Nav antennas: Do I need a triplexer, diplexer, or ?

I'm trying to figure out what I need.

My panel will have a Garmin 430W nav/com and an SL-30 nav/com.

I have a "cat's-whisker" VOR/ILS/GS antenna at the top of the vertical stabilzer.

I've read quite a few threads here about nav antenna wiring, but couldn't find anything that specifically addressed the way the signal must be fed with my setup, and the need for triplexer and/or duplexer splitters.

Very simply, all I want to do is to use the single nav antenna's signals (VOR, ILS, and GS) for both nav radios, without any kind of selector switch.

There are all kinds of splitters available -- for instance, Aircraft Spruce's P/N AV-571 is a VHF triplexer that allows the use of a VOR/ILS/GS antenna by 2 nav radios plus one GS receiver. Great, but I want the GS signal to go to both receivers. Do either the 430 or the SL-30 have a GS diplexer built in? If not, can I split the signal again, with a second splitter, without losing a bunch of signal strength? Does it matter that much?

I'm a bit confused here. Anybody wanna help me sort this out? Thanks!

You need a triplexer Buck - the 430 has a separate input for VOR and GS, and the SL-30 has one input for both. I've stared at the back of both these units recently, and know that's how they work!

Paul

nav antenna

Paul has it right. I have same setup, and was under the panel yesterday and reviewed the installation.

Thanks!

Paul and Tom:

Thanks for the quick replies! I had a feeling it would be easier than I was thinking.

Now I have to figure out the wiring on my 1.21 jigawatt flux capacitor...

I've got one of these here quadraplexers:



and I'm working on a 430W/SL30 installation. Obviously, I can't feed the NAV2/GS2 lines both into the SL30 because of the internal diplexer. What I'm not sure about is whether the quadraplexer filters the signal or not. In other words, if I feed the SL30 from the NAV2 output, and just don't hook up the GS2 output to anything, will the SL30 then be able to diplex the signal properly? I've gotten conflicting answers about this so far. The SL30 manual specifically says "DO NOT INSTALL AN EXTERNAL DIPLEXOR."

If the quadraplexer is in fact stripping the GS signal from the NAV outputs and vice versa, I'm either going to have to find an appropriate triplexer that does not, or buy a mini-circuits splitter/combiner and either split before the quadraplexer and run an unfiltered line to the SL30 (which will add an unacceptable amount of loss to the 430W's nav signal) or combine the NAV2/GS2 outs and feed the SL30 (which will add an unacceptable amount of loss to the SL30's nav signal).

Either way, if the quadraplexer filters, I'm buying more boxes.

Boy, you guys certainly ask timely questions. I am currently trying to figure this out myself. I have comant towel bar antennas joined with a comant signal combiner. I am also trying to connect to a 430 & SL30. Does anyone know what part number you need to split these back out for VOR, GLS, ETC.?

Stein told me that the quadraplexer like the one pictured will not work on my 430W/SL-30 installation. So, I am still trying to figure out how I can make this work.

dmaib said:

Stein told me that the quadraplexer like the one pictured will not work on my 430W/SL-30 installation. So, I am still trying to figure out how I can make this work.

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hm, that's not good... I emailed Stein last night about this, and got the following response:

Stein said:

Correct...the SL-30 does not need a Diplexer, but they have been used with success...albeit the overall signal strength will be a bit weaker. You can just feed it a standard Nav/Vor line out of the diplexer you have (no GS, just the VOR). Hook it up that way and you should be ok. It just needs to be the correct unit, and I think the Comat CI-505 "Triplexer" would be appropriate for your installation. Garmin may not recommend them, but I know people have used them and they've worked ok. Of course the best thing is a separate antenna, but if you can't then the next best is get the correct triplexer.

Hope this helps!

Cheers,
Stein

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I sorta took that to mean that what I had WOULD in fact work, but upon closer examination perhaps it won't. I'll email him back and clarify.

