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avee8tor13

Well Known Member
I can muster up about 30 inches from Garmin GTX327 to the antenna but am reluctant after reading posts and Garmin installation. Has anyone mounted their transponder antenna a little closer to their transponder?

Thank you for your response.


Oh, I noticed when measuring from the avionics stack to any location three feet out, there isn't a lot of options other than back down the fuselage. Others evidently have installed the antenna around the fuel filter/pump area in the tunnel (RV-10).
 
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I don't know about the 10, but on my 9A, I mounted the transponder antenna in the right forward corner on the bottom of the fuselage, near the fuel tank vent. This meets the 30 inches (barely), and it has worked fine.

greg
 
about 30 inches

My xponder antenna is about 30 inches away, front right corner near the firewall. Haven't noticed any problems.

With a firewall and hull of metal, the antenna is directly shielded from the transmitter. Shouldn't be a problem. On a glass plane, I would be concerned.
 
A fair number of builders have put it where Greg mentions, right up by the fuel vent outlet. However, I don't think this technically meets the 30" straight line distance (unless you have the tx on the far left of the panel), but those who have used it by the fuel vent have reported no problems with either the tx itself or reception with the antenna up by the gearleg (on tailwheel versions). Mine is there. We'll see if I have any problems. (FWIW, the above reports are for GTX320A's. Not sure if the 327 would be different.)
 
I tried a BNC union and stuck it right on the back of the rack in my wooden airplane, sounded simple. Pop, pop, pop. Guess who felt simple then?
 
alpinelakespilot2000 said:
...I don't think this technically meets the 30" straight line distance... (FWIW, the above reports are for GTX320A's. Not sure if the 327 would be different.)
Click to expand...
The 327 manual say 3 feet.
So... 1.2 Kw can't get through the holes in the front of our microwave window.... Anyone have any ideas how the 200 W from the transponder will make to through 3/16" of solid Al and interfere with the transponder...:confused:
 
Years ago we had a situation in a Pitts where the antenna was about 6" from the transponder. The very short antenna lead had a standing wave that radiated the signal back into the transponder causing it to "reply" to itself continuously. We installed a longer antenna lead looping it away from the transponder and solved the problem.
Sometimes it's the lead length and not necessarily the direct distance.
 
I'm on the centerline of the bottom skin, 8" aft of the firewall in my -4. I'd guess straight line distance is 24" or so.

However, my coax lead is at least 60", which is usually the critical issue.

An extra 6" at 200 watts is negligible for that total distance. But as Mel said, 6" short of the required coax length can be disastrous!

GTX-327 about 80 hours ago.
 
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My transponder antenna is right below my fuel selector. My transponder (KT-76A) is in the panel right above the fuel selector. There is less than 3-feet but there may be about 3-feet of coax.

It has not failed a transponder test for the past 2,230 hobbs hours over the past 11.5-years.
 
I upgraded my Cessna with a 327. When the installer was finished he had used the old transponder antenna location. When I bought the transponder for my RV I noticed the requirment of 36" so I measured the distance on the ol Cessna and it was more like 26". It worked fine for me and and went ahead and mounted the antenna on my RV which is more like 30". I mounted it under the #2 seat (7-A) behind the wing spare carry through. The comm antenna is mounted under the #1 seat (my seat) in aprox. the same location as the transponder on the other side. I used a dent created when I droped a bucking bar to locate the actual location to drill for the comm antenna. It's all still untested though since I haven't connected either radio to the coax hence the radios have never been switched on. Hope the smoke stays put :D.
 
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For info on my Maule I have a Garmin 330 remote unit feeding a GNS-480. The remote is just below my seat. The antenna is fairly close -- probably around 18 inches from that unit but the avionics guy put 3 ft of coax. The answer seems to be to use the required length of coax and not worry about the distance.
 
This is an opportune post!

I have read the material and was trying just this afternoon to locate where I will place my antenna's. It seems every decision latley is full of alternatives. Slows down the building process.

I think I will go in the front right corner near the fuel vent for my txpd antenna. I will use a loop in the cable.

Now I am trying to place my comm. Under the belly I know. Probably aft of the bagage compartment. I am concerned that will be too close the the ELT which is also aft of the baggage compartment on the left side.

I will check some previous posts for that and the GPS Antenna.
 
If anyone needs an install manual for any of the Garmin equipment let me know and I can get ahold of a copy and email you. In my experience when we install any transponders we normally use the length of coax called for in the install manual and make a large "drip loop" to provide the right dbM drop the equipment needs to feel.

Most of the time we will mount an upgraded version of whatever the customer wants in the same rack spot and the antennae are usually much less than 36" straight line distance.

The GTX-330 should be the same sensitivity as the 320 and 327, it just comes with Mode S, which on your FAR 91.413 certs will be more strict and has a whole extra list of stuff that needs checked for Mode S operation. The 330 also can provide TIS given proper equipment is installed and integrated correctly.
 
Like ?? I put the transponder antenna through the floor, just behind the firewall inboard of the fuel vent, on the right side.

The antenna was forward of the spar and actually bolts through the spar web.


This gave me the advantage of not having to run the antenna wire through the spar, taking up valuable space. Mine is a taildragger so I had extra holes in the spar cover support. These pieces are not structural so feel free to drill new holes or enlarge the existing holes, if you need.
 
I'm with Mel and Blandess, the length requirement is to avoid RF funnies through reflections back up the coax, so the important measurement is the length of the coax, the physical distance between txpdr and antenna is of secondary importance.

From the RF standpoint (in practical terms) there is no difference between mode A, C & S, and from the RF standpoint there is no difference between a 320, 327 & 328. The difference is in the data that is transmitted on the RF signal, its all on 1030 & 1090MHz, and all the boxes put out 200W. The 330 is similar but outputs 250W.

Pete
 
I would disagree...

penguin said:
I'm with Mel and Blandess, the length requirement is to avoid RF funnies through reflections back up the coax, so the important measurement is the length of the coax, the physical distance between txpdr and antenna is of secondary importance.

......

Pete
Click to expand...

..Garmin says...

"To prevent RF interference, the antenna must be physically mounted a minimum distance of 3 feet from the GTX ---"

Then they say to keep the co-ax as short as possible and as direct as possible.

You are assuming RF up the cable... the other possibility is for the strong RF pulse to enter the transponder box as leakage past any seals. If this is the case, it might come back to bite you in the future as the RF seals deteriorate with age and removal and replacement of the covers...:(

If Garmin really thought that the important measurement was the length of the co-ax, then they would have specified a minimum length...:rolleyes:
 
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