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Diversity antennas for transponders

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oskrypuch View Drop Down
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    Posted: 21 Aug 2018 at 10:30pm
It is possible that Transport Canada (TC) may require diversity antennas (even for small aircraft) for 1090 ADS-B OUT, when/if it becomes a required addition. They are using satellite for ADS-B reception, no ground stations are planned, nor 978, or ADS-B IN availability.

Problem is that the AXP340, and perhaps the AXP322, are not diversity antenna compatible.

Can you confirm or deny that, and if not, will we have any options other than a Lynx 9000?

* Orest



Edited by oskrypuch - 21 Aug 2018 at 10:31pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2018 at 7:55am
Years ago when I was making the decision of what model of NGT9000 to buy, I was told by either my shop or Lynx tech support or both (can't remember which) that the diversity model with two antennas is for large aircraft, which to me meant a biz jet size at least, and up, if not bigger.  Small piston aircraft do not shield a single antenna from traffic targets I was told.  Having been assured of that, I settled on the 9000+ model with a single antenna, and have not been disappointed.  

Since that was years ago, it might be worth asking again.  But it made sense: if two antennas were required to get a complete picture of traffic in GA airplanes, the install manual should say that, and the pilots guide should warn you that depending upon a non-diversity model of NGT for a complete traffic picture is a big mistake.  They don't say that.  Finally, I doubt L3 would offer a dysfunctional product that did not work well on any airplane, big or small.  I could be wrong of course, there are a lot of criminal CEOs out there who should be in jail.  I just don't see a reason to put L3 in that catagory.

If Transport Canada intends to require diversity just to simplify things from a regulatory standpoint, they may be doing a disservice to small GA operators.  If Canadian GA pilots have any input to the process, it might be a good idea to get that going just in case the rumors are for real and not based simply on paranoia.
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oskrypuch View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2018 at 9:05am
Note that I was musing about aircraft ADS-B OUT, not IN. Top/bottom antennas will likely work a little better even for a small aircraft to intercept incoming squawks, but for most instances seems good enough.

But for signals being sent out from the aircraft, the potential driving issue up here, is that ADB-B OUT is monitored by satellite only. Reception is much poorer. All airline traffic has been required to install top mounted antennas, to participate. Currently ADS-B is only required in the flight levels, and only in remote areas where there is no radar coverage. When they bring that requirement to all controlled airspace, it is unknown what they will require, in particular for small GA. 

It could end up being a horrible cost burden. 

* Orest



Edited by oskrypuch - 22 Aug 2018 at 9:15am
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tony View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tony Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2018 at 9:42am
Orest,  can you please enlighten me.  What is a diversity antenna? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2018 at 10:09am
Diversity = More than one antenna. Here, it means one on the bottom, like your normal transponder antenna, and one on the top of the aircraft, both being used by the same transceiver.

And, it is not as simple as just hooking up another antenna. The unit itself must be certified and capable in using both.

* Orest


Edited by oskrypuch - 22 Aug 2018 at 10:13am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2018 at 2:20pm
You're right Orest, I thought diversity referred to traffic antennas.  So I can see why in the US, the belly transponder antenna already in use by your current transponder can be re-used with the Lynx NGT 9000+ to good advantage, since the FAA continues to use ground-based receivers for ADS-B Out just as it has been doing for Mode A/C transponders.  It sounds like Canada has created a different system altogether.  Not sure why, but if the satellites cannot pickup the ADS-B Out signal through a small GA airplane, that's a big problem.  Not only for Canadian pilots, but for US pilots visiting Canada.  If Canadian ADS-B receivers cannot pickup your ADS-B Out signal, you're obviously not in compliance with Canada's ADS-B requirements.  I can see why Canada might make a top antenna a requirement if their system won't work without one.  So I'd say Canadian GA pilots need to get together and prove to Transport Canada that a C172 fuselage doesn't block the signal.  Or if it does, lobby for some ground-based receivers. 