I talked with Johnnie Blaine at Avionics Unlimited yesterday about hooking up my 430 & sl 30, I have comant towel bar antennas. He drew me a diagram which show using a comant power combiner to connect the two antennas, then using a "coax T" from the power combiner, from the "T" one leg goes to the SL30, the other leg goes to a Comant "CI 507", this in turn has one leg to Nav and one leg to the GS.

I want to give a "Big Plug" for Avionics Unlimited in Conroe, TX, they are the nicest people, always willing to answer questions and help in any way possible. They are also one or the biggest Garmin Avionics dealer in the nation. They also have a lot of offering on EBay that I think you could buy with confidence.

Chuck's way of doing it is the best way. David Maibs Quadraplexer will not work, and the Triplexer I referred to will work. What you guys are all assuming are the same parts are not. A "Diplexer, Triplexer and Quadraplexer" are all different. The SL-30 wants a separate antenna. If you can't do that, then a T before a Diplexer is next best. If not that, then a Triplexer will work. A Quadplexer is a no-no.

Hope that clears things up a bit.

Cheers,
Stein

Why not use a diplexer on the cat's whisker to feed the 430 and then add a Bob Archer wingtip antenna for the sl-30? That installation will provide the redundancy of a dual nav installation.

it only really clears things up in the sense that I'm probably stuck buying something else and waiting for it to get shipped before I can finish up this project.

I wish I could see a schematic for these magic little boxes. I am having a hard time seeing how the CI-505 triplexer is any different than the CI-1125 quadraplexer other than having one fewer GS output. The -505 is designed to feed 'Dual VOR / One Glideslope'. Nothing anywhere tells me that one or both of the VOR outputs of the -505 will deliver full spectrum signal that the SL-30 can use for both receivers. If the -505 is known to work, then the same conspicuous lack of information leads me to believe there's a reasonable chance that the -1125 will work just as well (with possibly a small amount more loss due to the additional split). I'm a bit uncomfortable spending $150 plus shipping (which will need to be overnight at this point) for something that may or may not be any better than what I already have.

As far along as this panel install is, I think I'm going to have to just fly it as is and see what sort of glideslope performance I get. If it's lacking, I guess I'll be back on my back ripping out wiring.

Since Stein builds panels by the dozen, I would listen to him because he should know what he is talking about.

tmbg said:

it only really clears things up in the sense that I'm probably stuck buying something else and waiting for it to get shipped before I can finish up this project.

I wish I could see a schematic for these magic little boxes. I am having a hard time seeing how the CI-505 triplexer is any different than the CI-1125 quadraplexer other than having one fewer GS output. The -505 is designed to feed 'Dual VOR / One Glideslope'. Nothing anywhere tells me that one or both of the VOR outputs of the -505 will deliver full spectrum signal that the SL-30 can use for both receivers. If the -505 is known to work, then the same conspicuous lack of information leads me to believe there's a reasonable chance that the -1125 will work just as well (with possibly a small amount more loss due to the additional split). I'm a bit uncomfortable spending $150 plus shipping (which will need to be overnight at this point) for something that may or may not be any better than what I already have.

As far along as this panel install is, I think I'm going to have to just fly it as is and see what sort of glideslope performance I get. If it's lacking, I guess I'll be back on my back ripping out wiring.

Click to expand...

Compromises

With a dedicated antenna for each input, practically all received input power from the antenna (minus line losses, etc.) will be available to each receiver, giving the best range of reception. A VOR/LOC (or NAV) antenna is optimized for the VOR/LOC frequency range of 108-118 Mhz, but does a fair job of also picking up the glideslope frequency range of 329-335 Mhz (~third harmonic of 108-118 Mhz). Ideally, you would have a separate GS antenna optimized for that frequency band, but that is a more expensive option. Since the GS signal is so localized to the approach end of the airport and range is not a huge issue, a nav antenna does an acceptable job of picking it up.

Any use of splitters, diplexers, or triplexers is a compromise. All of these devices provide impedance matching of outputs to inputs, minimizing impedance mismatch (reflection) losses.

