New Antenna Placement - Above Rollbar

I like the idea of having the radio behind the instrument panel so I tried to mount the J-pole antenna where I could disconnect/connect to it from the cockpit. I mounted the antenna by taping it to the windscreen at the top-most point, where it meets the rollbar. I only did one flight, but it doesn't look like it did too well. From the track you'll see that only every third or fourth point was received. I guess this makes sense since only a small portion of the antenna (roughly 1/3) is visible laterally - the rest is blocked by the steel of the roll bar.

Maybe it would work better if the antenna were taped to the inside curve of the roll bar, exposing maybe 2/3 of the antenna. The performance of the antenna when taped to the canopy behind the pilot's head was quite good, so these results were disappointing. I keep thinking that mounting the antenna in the wingtip should leave it blocked by the body of the airplane for one lateral direction, but Pete's results show that that is not the case. I guess that is where I need to put it.

Rick Grenwis

http://aprs.fi/?call=ka4hrd&mt=m&z=13&timerange=10800

Antenna Basics

Rick,

I'm not sure I understand how you taped your antenna in the plane, but it sounds like it's pretty close to the roll bar. Rule #1 when it comes to antennas is: don't put them anywhere near metal. Really your antenna should be about 19" away from any metal. That's probably not too practical, but if your antenna is within a few inches of metal, that's probably most of the problem.

The J-Pole antenna is nice if you don't have a ground plane (aka big sheet of metal) to work with, but it is about 3x as long as a 1/4 wave whip which makes it pretty tough to squeeze into an airplane, especially if you're trying to keep it away from other stuff while maintaining a vertical orientation. I think it's safe to say that Sam's 1/4 wave whip installation is about as good as it gets; if you don't want to put any holes in the plane, and you really want to avoid the wingtip antenna, you might try a little experiment with some coax. Strip the insulation back about 19 1/2"; cut the braid off at 1" and attach the braid to a nice ground point (Canopy rail, seat back, etc...)somewhere near the middle of the canopy. Tape the ~19 1/2" long center conductor up in the canopy as far away from metal as possible (Keep the end of the antenna a couple of inches away from the canopy frame). It won't be too pretty, but it will be removable, and I bet it works better than anything you've tried so far.

YMMV,
Paige

Bad antenna installation

Thanks Paige,

That's much more information than I ever understood about the way that antennas function. Here's another try at explaining how the antenna was mounted, and you'll see that you're exactly right about the problem.

The windscreen meets the 2" diameter rollbar and is riveted together at their tangent point of contact. In the V or the notch formed at their intersection, I pushed the twin lead as far as possible into the notch and taped the twinlead flat against the inside of the windscreen. Therefore, as you describe, one leg of the twin lead runs parallel to the steel rollbar, separated by only the insulation. As you describe, that, um, is less than optimum. It's amazing it worked at all.

Rick Grenwis

Rick, I hope that you heed good advice given

You may need to try your system in a repeater poor region soon so get it working right.

Paige, would that coax antenna option work horizontally in a wingtip?