[ArVeeNiner]

So, when I initially installed the cable between my transponder and my antenna I thought I could have cut it a bit longer than I had but I thought it was workable. Well, here I am, months down the line, making all the wire and cable runs all pretty and I'm just not happy with the bend radius at the transponder antenna. Man, if I could only stretch a couple more inches out of the cable I'd be much happier.

Rather than pull out the wire from all the nice bundles I've made and replace it, how about if I made myself a short cable extension at the antenna end? I'm thinking something with a male BNC on one end and a female on the other...no longer than about 4" long. Is that a fine and dandy thing to do? There probably is a bit of loss at every connection but how significant of a loss would I have?

Thanks

 

[gmcjetpilot]

Buy some more

How much is coaxial, $0.25-$0.50 RG58 (LMR195 better) or $2.05 RG400 (best).

Just get some more and make it longer. You can certainly make a spice with some male/female BNC connectors. May be there is some special splice.................................gross. A perfect place (another place) for a failure.

Also you lose some efficiency, a little not a lot but some.

My advice is suck it up and order some more and sell this piece on ebay or the VAF classified ads (or hang on wall as a tribute)


PS 4", oh my so close. One option may be take it out the bundle and re-route. Personally I don't bundle RF coaxial with any other wires if possible, in my opinion that's a BAD idea. You have pulses of 200 watts of non ionizing electromagnetic radio frequency waves shooting down that coaxial. With ships wires next to it (power, signal, audio) you may pick up nose. Hummmm if get noise in your power distribution or audio system, that "bundle" might be to blame.

 

[ArVeeNiner]

Gotta bite the bullet

Yea, after thinking about it over night I need to just pull it out and put in a longer piece. I won't be happy otherwise. Really though, it's not like the coax is guitar string tight but it's not the way I think it should be and if I get a bad connection in the future it will be way harder to fix. Right now I'm running RG58 but I can take this opportunity to run RG400 like I guess I should have done in the first place.

Not running this coax along with other wires is easier said than done. The antenna is located aft of the baggage compartment so it runs in the center tunnel along with strobe wires, aft position light power, trim power, and trim potentiometer wires. It also runs under the center cover between the firewall and the fuel valve. It's here that I do worry a bit about noise because the wires running to my headsets are here as well. With more slack in my transponder cable I was going to separate them as much as possible but that is only going to be about 4" max. Hopefully that is good enough.

I've tested my transmitter which has RG58 in the same area and it seems like I don't have any noise in the system. Of course all bets are off when I install and run the engine! We'll see.

Thanks!!

 

[Mel]

Do whatever makes you comfortable.

RG-58 works fine for the lengths we use in our airplanes and an extension cable will work fine as well. But, if you are more comfortable with the 400, then replace it.

 

[az_gila]

Unless...

RG-58 works fine for the lengths we use in our airplanes and an extension cable will work fine as well. But, if you are more comfortable with the 400, then replace it.

...you have a Garmin transponder, then you must use RG-400 or better - see the Installation Manual for acceptable types co-ax - to keep the transponder system within the Garmin TSO requirements...

Even with RG-400 there is a maximum length of 8.8 ft for a Garmin transponder.

To keep the TSO intact this is one case when you should RTM....

Garmin specifies a max. loss of 1.5 db in the cable. This does affect performance... too much loss and the sensitivity drops and your TCAS response range also drops... not a good thing.

 

[ArVeeNiner]

Gil:

Yea, I saw the 8.8' requirement in the Garmin manual. I have a GTX327. I went with RG58 based on the experience of some in my EAA chapter who haven't had problems. I never felt good about that. I'm slightly over 8.8' so the RG400 isn't good enough either. What took the wind out of my sails is that I couldn't find a source for the RG304, RG393, ECS type, or MIL-C-17 type recommended in the manual. Even Spuce only carries RG58, RG400, and RG142. You'd think that since Garmin recommends this type of cable for installations over 8.8' that it would be readily available out there. Any ideas who carries it? All I can find are parts houses that appear to supply aircraft builders like Boeing or Cessna or just specs.

 

[ArVeeNiner]

Cool dB Loss Calculator

http://www.timesmicrowave.com/cgi-bin/calculate.pl

Pretty neat. You can punch in all sorts of coax and it calculates loss. Garmin wants a max of 1.5 dB loss at 1090 MHz. It seems that at the lengths we're talking about, there isn't a big difference between RG58 and RG400. The RG400 gives you only about a 1% better effeciency at 10 feet than the RG58 at that frequency.

