mikeyj350
Well Known Member
(Posting here after some initial thoughts discussed in the RV-10 section.)
I'm trying to understand the requirements for installing a transponder antenna. I have a GTX45R and a CI-105 antenna. Due to some equipment changes (Originally I had planned for a separate GDL39R and GTX23ES), I have ended up with my GTX45R up in the panel and my CI-105 antenna in the tailcone, aft of the elevator bellcrank. I'm estimating about a 15' coax run for this configuration. I knew there was a *minimum* distance between the transponder and the antenna (3 feet physical separation), but I somehow missed and was unaware of a maximum distance until today. The GTX23ES manual actually does a much better job of explaining this than the GTX45R manual, but both do mention a maximum cable loss (the GTX23ES specifies 1.5dB and the GTX45R specifies 2.0dB).
Using various online calculators and spec sheets, I've pieced together some information on this setup, and would love some confirmation/advice/recommendations from the experts here:
(1) RG-400 and Amphenol 31-326-RFX connectors. This was the original plan, as it is the kind of cable I'm using for all other antennas in the airplane. With 250W at the transmitter and 15' of cable at 1090MHz, I lose 2.4dB over the cable, giving 143.8W at the antenna. This is below Garmin's 150W minimum power and 2.0dB maximum loss and is therefore not acceptable, I believe.
(2) RG-213 (MIL-C-17/74) with Amphenol 112562 connectors. Using the same calculators, this cable seems to have a 1.2dB loss over 15' at 1090MHz, giving 189.6W at the antenna, which is within spec. This cable also appears to be able to handle the necessary power (up to 270W).
So I guess my questions are: Can I use RG-213 for my long-ish transponder cable run? Is there another kind of cable that is an even better fit? Am I missing something else? Any other concerns with a 15-foot cable run?
I'm trying to understand the requirements for installing a transponder antenna. I have a GTX45R and a CI-105 antenna. Due to some equipment changes (Originally I had planned for a separate GDL39R and GTX23ES), I have ended up with my GTX45R up in the panel and my CI-105 antenna in the tailcone, aft of the elevator bellcrank. I'm estimating about a 15' coax run for this configuration. I knew there was a *minimum* distance between the transponder and the antenna (3 feet physical separation), but I somehow missed and was unaware of a maximum distance until today. The GTX23ES manual actually does a much better job of explaining this than the GTX45R manual, but both do mention a maximum cable loss (the GTX23ES specifies 1.5dB and the GTX45R specifies 2.0dB).
Using various online calculators and spec sheets, I've pieced together some information on this setup, and would love some confirmation/advice/recommendations from the experts here:
(1) RG-400 and Amphenol 31-326-RFX connectors. This was the original plan, as it is the kind of cable I'm using for all other antennas in the airplane. With 250W at the transmitter and 15' of cable at 1090MHz, I lose 2.4dB over the cable, giving 143.8W at the antenna. This is below Garmin's 150W minimum power and 2.0dB maximum loss and is therefore not acceptable, I believe.
(2) RG-213 (MIL-C-17/74) with Amphenol 112562 connectors. Using the same calculators, this cable seems to have a 1.2dB loss over 15' at 1090MHz, giving 189.6W at the antenna, which is within spec. This cable also appears to be able to handle the necessary power (up to 270W).
So I guess my questions are: Can I use RG-213 for my long-ish transponder cable run? Is there another kind of cable that is an even better fit? Am I missing something else? Any other concerns with a 15-foot cable run?
Last edited: