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Nav/LOC/GS Antenna Pair - Combiner Necessary?

mikeyj350

Well Known Member
Hi all,

I'm slowly starting to plan my antenna choices and placements as I get further into the fuselage kit of the -10. I'm starting from [near] zero knowledge on this subject and trying to learn as I go. I came across a "great deal" on a pair of Comant CI-120-G/S sharkfin antennas that I'd like to use for my VOR/LOC/GS. As I understand it, they are normally installed in pairs, then wired to a "combiner" which then (depending on your equipment) may need to be diplexed out into NAV and GS. Furthermore, if you want to run this signal to two NAV receivers, say dual 430's, you'd need to [quadraplex?] it out behind the panel. Well, the antennas I'm looking at don't come with the combiner, just the two antennas. Each antenna is stamped with the same part numbers (CI-120-GS), and each of course has their own BNC connector. Ok so here's my question:

If I want to run dual NAV receivers (dual GS, etc), is it an acceptable method to ditch the combiner and just run two diplexers instead, yielding two completely independent antenna runs to two independent NAV receivers? In my mind that sounds like the best idea, plus I don't have to buy a combiner :D Or, does the combiner/quadraplexer setup give you improved reception greater than the sum of each individual antenna? Failure modes of each?

Any clarification on this would be great... thanks in advance!
 
You need to join the two CI120 antennas together with either a combiner (normally supplied with the antennas) or a co-ax tee piece. You then run a single co-ax to a Comant CI1125 dual Nav/Gs diplexer to split the signals to feed the dual GNS430s. The CI120 antennas form a matched pair to create the correct polar diagram for optimal reception - you can't run a separate Nav receiver off just one half of the CI120 array.
 
Although I am sure those are nice antenna, you will find that most of us run the Archer antenna in the wing tips with very good success. I run a single Archer for a GTN650 to get LOC/VOR/GS with a single connection. No need for a splitter.
I believe the older 430w requires a splitter to get both LOC and GS out of the same antenna. You could use one in each tip if you want two Nav.
I get 100 miles with my Archer on a VOR.
 
Thanks

Thanks for the replies guys, I appreciate it. I figured there must be a reason they use the combiner but just wasn't sure if there was an alternate way to make it work.

As for the Archer nav antenna, I had considered this option a while back but to be truthful it seemed like many folks weren't getting the same performance you seem to be getting with yours Mark. I've read more than one write-up where the person is only marginally satisfied with it at long ranges and only recommends it for a secondary nav antenna... I guess that was enough for me to drop it as an option. I'm glad you're getting good performance with it though!
 
The bad news is that I only get about 45 miles Nav range on my 430w with the Archer wingtip ant on my -10.

The good news is that is about three times the range you'll ever need. With two GPS's (the 430w plus Skyview), the only purpose NAV serves is for LOC/GS.

When it comes to NAV, I'd go simple and cheap.
 
The CI-120-GS you have is not two antennas, this is one antenna consisting of two parts. For proper operation you need to use the combiner.
Running two seperate lines from each antenna part will just not work.
To use the Antenna with two GNS430 you need to have the CI1125 as well.

As an avionics inspector i often check airplanes with that kind of antennas and the performance is okay, but compared to the Archer Nav Antenna installed on our RV-9A they have less sensitivity. We have a reception of more than 80nm, which is excellent. The antenna is installed in the left wingtip, and I used RG142 coax.

I would install the Archer antenna because of the performance ( if correctly installed), weight, its cheap, and has no drag as it is hidden in the wingtip. Then use the CI1125 and you have a well performing system.

Btw, I have an unused (new) CI1125 bought from ACS, if you want to have it send me a PM.

Kai.
 
Thanks for the replies guys, I appreciate it. I figured there must be a reason they use the combiner but just wasn't sure if there was an alternate way to make it work.

As for the Archer nav antenna, I had considered this option a while back but to be truthful it seemed like many folks weren't getting the same performance you seem to be getting with yours Mark. I've read more than one write-up where the person is only marginally satisfied with it at long ranges and only recommends it for a secondary nav antenna... I guess that was enough for me to drop it as an option. I'm glad you're getting good performance with it though!

I think the posts you're referring to were people that were unhappy with the COMM Archer antenna, not the NAV. Archer's COMM antenna has to be vertically oriented, so there's not much room in the tip to do that. Therefore, the performance of those isn't as good as a whip antenna. The Archer NAV antenna on the other hand has provided good service in the eyes of most. I have two of them (one in each tip) and I'm rethinking that, because so many people have recommended not putting two NAV radios in a modern airplane. I haven't made the decision yet, but if I decide to get rid of one, I'll be looking to sell it on this site, more than likely. My 10 is not yet flying.
 
The Archer NAV antenna on the other hand has provided good service in the eyes of most. I have two of them (one in each tip) and I'm rethinking that, because so many people have recommended not putting two NAV radios in a modern airplane. I haven't made the decision yet, but if I decide to get rid of one, I'll be looking to sell it on this site, more than likely. My 10 is not yet flying.

Why two, instead of one and a splitter?
 
Two antennas and two NAV radios..

.. would equal better range and reliability (i.e. full ground based nav as a backup to GPS). Plus I learned to fly using VOR radials for navigation, using two radios (old habits I guess). I suppose an adequate backup could be one radio and one antenna, which I could flip between freqs to locate, if I had to. I'm not sure how robust the ability for ground-based nav would need to be, as a backup to GPS. Plus I'm not sure how many VORs are active in other countries outside the US. Anybody know how "useful" ground based nav is in other countries??
 
2 antennas vs 1 plus splitter

Better reliability? yes. Of course there's nothing out in the breeze so it probably goes from 99.9999 to 99.99999 %
Better range? Not necessarily. What matters is signal to noise; when you split the signal, you drop the signal strength, but you also drop the external noise just as much.
As always, only you can determine if the benefit is worth the cost (money, weight, construction time) which, in this case, is pretty small.
 
Better reliability? yes. Of course there's nothing out in the breeze so it probably goes from 99.9999 to 99.99999 %
Better range? Not necessarily. What matters is signal to noise; when you split the signal, you drop the signal strength, but you also drop the external noise just as much.
As always, only you can determine if the benefit is worth the cost (money, weight, construction time) which, in this case, is pretty small.

Bob,

To help me make that decision, do you know how "available" ground-based navigation infrastructure is in other countries. I plan to travel quite a bit in my RV-10, potentially across the Atlantic. My goal is to have reliable means of IFR flying in all countries, if possible.
 
It's hard to argue that with GPS as a primary nav source (triple redundant at a minimum with 2x experimental receivers on the displays and at least one TSO'd unit... not even counting tablets and phones with apps like ForeFlight) it's going to be awfully hard to get lost enroute. I agree with Myron, 99% of the time I'd only be using it to fly a LOC/ILS, and then hopefully range isn't really an issue. A counterpoint to that though is why sacrifice signal strength when "we have the technology"?

I guess it's hard for me to embrace the idea of my shiny new RV-10 not being able to pull in a VOR as well as the 1976 Archer II I'm flying now. I mean I'll have close to as much money in avionics alone into the 10 as the Archer is probably worth in total! Even if VOR's aren't the norm anymore (and even increasingly less so), the ex-avionics engineer in me still wants to ask about worst-case scenarios, GPS outages, weak ILS signals, etc etc.

Anyway, I guess I'm drifting a bit from the initial question. I definitely appreciate everyone's input, even if now I feel like I'm more confused than when I started. I hear some people have excellent performance out of their Archer NAV antenna, some don't, and now I also hear one data point where the CI120's aren't even that good of a performer! Ugh what to do :(

I think I should just build this thing to be a NORDO.
 
The thing is, that with EFIS and GPS, EVERYTHING navigationally that you can do with a ground based navaid, (i.e. VOR, NDB) you can replicate and replace with the GPS. The only exception is the higher sensitivity (and slightly lower minimums) of a localizer/GS.

For any routine navigation that you would be doing with a nav/vor, a GPS plot of the fix gets sent to the CDI and/or RMI needle in the HSI and functions exactly the same, including intercepting radials or identifying cross fixes. From the flying the HSI standpoint, there is no difference. The only operational difference is that the fix selection is by name instead of frequency.

Two GPS's for redundancy plus one Nav for ILS capability is perfect for today's environment. Since you have to have an IFR certified GPS, a box that has both integrated like a Garmin is ideal. Installing more than one NAV today would be akin to installing more than one ADF 20 years ago.

I've flown for airliners for 30 years and the times that I've needed something less than GPS approach minimums in order to get in could be counted on both hands and two thirds of those were even below ILS minimums and accomplished on a HUD. I have ILS capability in my -10 and have no problem flying IFR, especially fog, which is where you get low approach minimums, and if I ever get to log more than one or two actual ILS approaches at lower than GPS/Vnav minimums in my 10, I'll be amazed. I actually look forward to that day and will even seek them out just for the challenge, but they are as rare as hen's teeth.

Edited note: I didn't see Mike's last post until after I had composed mine.
 
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Hello Mike,

the CI120 is a good antenna, but the archer is better if you don't have to use TSO certified products.
Today I checked the range again, at an altitude of 3000ft I had a range of 30nm with both receivers (GTN and SL30). At 7000ft I had 80nm.

Speaking of other countries, here in Germany we still have lots of VORs.
 
Archer Antenna

For background - I inadvertently swapped the Archer & Comm connectors to my GTN650. The comm reception was very poor & the I lost contact with ATC just as I passed the 10 mile vail (sp?). My experience: Archer is a poor Comm antenna. (I landed at the first opportunity and swapped the connectors).

The 430W needs a splitter as noted earlier & the 650 has the splitter included internally.
 
For background - I inadvertently swapped the Archer & Comm connectors to my GTN650. The comm reception was very poor & the I lost contact with ATC just as I passed the 10 mile vail (sp?). My experience: Archer is a poor Comm antenna. (I landed at the first opportunity and swapped the connectors).

The 430W needs a splitter as noted earlier & the 650 has the splitter included internally.

No surprise. VORs are horizontally polarized, coms are vertical. I have an Archer clone as an antenna for my backup com, it is bent downward as much as possible in a 10 wingtip. Not as good as the external antenna, but better than 10 miles!
 
Correction

I was talking to Tampa Approach out of Sarasota. So, the air distance is 30 to 35 nm. Still nothing spectacular. The drop-off at the edge of the vail was dramatic. So, at most 40 nm.
 
Thanks Guys!

Looks like I'll be selling one of my Archer wingtip NAV antennas now. You interested, Mike?
 
Hey Mike... appreciate the offer but I think I'll pass. I think I'm going to give this blade set a try (using a combiner and diplexer as necessary, of course) and go from there. I am truly thankful for everyone's opinions, advice and wisdom on this topic! Here's to hoping I'm satisfied with my choice in a few years :)
 
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