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steve(at)tomasara.com
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 Posted: Fri Aug 05, 2016 7:38 pm Post subject: Garmin GTX 335 Install nonsense(?) |
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Greetings all,
I'll soon be installing a GTX335 transponder in my LongEz. The (stc) install manual has some items I find curious related to the antennas.
While it does say if I'm replacing a different GTX transponder (I'm replacing a 327) the existing transponder antenna is fine, it ALSO says that the minimum radius for the transponder antenna ground plane is 12"! Mine is 2.7" (1/4 wave at 1090 MHz). Anyone have any knowledge of what they are trying to optimize?
Also it goes on to say the minimum radius for the gps antenna ground plane is 7.5"! The gps antenna that goes with the GNS250XL has no ground plane at all... Anyone have any idea what they are after here?
Also, the manual goes on and on about all the bonding required between antenna ground planes, the avionics rack etc. My aircraft is a LongEz (i.e. fiberglass) and I don't have any explicit "bonding" at all (though I DO have very carefully planned and implemented power returns etc. In fact, my transponder ground plane is encapsulated in fiberglass (except where it is electrically connected to the coax connector).
My experience and existing knowledge tempts me to ignore all of the above but, on the other hand, I'm still learning things that surprise me everyday.
Can someone enlighten me as to the technical benefits to be had by changing my existing configuration to be in line with what Garmin is mandating in their installation manual?
Thanks in advance!
Steve Stearns
LongEz N45FC
Boulder/Longmont CO
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Sat Aug 06, 2016 7:04 am Post subject: Re: Garmin GTX 335 Install nonsense(?) |
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First, and most important, and Im sure you know that TSO installation instructions do not apply to you.
Theoretically, the benefit of ground planes increases with size, in the limit, a ground (surface of the earth) antenna ground plane which is the entire earth is optimal. However the benefit decreases exponentially with size. Modern "active" gps antennae have a ground plane which is a fraction of an inch on a side and they are designed to work just fine. It does have a ground plane, it is just encapsulated inside that plastic lump. Transponder transmitter antennae have slightly more critical requirements to optimize impedance load and transmission pattern. If your transponder performance was OK with existing antenna, it will almost certainly work with the new one.
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steve(at)tomasara.com
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| Posted: Mon Aug 08, 2016 2:06 pm Post subject: Garmin GTX 335 Install nonsense(?) |
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Ira, Bob N. and the group:
I appreciate the reply. I do know the TSO installation instructions are not obligatory but they also are the only installation instructions I received.
I understand and appreciate your information regarding ground planes. Where I am still interested in knowing more is the magnitude of the differences between what they specify and what I have already installed. Sure, bigger is better from an RF standpoint but would I or the FAA notice the difference in a real life A/B comparison?
Bob, do you have any thoughts on this? I consider you one of the better people out there to sift the meaningful theoretical benefits out from all of the inconsequential ones...
Also, anyone want to render an opinion on all of the specified bonding? My background leads me to conclude that "bonding" a 1/4 wave-radius ground plane on my transponder antenna is a decidedly bad idea...
Steve Stearns
LongEz N45FC
Boulder/Longmont CO
Ira wrote:First, and most important, and Im sure you know that TSO installation instructionsdo not apply to you.Theoretically, the benefit of ground planes increases with size, in the limit,a ground (surface of the earth) antenna ground plane which is the entire earthis optimal. However the benefit decreases exponentially with size. Modern "active"gps antennae have a ground plane which is a fraction of an inch on a sideand they are designed to work just fine. It does have a ground plane, it isjust encapsulated inside that plastic lump. Transponder transmitter antennaehave slightly more critical requirements to optimize impedance load and transmissionpattern. If your transponder performance was OK with existing antenna,it will almost certainly work with the new one.
Steve original wrote:
I'll soon be installing a GTX335 transponder in my LongEz. The (stc) install manual has some items I find curious related to the antennas.
While it does say if I'm replacing a different GTX transponder (I'm replacing a 327) the existing transponder antenna is fine, it ALSO says that the minimum radius for the transponder antenna ground plane is 12"! Mine is 2.7" (1/4 wave at 1090 MHz). Anyone have any knowledge of what they are trying to optimize?
Also it goes on to say the minimum radius for the gps antenna ground plane is 7.5"! The gps antenna that goes with the GNS250XL has no ground plane at all... Anyone have any idea what they are after here?
Also, the manual goes on and on about all the bonding required between antenna ground planes, the avionics rack etc. My aircraft is a LongEz (i.e. fiberglass) and I don't have any explicit "bonding" at all (though I DO have very carefully planned and implemented power returns etc. In fact, my transponder ground plane is encapsulated in fiberglass (except where it is electrically connected to the coax connector).
My experience and existing knowledge tempts me to ignore all of the above but, on the other hand, I'm still learning things that surprise me everyday.
Can someone enlighten me as to the technical benefits to be had by changing my existing configuration to be in line with what Garmin is mandating in their installation manual?
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