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Home made glide slope antenna

Don Jones

Well Known Member
Has anyone tried the stripped coax glideslope antenna in a gear leg fairing?
I have been considering where to put one. Thinking either the cowl or gearleg fairing. I don't want to use a diplexer to split my vor antenna to feed the glideslope to my 430. I suspect it might cause the signal to be directional, just thought someone here might have tried it. Paul??
 
I seriously doubt it would work at all in the gear leg fairing. It would be way to close to the gear leg. The engine cowl may work, but it would need to be kept away from metal as much as possible. Neither place is really ideal. It may be worth a try, but a splitter is a lot easier. A splitter will not add directivity to an antenna. What are you doing for the nav antenna?
 
As Bill said, you want to keep it away from metal. Also as horizontal as possible. That makes the gear leg a poor choice.

I use an Archer antenna in the wingtip for localizer, VOR, and GS with an SL30 which has an internal spliter. Works fine.
 
Along roll bar

Why not use a splitter.. they work just fine..

I am not flying yet but plan to do like Paul Dye and run the stripped coax along the forward side of the windscreen, just in front of the roll bar. This has worked well for him so I will give it a try.

Cheers
 
Mine has worked just great tucked along the roll-bar on the -8, and on the -3, we id exactly what you are suggesting (running it down the gear leg) and it has done just fine as well.

For those wondering why the spliter....the wingtip Nav antenna is extremely finicky in real-world installations, and the signal strength it delivers can end up marginal. Split it, and oftentimes, you don't' have enough left to drive the receivers. I started that way on the -8, and finally gave up - the simple home-made GS antenna fixed gave me better GS, and also better VOR range.

Paul
 
Several people have had lousy glide slope reception with the Archer antenna in the wing tip like I have when using a splitter on the Garmin 430. Paul Dye went throuth it as well as Scott Card. Paul ran one up his roll bar and it made a huge difference.
Here is the thread http://www.vansairforce.com/community/showthread.php?t=40633

I don't have a roll bar, so I wondered if anyone had tried the gear leg fairing idea.
 
I used a variation of the stripped coax GS antenna in the RV-8... made a 16" long strip of aluminum, about 3/8" wide and fiberglassed it to the floor of the right wingtip, sitcking as much straight outward as it could get, polarized horizontally. Attached the coax center conductor to the inboad end of the strip and grounded the coax to the outer wing rib. It seems to work just fine with the Garmin 430, cost was virtually noting. The coax was the only cost item.
 
Neal's example shows how sensitive modern receivers are; almost anything will work, if it can see the transmitter without looking thru a lot of metal. 16" is "the wrong" length.

For an ideal antenna (that is, an infinite ground plane instead of the finite wing rib for a ground) the length of the antenna element should be about 9". (Remember the "bow tie" GS antennas, high up on the windscreen of 1970-era Cessnas?). 27" also works, and is what you want for the VOR frequencies.
 
I chose 16" as the length from Paul Dye's stripped coax homemade GS antenna example where he stripped 16" of shield away from the coax.

At the ~330MHz of the glideslope band, an online "whip antenna" calculator gave around 17" length for a 1/2 wave whip. I've got at least an inch of coax center conductor exposed from the shield before it connects to the end of the aluminum strip with a ring terminal and a 4-40 screw and nut.

So... is my homemade glideslope antenna not essentially a 1/2 wave, horizontally-polarized "whip antenna", sticking outward from the end rib of the wing?
 
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I am not an RF expert Neal, but I got the 16" dimension from guys who are...and it was confirmed by several other Ham's and the like here and elsewhere. Bottom line is that it seems to work quite well as a 1/2 Wave antenna.

Paul
 
Supposedly, 1/2 wave whip antennas are more tolerant of odd-shaped, and improper sized ground planes, than a 1/4 wave whip which really needs a properly sized and shaped ground plane "underneath" it to achieve best propagation pattern.... or so I've read on several google search results for antennas. Given that the outer wing rib (and the rest of the airplane behind it) is serving as the ground plane in my example, it's definitely an odd-shaped and wrong-sized ground plane for any whip antenna for 330MHz.

I've also read that if you intend to transmit from a whip antenna, that with a 1/4 wave whip, you don't so much need any kind of impedance matching device between the end of the coax and the antenna itself, but with a 1/2 wave or 5/8 wave whip, you really need some kind of impedance or gamma match to get low SWR.... be we ain't transmitting a thing with these GS antennas so I don't think that really matters much as long as the GS receiver gets enough of an earful of RF to lock on at least 10 nm out.

I'm not really any kind of a serious RF guy either... but back in my teenage years in the late 1970's, I was a CB radio hack and even had a novice class ham license. Today I mostly only deal with licensed and unlicensed microwave network gear (900MHz to 11GHz) and all that is factory-made stuff.
 
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As I said, it's a tribute to how well the receivers work.
A "half wave whip" is a very poor match to 50 ohm coax. Most of the signal will be reflected back. A quarter wave whip against an ideal ground plane is 36 ohms impedance, so a pretty good match to 50 ohm coax. (The non ideal ground in real life makes 50 ohm coax even better).

The Archer antennas are a bent quarter wave.
 
Mine is the stripped coax shoved down the front of my gear leg faring. It works great. I have made ILS landings at numerous airports.
 
Mine is the stripped coax shoved down the front of my gear leg faring. It works great. I have made ILS landings at numerous airports.

Thanks Tom, that's what I was looking for. I figured if it worked next to Paul's rollbar it should work in a gear leg fairing.
 
Another option

For my glide slope antenna, I stripped a section of coax and glassed it into the inside bottom of the cowl horizontally. Works great. I've got a quick disconnect for when the bottom cowl comes off.
 
silly question, but does the striped coax being used for the GS antenna also double for as the VOR/LOC antenna.
 
silly question, but does the striped coax being used for the GS antenna also double for as the VOR/LOC antenna.

I suppose you could. I used the stripped coax in one wingtip for G/S and have the Archer in the other wing tip for VOR. Works terrific!
 
You could put a vor/gs splitter in near the nav radio and not need a seperate gs antenna. The losses in dbm depend on the quality of the splitter.
 
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Antenna which are resonant and well matched to 50 ohm coax (e.g., a 1/4 wave whip over a ground plane, or a half wave dipole (like the cat's whiskers) also work well at odd harmonics, e.g., a 3/4 wave whip over a ground plane, or a 3/2 wave dipole. Since GS frequencies are 3 times higher than VOR frequencies (and GS wavelengths 3 times shorter), VOR antennas also work well for GS reception. The reverse is not true.
A half wave at 112 MHz is about 53", and that's the length (total length) you should make a "stripped coax 1/2 wave antenna" for VOR reception. Also note that GS signals are only of interest when they're more or less in front of you. Using the halfwave dipole taped to the roll bar may give poor reception in the direction of the wingtips (that's why the cat's whiskers are "bent").
 
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