Transponder

02-20-2012, 04:58 PM

Mike | Transponder

Where do you all have your transponder antenna located on/in your plane?

Traveling from NY to Fl and back I noticed that ATC was receiving intermittent signals from my transponder. This is due to the location of the antenna in the starboard nose of the plane. I noticed with a scope and a small length of wire..... as I walked around the plane the strongest signal was from the 12 o clock to the 4 o'clock position, basically out the copilots side. It seems that the NLG and the radio stack stops the signal from getting out the pilots side of the plane.

There are several problems associated with the placement of the antenna.

Antenna Position (mounting) - Since we have a composite plane we need to choose an antenna that doesn't need a ground plane. The problem with this style of antenna is that the distance of the transmitted signal is generally less than that of an antenna with a ground plane. If we choose an antenna that needs a ground plane, one will need to add a thin plate of metal or foil tape to the back of the antenna. (ground plane would be the metal airframe in most cases)

Line loss (coax) - Line loss is a big problem. The further away from the transponder with the antenna the less power will be supplied to the antenna. RG400 max run is 6'1.3" while ECS 310801 is 30'.

I am in the process of relocating a new antenna in the rear of the plane.

Well that's the short story.....any more out there?

Mike

02-20-2012, 06:36 PM

Fred Lohr

interesting. jack ardoyno and I had a lot of complaints from ATC on lack of transponder signal on the way down to florida last fall. my problem turned out to be, in part, one or two inches of water in the rt nose gear compartment that houses the ground plate (under battery) and antenna. once the water was drained at our halfway stop, and the antenna was no longer under water, it worked a lot better. the only other time this happened was when a bag of lead shot i had in front for ballast broke on a water takeoff, and all the bb's ran down around the antennae. It must be hard to transmit thru lead, cause the transponder kept overheating and quitting, which also happened with the water.

so the next best place might be project down thru the the floor next to the right sided support beam. The grounding plate, (aluminum disk ) could sit on top of the floor with the approx three inch antenna projecting downward. Under the foward part of passenger seat, closer to the midline would be nice so the cable is short and out of the way. Is there enough depth there? plate could be raised up a bit so tip of antennae does not project thru hull but is just at the inside surface of hull. what do you think.

What do you think

02-20-2012, 10:25 PM

Russ Kotlarek | Antenna Placement

Please note the Transponder is one radio that supplies an enormous amount of power >200W on a frequency of 1090 Mhz. I addition it is a transmitter that continually is transmitting. The radiation from this transmitter specifically is rather unhealthy to have too close to crew or passengers.

An artificial ground plane as Mike suggested is a great way to boost its effectiveness. Indeed the longer the cable the greater the cable loss even more so depending on the type of cable used. The newer style aviation grade low loss cable does pretty well but keeping it to the minimal length and connectors is something to strive for. But keeping it away from you is even a better thing to strive for.

Comms only produce < 25 Watts and at a frequency about 10 times lower and rarely used in flight so sitting near that one is not nearly as bad.

Interesting enough the best location for the Lake amphib is on the bottom of the hull behind the step. Yep a hole in the bottom. I do not have mine there for obvious reasons but I guess it works. Anything that shorts out the antenna like water or lead shot obviously will hamper the output but could easily drive the final output circuit of the Transponder into failure so the environment is also important.

I do not have the answer but in making this decision these points might be important. How about a normal 4" or so Transponder antenna mounted on a light weight aluminum plate or foil inside the wing for a good ground plane bonded to bottom of the top skin inside?

Russ

02-20-2012, 10:58 PM

keithw | Transponder antena

My transponder antena is directly below the pilots seat. It has a metal disc typ ground plane attached and works very well. At the time of installation I was told that all radiation would be down and away from pilot and others. I have had very good trans/rec. performance. Keith N80CC P S No bb s or water prob I just drilled a small hole in the floor below the pilots seat (approx center) and then had antena pushed thru with the ground plane flush mtd on the floor. Carpet is installed around cable mounting end.

03-02-2014, 09:49 PM

Rusty | Xpndr antenna

Quote:

Originally Posted by keithw View Post

Did you buy that antenna, or make it yourself? I just received my complete avionics package from GRT and find nothing that looks like a transponder antenna. Back in the dark ages when I started this project, Dean Rickerson advised a copper ground plane in the top of the vertical tail to which I expected to attach one rod down for transponder, one rod up for GPS. Other comments here say keep the coax from xpndr to antenna short. Using a Trig remote xpndr, I could mount it back there, but your idea sounds better. Any concerns about that high frequency high energy source running continuously under the family jewels?

03-06-2014, 04:51 PM

Mike

At our age.... Hell no.

They are starting to make remote ponders and comms...best way to go

03-09-2014, 11:09 PM

Larry Brunzlick

I placed my transponder antenna about as far behind bulkhead 212 as it can be. Just in front of the water rudder housing on the starboard side. I installed it through the tailcone access cover under the air rudder. I used RG400 coax antenna wire and so far ATC has not reported any difficulty receiving my signal. I used ADVANCED AIRCRAFT ELECTRONICS HIGH GAIN TRANSPONDER ANTENNA (L2) from aircraft spruce.

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03-11-2014, 02:38 AM

Larry Brunzlick

no ground plane and it has to be placed vertically

06-24-2014, 01:40 PM

Rusty

Remote xpndr & remote antenna looks like the way to go.

My xpndr is planned for remote. My water rudder will be external operated by external push rod from bottom of rudder. Butt end of tail cone should stay dry.

I asked GRT about energy requirement for wire size & he reminded me that Xpndr doesn't xmit continuously, but when interrogated.

06-27-2014, 09:28 AM

Planemakers

Hi guys,

One more option: We've mounted one of those metal q-tip type transponder antennas to one of the metal cover plates half way out on one wing. The run is long but not too long. We've done this a few times and as far as I know they have worked pretty well. Just another option.

John J

06-29-2014, 05:56 PM

Mike

John

What coax did u use? Wondering if the regular rg coax is good or did u use the low loss stuff

07-02-2014, 09:55 AM

Planemakers

Hello,

I believe we used RG-58, the normal, black-covered stuff with the strand core, not the solid core. We have also used the tan colored cable but I can't remember the number.

Someone correct me if I'm wrong but I believe the "lose" once you get out past the recommended lengths is tiny and in order to significantly and noticeably harm the signal you would need to use a cable longer than you could use in a Seawind.

Thanks and have a great day,

John J

07-04-2014, 03:46 PM

Mike

Rg-8 is the thick cable, rg-58 is the thinner cable, rg-400? Is the low loss cable which is usually tanish in color.

The length does matter..... More coax=greater line loss (less power getting to the antenna)

07-08-2014, 06:12 PM

Ed Lynch | Transponder and cable loss

I put my transponder in the Seawind recommended location out in the wing tip, used RG-58 cable, and have had NO transponder problems. Don't invent a problem where there is none! Ed

07-10-2014, 08:12 AM

Planemakers

Hi Folks,

We've used RG-58 and RG-400 (Thanks Mike) You are right Ed. We've put the xponder antenna out in the wing tip too without a problem. The lose is tiny (read insignificant) until you get out to 100 ft or more of cable. Down the longeron and out the wing is around 35 ft and shouldn't be a problem.

Have a great day,

John J