 

[az_gila]

The RG-400...

http://www.timesmicrowave.com/cgi-bin/calculate.pl

Pretty neat. You can punch in all sorts of coax and it calculates loss. Garmin wants a max of 1.5 dB loss at 1090 MHz. It seems that at the lengths we're talking about, there isn't a big difference between RG58 and RG400. The RG400 gives you only about a 1% better effeciency at 10 feet than the RG58 at that frequency.

...is double shielded, which is a requirement in the GTX-327 Installation Manual - RG-58 is not.

The 1.5 dB maximum appears to be for the cable assembly which would then include the two BNC connectors.

Since sensitivity would affect TCAS performance, it is important...

 

[az_gila]

buying RG-393 ...

Gil:

Yea, I saw the 8.8' requirement in the Garmin manual. I have a GTX327. I went with RG58 based on the experience of some in my EAA chapter who haven't had problems. I never felt good about that. I'm slightly over 8.8' so the RG400 isn't good enough either. What took the wind out of my sails is that I couldn't find a source for the RG304, RG393, ECS type, or MIL-C-17 type recommended in the manual. Even Spuce only carries RG58, RG400, and RG142. You'd think that since Garmin recommends this type of cable for installations over 8.8' that it would be readily available out there. Any ideas who carries it? All I can find are parts houses that appear to supply aircraft builders like Boeing or Cessna or just specs.

RG-393 does seem to be advertised...

Here at $7 per foot

http://www.surplussales.com/Antennas/Antennas-10.html

...and new here at around $8 per foot

http://www.tessco.com/products/displayProductInfo.do?sku=57757&eventPage=1

Here at $7 per foot...

http://www.therfc.com/coax.htm

Perhaps the pricey bit has something to do with the silver in it...

 

[gmcjetpilot]

close enough

It will work, Garmin is asking for the ideal. I can assure to coaxial runs in the Boeing are way longer. so what if it only can get out 190 miles at fl390, not 200 miles.

I personally do like to keep the antennas fwd of spar, shorter runs.

noise wise you should be ok, you shall see.

rg-393, ha ha and you think I'm crazy............rg58/rg400 will work and you will not be able to tell the difference. with that said follow the manual (my lawyer told me to say that).

 

[az_gila]

I'm surprised...

It will work, Garmin is asking for the ideal. I can assure to coaxial runs in the Boeing are way longer. so what if it only can get out 190 miles at fl390, not 200 miles.

I personally do like to keep the antennas fwd of spar, shorter runs.

noise wise you should be ok, you shall see.

rg-393, ha ha and you think I'm crazy............rg58/rg400 will work and you will not be able to tell the difference. with that said follow the manual (my lawyer told me to say that).

...at your response George.

You are an airline pilot, correct?

Garmin give the maximum losses acceptable for their transponder TSO'd system to work as advertised.

They require a 1.5 db maximum loss in the cable assembly (including connectors), and this is at the high frequency of around 1000 Mhz, not the VHF frequencies we use in comm/nav systems.

At these frequencies, losses are easy to create, and their effects may be more noticeable.

While I definitely agree that 190 miles vs 200 miles to ATC has no effect - as an ATP pilot, I would think you would want your TCAS system to work correctly.

An extra loss of a mere 1 db in the cable reduces your TCAS detection range by 21%, and a 2 db loss reduces it by 37%, and these numbers are based on the target's (our RVs) reciever losses, and only get worse if other losses are taken into account.

Transponder reciever sensitivity can be easily checked on the bench at your avionics shop, but the 2 year "in-the-plane" transponder check is a gross check and would not detect the level of losses we are talking about here - the bench test is only at the input to the transponder box. That is why Garmin specifies a line loss maximum and the need for a TSO'd antenna - this enables then to specify that the system meets the TSO requirements.

You might do as you say for line connections, but Garmin would like you to limit the line losses, and the FARs require that your transponder system meets TSO specifications.

This is one area where our transponders must meet TSO specifications (note, I did not say be TSO'd) even though we are in the Experimental category.

Based on TCAS performance, I would have thought that you would support the text of the approved Installation Manuals.

Builders, make sure your transponder antenna cable is top notch and the connectors are correctly installed. This probably would be a good cable to get professionally made.

PS ... I bet Boeing checks the dB loss in every transponder cable in the plane before it leaves the factory....

PPS ...my lawyer was not consulted, but the fact remains that the FARs are not optional....