Firestik kw-7

You're right in that the length would make it easier to tune. At 7ft length, it's nearly the "ideal" standard quarterwave length (108") so nearly none of its electrical length has been compensated for in its physical length by using a loading coil.

It will present a pretty big sail area, though, and a few members have mentioned problems with the single perch mount stripping out (or out of) the threads on the body. Plus, even with its extended length, there's no more metallic ground plane on the cab, so it'll still be inefficient. Less inefficient probably, but still not ideal.

One way to get around not having a decent ground plane is by use of a proper capacitance hat, adding series capacitance to the radiating element. As the physical length gets longer & longer, the needed hat size shrinks, until you get to the length of a halfwave element, and the hat is no longer necessary. Unfortunately, for CB (11 meters) the physical length at that point is around 19ft, problematic to say the least.

So you *may* get better performance than a 4- or 5ft antenna, but still have no ground plane to "work against."

Use the Forum Search utility to look for "ground plane" as a topic. There've been a few workable solutions (done to death, actually) including one by yours truly that suggests an easy & affordable way to make a vertical dipole out of a mirror mount (a perch should be OK, too, if the threads are tight enough.)

When in doubt, revert to the standard antenna formula: "As much resonant physical length as you can get, as high as you can get it, to the most ground plane at the feed point as you can manage, as far from vertical metallic elements as you can get it."

Either that, or something else. <--------- (Handlebar's Universal Disclaimer, ®1981 Handlebar Enterprises Int'l, Inc.)

 

Some think the passenger side is better because there is less movement from the door opening and closing.

The big problem is the antenna gets hit more often than it is on the driver side. Secondly, the antenna is blocked by the trailer when it is on the passenger side.

Leon
kc0iv

 

As long as you simply tune for the lowest reflected voltage on the SWR meter(s), then the actual numeric value doesn't matter much. The coax length 18ft myth is pretty much that: it assumes a 100% "velocity factor" for the coax, and that just doesn't exist. The idea for the measured-length purists is to accurately show the value of the SWR at the meter, but in truth the coax would have to be substantially shorter for that to hold. And it's nearly impossible to get the kind of precision in the length that's computed to be dead-on; a small deviation would invalidate any measurements except (you guessed it), using the shortest practicable length. I actually leave a foot or so at the radio end to allow for getting an antenna analyzer (or SWR meter) between the radio and the coax, and for having enough slack for the folks who are constantly measuring their SWR or power output to eventually work the shield loose at the connector, and then replace the connector a couple of times, as needed.

On the other hand, as long as you've got enough coax to reach from one end to the other, then virtually any excess length (within reason; don't use a 100ft chunk for a 15ft run) won't effectively reduce power output or receive sensitivity. The value of loss can be computed easily, and with sufficiently advanced service equipment it can be measured, but in the real world (on the air) it'll never be noticed on either transmit or receive. I wouldn't cut off a foot or two of coax and then have to buy and correctly install a new connector just to gain a small fraction of the signal. Likewise, cutting off a perfectly good nickel plated connector in favor of one with silver and/or gold plating will never be noticed at frequencies as low as CB lives in. Silver *is* easier to solder, though.

RG-8 in its various forms (including RG-213) will only benefit you when higher power (or much higher frequency) is being used. At my house, I feed a multiband vertical antenna 110 feet away from the shack window with a length of buried RG-8x with almost no power loss compared to RG-213, and it's way cheaper. It you're moving up above a couple hundred watts, which remember will not benefit your received signal a bit, I'd think about something larger, but for anything below 50 ft and 50 MHz you'll be hard pressed to do better than RG-58.

Using the passenger side for a side mount above a symmetrical ground plane will slightly favor signals towards and from the driver's side, but it'll also put it closer to overhanging thingies like tree limbs and pole mounted traffic control devices. Without any real ground plane, like on a lot of plastic cabs, you'll have a hard enough time trying to achieve a decent match than any theoretical gain towards oncoming traffic in the lane to the left.

Before you start tossing out perfectly good coax, I'd suggest trying that CB out with a known good install on a pickup or passenger vehicle and see if you get a better match there. If you do, then something is caddywampus (technical term there) with your antenna system. If you have the luxury of a real coax connector at the antenna end of the coax, fasten a calibrated dummy load on the antenna end of the coax and see if you get a better match. If you do, then it means the radio, coax, and meter are healthy. If it's still bad, on the other hand, then something is amiss; put the dummy load on the antenna end of the coax and put the wattmeter right after the radio. If it's still bad, you have bad coax -- at the very least. You can't rule out a bad antenna until the coax is good, though, or if you can swap your antenna onto the vehicle that you used earlier to test your radio on the air. If *that* system goes from great to terrible just by swapping your antenna for his/her/its antenna, you've found the likely culprit.

Just remember to use the scientific method: change only one variable at a time, or you'll be chasing gremlins forever.

Hope that made sense. I was reluctant to throw in the term "caddywampus", but you seem technically aware enoughto handle it

73

 

Here is my last post in a thread I started about the Cascadia. I am making a support out of Lexan to stabilize the antenna. It should work. A top loaded antenna in the 7ft range is the way to go on these trucks from what I've seen so far.

http://www.thetruckersreport.com/truckingindustryforum/cb-radio-forum/249124-cascadia-antenna-solutions.html

QUOTE:
"Well I was in W. Memphis Thursday and stopped in at Ray's CB Shop. I bought a 7ft SkipShooter ($35) and installed it on the mirror perch mount. The SWR was below 1.5:1 from 26.515 (40 channels below ch. 1) to 27.405 (ch.40). O.K. I'm happy with that! However, this antenna is too flimsy for it's length. It needs to be made stiffer. It flops around in the wind so much it won't last very long unless you support it somehow a few feet above the base. I will try to come up with something to do that and give it a try. Beware if you use automatic truck washers.......you will have to remove this antenna before running it thru the wash bay, or risk breaking it.

I leaned it forward some so that at 65 MPH it was pretty much standing straight up. If I left it straight up, it would blow back past the mirror and seemed flop around even more. Incidentally, the mirror has no effect on the SWR when the antenna gets close to it (the mirror is mostly plastic); but if the antenna moves toward the body of the truck, the SWR goes up dramatically. So when using the perch mount, move the antenna as far away from the truck body as possible."

 

Longer is better and getting the "load" above the cab is key. When that antenna goes south, try a longer one. Mine (the 7ft) tuned under 1.5:1 over at least 80 channels...that was all I checked so it would be even more broad banded that that........You didn't say what your SWR was over the the 40 channel CB band?