Do short antennas work ?

[PtownBubba]

A friend here has a Cobra 200 mobile, and is putting on a pair of co-phased antennas on his sons pick-up,,,just like discussed here, but they realize that co-phase is not real efficient on a pickup,,, so one antenna is just a dud,, not hooked up. They ran a single coax to the right side antenna. These antennas are mounted in the stake hole behind cab. Now,,,, [ ugh ] they want to use 2 foot fiberglass antennas, so truck can go in and out of garage without hitting.

Help me out here,,,,,, I do not think 2 foot antennas are good to use "anywhere",,, especially when married to a high horsepower radio like the Cobra 200,,,,am I wrong ?

The 2-footers will just barely stick above the cab's roof, if at all.

Not good ,,,, need 60% or more antenna above vehicle, right ? Even if they somehow get the SWR down to a decent level,,, the short antenna,,,,, mounted low,,, behind the cab,,,,, can not rock and roll,,, can it ?? It is their truck, so I just kept quiet,,, but hate to see them work at it and end up with a crappy set-up. If they insist on the stake hole mount,, I would use a "normal" or longer, good antenna,, with a quick disconnect,,,, or,,,,,,,,, the ole' 102 SS whip, and just clip it down when going in garage. Has anyone had success with 2 foot antennas ?

[Sandbagger]

Apr 21, 2008 19:02:17 GMT -5 PtownBubba said:

A friend here has a Cobra 200 mobile, and is putting on a pair of co-phased antennas on his sons pick-up,,,just like discussed here, but they realize that co-phase is not real efficient on a pickup,,, so one antenna is just a dud,, not hooked up. They ran a single coax to the right side antenna. These antennas are mounted in the stake hole behind cab. Now,,,, [ ugh ] they want to use 2 foot fiberglass antennas, so truck can go in and out of garage without hitting.

Help me out here,,,,,, I do not think 2 foot antennas are good to use "anywhere",,, especially when married to a high horsepower radio like the Cobra 200,,,,am I wrong ?

The 2-footers will just barely stick above the cab's roof, if at all.

Not good ,,,, need 60% or more antenna above vehicle, right ? Even if they somehow get the SWR down to a decent level,,, the short antenna,,,,, mounted low,,, behind the cab,,,,, can not rock and roll,,, can it ?? It is their truck, so I just kept quiet,,, but hate to see them work at it and end up with a crappy set-up. If they insist on the stake hole mount,, I would use a "normal" or longer, good antenna,, with a quick disconnect,,,, or,,,,,,,,, the ole' 102 SS whip, and just clip it down when going in garage. Has anyone had success with 2 foot antennas ?

You are not wrong in your thinking. Generally speaking, the shorter an antenna is, the less surface area there is to radiate, which translates into poorer performance. Add in the effects of close proximity to a mass of metal like the truck's cab, and not only do you have a less efficient radiator, you also have a distorted radiation pattern.

The most effective antenna for a mobile is a full 1/4 wave (9') whip. The best place to mount an antenna is at the highest point on the vehicle. Practical considerations force us to compromise on these two points a bit. To reduce the problems with hitting obstructions, we can either run a shortened antenna (as long as possible without striking overhead objects) in the center of the highest point, or we move the full length antenna to a lower mounting point. Each method will cost us in performance in some way. Keeping a shortened antenna in the middle of the highest point will keep a fairly uniform omni-directional pattern, but will lose some radiation gain. The full 1/4 wave mounted on, say, the rear bumper, will give you full gain, but the radiation pattern will be lopsided and favor one direction at the expense of others.

Like they say, there's no such thing as a free lunch. There are always compromises to be made. You have to decide which way makes more sense.

[PtownBubba]

Thank you S-bagger for your reply. I sure don't think the guys will be happy with the performance of a 2 foot antenna where they mounted it. They normally use mag mounted antennas, I also think that with high powered radios, sometimes a mag mount can get you in trouble. I guess they need to decide, if they want the garage door clearance, they will sacrifice performance. I think they feel if they get the swr down to say 1.2 or better, they can transmit fine. I do not think they will see that low of swr behind the cab anyway. Oh well,,,, thanks again for your great info !

[JesseJamesDallas]

Heck...I would let them go a head and use the 2' antenna's...Tell them they sound "Loud & Proud!" Then go and chalange them to a "Key Down"...Winner take all! ;D