As you say Orest, the problem is not the cost of the additional antenna, it's the extra charge L3 adds to a 9000+ to make it capable of using two transponder antennas for ADS-B Out.  If you get a 9000+ or already have a SkyWatch, the diversity model can use the traffic antenna on the cabin roof for the second transponder antenna, so that's not even a factor.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2018 at 3:06pm
There is talk about having exemptions for small GA US traffic. Not only do they not have top side antenna, but many are 978 and would be invisible in any case. The satellites only pickup 1090 OUT, 978 is a US thing only.

But, if there is an exemption for 1090 small GA US traffic, why not Canadian as well. COPA is certainly active in this regard, but have only so much sway.

And the cost would be potentially trashing a perfectly well operating 1090 Mode S ADS-B OUT transponder, to install a 9000D. I don't think there are any other diversity capable transponders right now, and the Canadian market is so tiny (about 1% of the US), seems unlikely that anyone would rush to provide it.

It is gnarly.

* Orest



Edited by oskrypuch - 22 Aug 2018 at 3:07pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSteve Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2018 at 10:46am
Originally posted by oskrypuch oskrypuch wrote:

... the AXP340, and perhaps the AXP322, are not diversity antenna compatible.

Can you confirm or deny that, and if not, will we have any options other than a Lynx 9000?
Correct, neither the AXP340 nor the AXP322 have diversity antenna support.  I'm not sure what other options exist besides the Lynx 9000.
Steve Lindsley
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2018 at 3:42pm
Diversity as I understand it requires two receivers.  Is this necessary or could it be done with two antennas and a hybrid splitter?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2018 at 4:13pm
Originally posted by Paul Paul wrote:

Diversity as I understand it requires two receivers.

Assuming you meant transmitters, not necessarily two of them, no.  The Diversity model of the L3 Lynx NGT9000 is a single box that connects to two separate antennas for ADS-B Out.

I had thought that L3's "diversity" option referred to two TAS traffic antennas for the TAS traffic receiver for the NGT9000+ model.  It turns out that the L3 diversity option refers to two separate ADS-B antennas feeding one ADS-B receiver.


Edited by Catani - 24 Aug 2018 at 4:16pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2018 at 5:01pm
Well, perhaps it is possible to output from a single transmitter, to two antennas, creating the diversity array that may be required up here -- using some sort of device in between. 

Of course, the device would probably cost as much as a transponder.

* Orest



Edited by oskrypuch - 24 Aug 2018 at 5:02pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul Quote  Post ReplyReply Direct Link To This Post Posted: 25 Aug 2018 at 8:40am
If a single transmitter or receiver can provide diversity then the term doesn't mean what I understand it to mean.

A single transmitter has one antenna.  This antenna may be made up of multiple elements.  The result may be directional and it may have multiple lobes.  Some people might call the individual fed elements antennas and say that the result is an antenna system but this is just semantics.

For diversity in reception you would have two or more receivers each connected to an antenna or antenna system.  The output of each receiver is used separately.  In the case of a transponder an inquiry request received by any receiver would cause the transmitter to respond.

I haven't looked at the Lynx specifications but if it really has only one receiver then it is not actually diversity.  However the advantage of diversity is that it can be used to detect weaker signals and that may not be needed in a transponder.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 25 Aug 2018 at 2:05pm
Originally posted by Paul Paul wrote:

If a single transmitter or receiver can provide diversity then the term doesn't mean what I understand it to mean.

A single transmitter has one antenna.  This antenna may be made up of multiple elements.  The result may be directional and it may have multiple lobes.  Some people might call the individual fed elements antennas and say that the result is an antenna system but this is just semantics.

For diversity in reception you would have two or more receivers each connected to an antenna or antenna system.  The output of each receiver is used separately.  In the case of a transponder an inquiry request received by any receiver would cause the transmitter to respond.

I haven't looked at the Lynx specifications but if it really has only one receiver then it is not actually diversity.  However the advantage of diversity is that it can be used to detect weaker signals and that may not be needed in a transponder.

