Where to mount a transponder antenna

[RobO]

I am trying to figure out a good place to mount my transponder antenna and its associated 5-1/2 inch ground plane on my Dominator. For those of you with transponders, where did you mount your antennas and why there?

Thanks,

Rob

[iconnary]

Out of curiousity, why the need for a transponder? As I understand the regs, you only need a transponder for Class A/B/C airspace. I can't imagine a gyro even getting close to Class A (FL180?). Class B and C airports seem like they'd be far too busy to tolerate an aircraft with a top speed of 80-90mph.

Does anyone here fly out of a Class C airport?

Just curious,
Iven

[gyropilot]

Out of curiousity, why the need for a transponder?Iven,

Per FAR 91.215(b)(2), I was required to install a transponder on my GyroBee because I operate within the 30nm "Mode-C veil" around SeaTac Intl. Airport (even though I don't fly into Class A, B, or C airspace).

This requirement applies to any of the 35 airports currently listed in Appendix-D of FAR Part 91.

Atlanta and Denver International Airports are two of them.

Regards,

John L.

[iconnary]

Ah yes, the mode C veil. I shame my instructor by having forgotten about that already (just got my PPL last month).

However, the FAR/AIM gives this exception to the mode C veil :

"However, an aircraft that was not originally certificated with an engine-driven electrical system or which has not subsequently been certified with a system installed may conduct operations within a Mode C veil provided the aircraft remains outside Class A, B or C airspace; and below the altitude of the ceiling of a Class B or Class C airspace area designated for an airport or 10,000 feet MSL, whichever is lower."


Is a gyro with a nav/landing light or electric prerotator considered to be an aircraft with an engine-driven electrical system?

Sorry to have hijacked your thread Rob0, I'll start a new thread for my questions.

[gyropilot]

Is a gyro with a nav/landing light or electric prerotator considered to be an aircraft with an engine-driven electrical system?Iven,

It's not what accessories you have or how you use electricity, but whether or not you have an "engine driven electrical system" on your aircraft.

To the best of my knowledge, the phrase "engine driven electrical system" isn't clearly defined in readily available FAA literature. But any reasonable person who isn't trying to find a loophole to operate under can take that simple phrase at face value. I spent a lot of effort trying to find a loophole and failed. :)

I tried to get a clarification of exactly what a "system" is in the regulation (hoping that it had to include a battery, which I don't have on my GyroBee).

I wrote the local FSDO director for an exemption. No luck!

In the end, the local FAA MIDO and FSDO told me to get with the "spirit" of the regulation and install the damn transponder on my gyroplane! And ya know, now that I have it, I feel good I'm doing my little part for overall airspace safety.

So basically if you have an alternator or generator driven by the engine, then the requirements of FAR 91.215(b)(2) apply to your aircraft.

Of course this doesn't apply to an UL aircraft operating under FAR Part 103.

Best regards,

John L.

[ventana7]

I have a small aluminum plate mounted on the front of the keel of my AAI modified RAF with the tiny golf tee type transponder antenna on it and it works fine. Whenever I fly into class B or C or even thru the Washington DC ADIZ they have trouble hearing my comm radio but always get my squak.

I can take a picture of it if you want.

Rob

[KenSandyEggo]

I've flown my gyro close to 500 hours strictly in Class C and B, and I'm in the 30 mile veil. I took my little buddy Stefanos into Lindbergh International a few years ago for lunch with nary a problem. I did have a transponder and encoder. Here's a link to numerous photos of that adventure....pre-S-H conversion and pre-J2 ownership.

http://www.geocities.com/Area51/Stargate/7195/New_Ignition/

[RobO]

I will be based under, and flying within, Denver International’s 30nm mode C veil. And, going for an exemption is reportedly difficult...now. I have the transponder and encoder installed and all ready to go. I just need to install the antenna & ground plane and run the line.

Rob D., I’ve seen how RAFs and Sparrowhawks have mounted them under the keel. With it sticking down like that, I’m concerned about dinging the antenna while moving the aircraft. I also wonder about dinging it on a grass field. I would like to see your pics to see how you did it.

Thanks,

Rob

[gyropilot]

Rob,

On my GyroBee I mounted the transponder antenna under almost between the main landing gear. In this location, there's little possibility of it getting damaged on anything (unless the main landing gear collapse... and then I have bigger worries).

