Long transponder cable - RG-213 instead of RG-400?
 

(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 think you'd be better off with RG142: https://www.pasternack.com/images/Pr...F/RG142B-U.pdf

RG213 has a PVC jacket (no flame-resistant) and is pretty bulky. RG142 has pretty much replaced it in certified avionics and it has a dual screen braid.

Frm GTX 330 manual
 

Watch out, some of the cables get really thick and connectors can be difficult to obtain $$


Max. Length (feet)---- ECS Type---- MIL-C-17 Type----- RG Type
8.8 ------------------------- M17/128----------------------------------RG400
10.0------------------------ 3C142B
12.5 ---------------------------------------------M17/112 -------------RG304
17.0 ------------------------311601---------- M17/127 ------------RG393
21.0 ------------------------311501
27.0 ------------------------311201
41.0 ------------------------310801

Thanks guys. I had considered RG142, however at least according to this calculator it doesn't meet the specs I'm after with respect to loss. (It's close, but still 2.1dB loss. Good point about the PVC jacket, too though.

As far as thickness and $$, yes that was considered as well. Cost-wise RG-213 isn't great but for one 15-ft run it's not going to break the bank. It is also quite a bit thicker than RG-400 (0.405" dia vs. 0.195"), but again for just one run that shouldn't be too big of a deal.

JD There is a similar chart in the GTX23ES manual as well, along with a note that says "Note that any 50 Ω, double shielded coaxial cable assembly that meets airworthiness requirements and the 1.5 dB maximum loss figure (including connectors) may be used."

Would you not consider moving that Transponder antenna forward enough to satisfy the loss minimum? Most rod mounted antenna have a small single hole that would be easy to close up on a 10.

L-com had RG-213 with BNC connectors
 

I was thinking that you wouldn't be able to use BNC connectors on RG-213 due to the diameter, but L-com stocks RG-213 cable assemblies with BNC straight connectors. $28 for a 15' cable. I'm not sure if they offer 90 degree ends, but you can call them and ask...or use an adapter.

I used RG-142B on my transponder, and be aware that it has a solid center conductor and is somewhat stiff.

Mike,

I went through the same trades and couldn't find any low-loss coax that wasn't expensive, thick, inflexible and/or readily available. So, I stayed with RG-400 and placed the antenna just behind the firewall, away from the exhaust and on the opposite side of the airplane from my GTX-45R which will be mounted under the subpanel. That location keeps the line and connector losses within Garmin's requirements but I'm probably on the ragged hairy edge of that 3-foot minimum - maybe the Garmin experts can chime in on the reason for that minimum distance?

That probably won't help solve your problem, but at least it's a data point.

Dave

If I have to, I have to, but I'd really prefer not to, since I've already made the necessary cutouts and installed doublers not only for the transponder but for the CI-122 COM antenna. If I move the CI-105 up front, I will likely have to relocate my COM antenna as well. It's all doable, but it would be much easier and allow me to stick with the original layout if I can keep the transponder antenna in the back. I had even thought about installing the GTX45R in back, but with four RS232 runs and an ethernet cable run going all the way to the panel, I think I'm in better shape if I just have to run one fat antenna cable from the front to the back...

Appreciate the info! I did actually check on that myself as well, I was planning on purchasing the cable in bulk and then getting the crimp connectors (Amphenol 112562) from Mouser or Digikey for around $8 apiece. The RG-213 actually does have a stranded center conductor, so hopefully it will still have some flexibility in spite of its thickness. I don't have any sharp bends or anything in any of my runs, so we should be all set. I'm really just curious if others have done something like this or not... From everything I've researched it sounds like it is acceptable, but it would be great if I found someone who actually did this already. One person brought up the PVC jacketing, that does raise a flag but I believe it is still an aviation-grade cable, so it must qualify in some way.

Thanks for all the responses and data points! I think I might reach out to the Garmin folks if I can't raise them on here and see what they have to say, and get a final thumbs-up or -down.

Caution using the chart data
 

Using the chart, I did a 15ft run. It worked for the 330 but after replacing the 330 with a Dynon (for ADSB)The unit would not pass a mode S test. I had to relocated the Trig to fix the problem.

RG-393
 

I've used RG-393 in a couple of larger aircraft (not RV's). It has the same physical qualities as RG-400 (double shield, silver plate, FEP jacket - not PVC) but a larger diameter (same as RG-213 I believe). It is expensive, I pay around $6/ft. At 1000 MHz, 15' has a 1.125 dB loss.