 
No, the Lynx's extra-cost "diversity" feature is not for reception.  The Lynx "diversity" model uses two antennas for transmission of ADS-B Out transponder signals: one antenna on the bottom of the aircraft and the other on top.  If it's a Lynx box without diversity, it uses only one antenna for that purpose, usually on the bottom of the aircraft.  There are no options for increasing the number of antennas used for reception, unless you count the need for a traffic antenna if the TAS option for the box is purchased.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul Quote  Post ReplyReply Direct Link To This Post Posted: 25 Aug 2018 at 6:13pm
The answers are in DO-181.  I don't have a recent copy but I doubt this has changed.

Two receivers and two antennas are necessary.  Each receiver listens to one antenna.  When an interrogation is received the transponder will respond using the antenna where the interrogation signal was strongest.  ADSB message broadcasts will alternate between the two antennas.

There are requirements for the relative locations of the two antennas and the isolation between them.

It is not possible to meet these requirements with a non-diversity transponder and a splitter feeding two antennas.

I tried a quick Google search and the only transponders I found which would be suitable for a small airplane are the Lynx and the Garmin GTX 330D (ES version).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 27 Aug 2018 at 4:57pm
If you mounted the single AXP340 antenna on the top, wouldn't that solve the Canadian issue?

Understood, though, that would pose issues with reception in the US where ground-based systems are used to receive ADS-B OUT.



Edited by chflyer - 27 Aug 2018 at 4:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul Quote  Post ReplyReply Direct Link To This Post Posted: 27 Aug 2018 at 5:15pm
You can't mount the antenna on top for two reasons:

First is legal: DO-181 says that a single antenna must be mounted on the bottom of the aircraft.

Second is practical: The transponder is not just there for ADSB out.  It is also responding to ground radar (mode C).   ATC needs it to know your altitude.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 31 Aug 2018 at 10:38am
uavionix is saying that the 2nd reason could be met by a 1090MHz version of their wing-tip solution. Don't know how that would meet the DO-181 wording though ..... does it really say that it needs to be on the bottom of the aircraft?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul Quote  Post ReplyReply Direct Link To This Post Posted: 31 Aug 2018 at 8:53pm
This stuff gets complicated.  Parts are covered in SA-151 and SA-165.  And the copy of DO-181 I'm looking at is obsolete.

If the wording in this copy of DO-181 is still current, it says under Single Antenna "The antenna shall be installed on the bottom of the aircraft as close to the longitudinal axis of the aircraft as possible." (emphasis in the original).

Perhaps Avidyne has someone who is conversant with these documents and who has a current copy and could comment on this?  I don't want to spread incorrect information and as I said before this stuff gets complicated.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2018 at 12:20pm
Originally posted by AviSteve AviSteve wrote:

Originally posted by oskrypuch oskrypuch wrote:

... the AXP340, and perhaps the AXP322, are not diversity antenna compatible.

Can you confirm or deny that, and if not, will we have any options other than a Lynx 9000?
Correct, neither the AXP340 nor the AXP322 have diversity antenna support.  I'm not sure what other options exist besides the Lynx 9000.

Having said that Steve, I would hope that Avidyne has plans to provide the support (understanding that Avidyne may not announce such plans before availability).

There are 2 elements of the diversity antenna discussion (which relates to ADS-B OUT .... i.e. AXP340/322):
- one is ATC surveillance via satellite rather than ground-based, which could well be the primary reason why Nav Canada has invested in Aireon. It would be much cheaper than starting today to build up a ground-based ADS-B tracking infrastructure to replace radar as has been done in the US.
- the other, and perhaps of equal or more interest to GA pilots for SAR, is the potential to replace all ELT's with satellite-based ADS-B surveillance.

In this context, I suspect that Canada (and perhaps also Europe) could well drop plans/requirements for ELT and require diverse antenna ADS-B OUT instead (note Ireland, UK, Denmark, Italy, and now Brazil ANSP involvement with Aireon, in addition to NavCanada). 

The latter makes a lot of sense, given the unreliability of ELT use (antenna breaks at impact, not turned on, simply fails when needed, etc), IF diverse antenna support/installation cost could be equal or less than an ELT.


Vince
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