The GyroBee has a split keel with the tail boom tube overlapping. An open end of the tail boom is accessible to the front of the gyro. I simply drilled the appropriate sized hole in the underside of the tail boom tube right near the open end and mounted the antenna there pointing down. The coax with BNC connector goes into the open end of the tube and is easily accessible for antenna removal.

I didn't install any additional ground plane and the system seems to work well. It always passes the biennial transponder signal strength check and with enough altitude, I see the transponder replying to ATC radar queries.

John L.

[RobO]

Thanks John. The nice thing about the gyrobee in this situation is that the gear and keel don't hang down like the Dominator's. Once you're up high enough, none of that stuff interferes. Maybe the Dominator's keel and gear don't matter. I don't know. But, it's nice to know the ground plane may not be essential in some installations.

I'll probably just install it under the seat/keel somewhere and see how it tests out. If that doesn't cut it, I'll move it to the bottom of the rear keel and hope I don't catch it on something.

Rob

[Udi]

Sounds like you are getting close, Rob! Do you have pictures to post?

Udi

[JRB549]

Ken, you need to post that site in the photo section those are some nice pictures

[KenSandyEggo]

If you click on "Parent Directory at the top, you'll get hundreds more....or my old website. There's a photo link at the bottom. It's screwed up and I can't get in to modify it, so it just stays as it is.

http://www.geocities.com/kenj_sandyeggo/KensWebPage.html?942383509800

[RobO]

Udi, I was close two fora ago :) In theory, I still need to:

1. Prime, paint, and install tail & control cables
2. Order and install pre-rotator flex shaft
3. Transponder antenna & cable run
4. Install prop
5. Install/hang test rotor head
6. Construct & install control tubes/ferrules

That's about it mechanically- not too much. I'm very happy with things so far though. I still have the regulation stuff like tail numbers, placards, experimental markings, weight/balance, logbooks, data plate, and of course inspection.

It's always an issue of time. I'll try to post some pictures this weekend in the Builders Corner.

Rob

[PW_Plack]

Transponder antennas on gyros with stepped keels face compromises. Mount it forward of the step, and it will be shadowed by the rear keel and gear. Mount it farther aft, on the low part, and it becomes a ditch witch. I guess just try to get it on the lower keel, as close to the main gear as possible while keeping it low, for the best approach/departure angles.

[KenSandyEggo]

I used 2 Advanced Electronics ribbon antennae in my old gyro. The com antenna was the best I've ever used. I can't tell about the TX antenna though. There was no coverage below 1800' MSL where I flew and I never much got that high. I had it mounted against the cabin side, just aft of the right side, right rudder-pedal.

[Caribean_gyro]

m,ount between radio and transponder antena has to be at least 6 feet. DOnt mount the xpndr antenna near any edge of the fuselage. Goundplane should be at least 8 square inches. The cable has to be the low loss type. dont mess with it. I didi it and was getting no signal. Once all mounted hire a guy to do the filed test and certification. This could cost you around 150.00 dollas.

Is good to see that blinking light working. give me some peace in mind
ChuckP

[RobO]

Thanks Chuck. My groundplane has adequate area and I will be mounting it under the rear keel this weekend when I get home. Cable selection should be no problem. My comm radio is a handheld, so it is where where it will be :)

For anyone else who hasn't seen it, there is a useful and very easy to read publication that covers aircraft electrical/radio systems:

http://www.aeroelectric.com/Catalog/pub/pub.html#aec9

Rob

[KenSandyEggo]

I saw an article awhile back in a mag by Jim Weir for a ground plane for a transponder antenna in fiberglass craft. It was a pie-plate cut into an octagon with tin-snips. He said the peach pie-tins work best. I think it was 8 inches in diameter.

[RobO]

Transponders only transmit at a high wattage for a brief period of time. It isn't putting that out at a constant rate (thank goodness).

Assuming 300 watts, that means 25 amps of current at an assumed straight 12 volts. But, because of the brief duty cycle, wiring and fusing for far less than 25+ amps is fine.

My understanding is that a 5-1/2" round groundplane of copper (preferable) or aluminum is the proper configuration to make the antenna system resonant. I'm not sure how tying the entire aircraft frame in as the ground plane helps with this.

I'm not a strong electronics guy, so perhaps I wandered somewhere when I was figuring this stuff out.

Rob

[RobO]

Thanks Ken. Now that you mention it, I do remember seeing that same article. I will try to look it up.

I wonder how it knows that peaches are better...

Rob

[KenSandyEggo]

Maybe it was apple.