but we'll just pay attention to two), it has lobes where the signal is stronger and weaker. This pattern is determined by a variety of things, including both the antenna design and the surrounding materials. An antenna on top of a tower is pretty close to perfect because there isn't much material around it. An antenna mounted on your Jeep tailgate, license plate bracket, spare tire, etc, not so perfect. All of that mass of metal will affect the direction that the lobes form--what direction you can talk and hear farthest. Of course, most of this gain in one direction is given up in another direction. So, maybe you can hear people (and they can hear you because this works both directions) on the left side of the Jeep, and not so well on the right side. It should be noted that, in general, the mass of metal of the Jeep will direct most of the energy in that direction--not block it. I know it sounds counterintuitive, but it's RF. In general, you'll have a circular pattern around the Jeep, with a large lobe out towards the front, if your antenna is mounted on top of the spare tire rack. Mount it on the license plate bracket, and that pattern will shirt forwards and out the right, front corner of the vehicle.

So, how, if you're a trucker, can you take advantage of this? Well, for truckers on Interstates, having most of your performance towards the front of the semi, so you can talk to someone further down the highway, is an advantage. It's not so good to be able to talk out to the sides, since most of the time someone you want to talk to is either in front of you or behind you on the highway. So, dual (technically called cophased) antennas work to reshape what is in general a circular pattern (when looked at from the top), to a pattern with large lobes up front and behind. That means you can talk further up and down the highway, but with reduced performance out the sides.

I hope that helps.

 

What Mark forgot to say tho, is that co-phased (dual) antennas only work properly if they are set at a certain distance between them where they work off each other and resonate properly. Normally, on a small vehicle, you can't achieve that ideal distance. So it's pretty much a waste of time.

 

Two weekends ago was my first real use of my 2m ham radio. I was SO impressed I'm seriously considering dumping the CB permanently.

Guys on I-5 were crystal clear at over 50 miles away, but the CB was good up through spitting range (i.e. almost to the Jeep behind me).

I had no idea that ham worked so well.

KJ6DMD

 

Perfect fellas, that makes perfect sense after the explanation I have just seen this setup on a few rigs around town and was trying to understand the purpose. Now I know

 

You know, I had that in there, in fact. But, I went skulking about for an antenna lobe pattern chart to illustrate what I meant (didn't come up with one before I had an errand to run) and while visiting the Firestik website they say that they have experienced increased gain with antennas as close as four feet:

"32. Some people believe that co-phased antennas must be separated by a minimum of nine (9) feet. We have successfully used co-phase antenna systems with spacing as little as four (4) feet. Space alters the pattern and not always negatively. Each vehicle will be different."

From http://www.firestik.com/Tech_Docs/63Things.htm

So, while I'd always heard what you wrote, Dave, I decided to remove my words saying the same thing. I guess folks can try things out and see what happens. When it comes to RF, it's all FM (frelling magic--I've been watching four seasons of Farscape the past few weeks. LOL).

P.S.: Here are some antenna lobe pattern charts to illustrate what I wrote about above. Not for you, Dave, for those who haven't seen one before ...

http://www.vias.org/wirelessnetw/wndw_06_05_05.html

 

Back when I was much younger (still in high school) and I had my ham license, I lived in Chula Vista (the one south of San Diego, not east of Birmingham). I used a home-built, 16 element Yagi array mounted on my dad's old TV antenna pole--probably about 25 feet up. 10 watt 2-meter rig. And when ducting would occur over the Pacific, I could talk 59 to Lompoc--well over 200 miles. It was pretty cool.

In the Jeep, on CB you should be able to talk a mile or two reliably on the Interstate. On the trail, you should be able to talk to anyone you're running with unless you put some obstacle between the two of you (same problem with 2-meters, BTW). It sounds like you're getting more the lackluster performance typical of handheld FRS radios. I'd say check out your antenna system, and if that checks out, consider moving it. The one advantage of CB over ham is that most Jeepers will have one if they're heading out on trails.

 

Yeah, I had a Cobra handheld with an external antenna that was about 12". I wasn't able to hear the lead Jeep in most cases unless I was able to see him. I went to my current setup and have no problems picking up the signals even if they are on the other side of the ridge (depending on the distance).

 

When you get along ridges, knife-edge refraction can come into play and cause a signal that might not normally work to be heard. This is more pronounced on higher frequencies. Like I wrote above: FM.