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/ProductPDF/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

DennisRhodes said:

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.

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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...

snopercod said:

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.

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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

mikeyj350 said:

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."

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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.

Mike F said:

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.

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Hey Mike, thanks a ton for the info! I hadn't come across RG-393 but with a little more research it looks like it might be perfect! Diameter-wise yeah it is still pretty fat but still a smidge smaller than RG-213 (0.39" vs. 0.405")...at least it isn't bigger...

Cost-wise yeah if I can get it at $6/foot and two $8 connectors (looks like they share some part numbers with the 213) it's not ideal but at least not terrible for just one cable.

I've already got a message into the G3Xpert folks (asking about the 213), we'll see what they have to say too but I think I found a winner here.

Again, appreciate all the help from the VAF brain trust!

Longer coax is never a good substitute for optimal antenna placement. I say this based on too many years of dealing with very expensive and cumbersome antenna configurations on a variety of special-purpose aircraft.

I'm not sure where you've mounted your comm antenna. Obtaining the minimum distance between comm and transponder is important, but on an aircraft of this size should be easily accomplished. I've got both a belly-mount comm and transponder antenna on our Sportsman - no problem getting the separation. The RV10 should be pretty easy in this regard.

Big fat coax is great stuff, but its lifecycle costs are VERY high. Sure, you'll never have to touch it, until that day you find it's been chafed by unseen contact with a sharp edge and it has to be replaced. Or a connector corrodes and has to be replaced. All these things are very good reasons to stick with the cheapest, simplest, most common coax cable and connectors.

Check out LMR-240-UF from Times Microwave.
Times Coax Loss Calculator

It's 0.240" diameter, has a double shield (foil+braid), costs <$1/ft, uses readily available connectors, and shows 1.5-1.7dB loss for your 15' run. Do make sure you get the Ultraflex version (LMR-240-UF) as it uses a stranded center conductor instead of solid. There are even places that will make up the whole assembly for you for reasonable cost. The PVC jacket isn't really a big deal, just install it so no chafing is happening.

David

Transponder Antenna Location

"Longer coax is never a good substitute for optimal antenna placement. I say this based on too many years of dealing with very expensive and cumbersome antenna configurations on a variety of special-purpose aircraft."

I have to agree with this statement. My only regret with my -10 build was not paying more attention to weight reduction/elimination. While you may find the right 15' cable to minimize attenuation, the real solution is to find a location to eliminate 10-12' of unnecessary cable. Other considerations are ensuring the right size ground plane, eliminating shadowing, and ease of accessibility.

Brian

Transponder Coax

Can RG58 coax be used for Transponder? I'm using RG400 for radios and GPS and I've run out. But I have a 5 foot piece of RG58 which would fit for my transponder. Is RG58 good enough for transponder or do I need to buy more RG400. I noted that my ELT came with a piece of RG58.
thanks

sglynn said:

Can RG58 coax be used for Transponder? I'm using RG400 for radios and GPS and I've run out. But I have a 5 foot piece of RG58 which would fit for my transponder. Is RG58 good enough for transponder or do I need to buy more RG400.

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Only if it were 1960 would it be good enough. . RG 58- PVC jacketed therefore burns well, and many brands have very poor shielding leading to interference with other systems. Get RG 142 or RG400 depending on length.

Quality RG-58 coax (think Belden) actually has slightly less loss per foot than RG-400. I used RG-58 in my first plane and all is well after 18 years.

I now use RG-400 in my builds as it is more mechanically robust and I got over the high price tag. For our applications, the double shield is really not needed, but it does not hurt either.

Carl

sglynn said:

Can RG58 coax be used for Transponder? I'm using RG400 for radios and GPS and I've run out. But I have a 5 foot piece of RG58 which would fit for my transponder. Is RG58 good enough for transponder or do I need to buy more RG400. I noted that my ELT came with a piece of RG58.
thanks

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Short answer: Yes it can be used.
Better answer: You should use RG-400.

But a 5ft run isn't going to make much of a difference. Depending on the RG58 used, it might be about .25 of a db. You loose more than that with each connector.