What is often referred to as a "splitter" (ex: Comant CI-5120 or Mini-Circuits ZFSC-2-1) splits the entire received signal to two separate outputs, each output including both the VOR/LOC and GS components and about half the power or a little less (-3 to -4 dB loss) of the received signal. This will reduce the range of the received signal versus a dedicated antenna.

A "diplexer" (ex: Comant CI-507 or RAMI AV-570) is used to break out the separate frequency ranges to two separate outputs (one for VOR/LOC and one for GS) by filtering out each component (with usually less than -1 dB loss for each). In other words the GS signal on the GS output of a diplexer will only suffer about -1dB loss of the input GS component, but the VOR/LOC signal on the GS output with be filtered out by about -30 dB or more, and vice-versa for the VOR/LOC output of the splitter. The SL-30 has a built-in "diplexer", the GNS-430 does not.

A "triplexer" (ex: Comant CI-505 or RAMI AV-571) is a combination of the two. It has one input connector from the antenna, two VOR/LOC outputs and one GS output. I'm not sure if the signals are "diplexed" first to the GS component and VOR/LOC component, and then the VOR/LOC further "split" to two outputs, or if it is split first and then diplexed. There is a difference. If it is diplexed first (which I suspect it is), the GS component of each of the VOR/LOC outputs would be at a very low level (-30 dB) compared to the input, but the GS component at the GS output would only be down about -1 dB). If it is split first (which I doubt) then the output at the GS output would be down about -4 dB as would each of the VOR/LOC outputs (but each of the VOR/LOC outputs would contain a GS component). If somebody knows more about how the "triplexer" actualy splits the signals, please pipe in.

So, the best compromise in my mind when connecting a single nav antenna to a GNS-430/530 and SL-30, would be to connect the nav antenna to the input of a "splitter" (I used the Mini-Circuits ZFSC-2-1) with one of the outputs connected to the SL-30 (with its built in "diplexer") and the other output connected to the input of a "diplexer" (I used the RAMI AV-570) and the two outputs of the diplexer ("VOR/LOC" and "GS") connected to the corresponding antenna inputs of the GNS-430/530. I believe this is what Stein and others have recommended as well. But hopefully now you know why

thanks paul, at least you understand what it is I'm getting at...

I've got a pretty extensive background in radio, so I know what it is I'm trying to accomplish, I just don't know exactly how these magic black boxes with zero documentation achieve what they claim to achieve. I even tried to call comant and talk to their tech people, but noone answers the phone over there.


I considered just setting up a test rig and measuring how the thing behaves, but I only have a 100MHz scope at the moment, and I'm not sure how fast my frequency generator is.

I figured out how I need to do it. I bought a mini-circuits ZFSC-2-1 power splitter/combiner off ebay (cheaply)... it's the unit recommended in the SL30 manual, and was recommended to me by another avionics guy...

Ok, assuming all devices have a 0.5dB insertion loss (which is pretty accurate):

If I went out and bought a diplexer, then I'd have a 3.5dB split before a 3.5dB diplex into the 430W, for a total 7dB loss, and a 3.5dB split before the 3.5dB internal diplexer in the SL30 for a total 7dB loss.

If I cheaped out and used my existing quadraplexer, I'd have a 3.5dB split before a 6.5dB quadplex into the 430 for a 10dB loss (yuck), and the same 7dB loss into the SL30.

However, I can quadplex first, which gives me the 6.5dB loss into the 430, and then use the ZFSC as a combiner rather than a splitter, which actually gives me a 3dB GAIN instead of loss, for a net gain of 2.5dB. Then we have the 3.5dB loss in the SL30's internal diplexer for a net loss of 7dB and no more equipment to purchase.


I'll report back with results when I get it all together

Antenna theory.

Deleted answered previously

aw George, you could have left it up there... it was good info.

just wanted to report back and say that the setup I described above works flawlessly. It allowed me to use my existing magic box and only purchase that mini-circuits combiner for $28 shipped.


Also, I confirmed that the quadraplexer is indeed a low pass and high pass filter.