GTX 345R with top antenna only (Canada)
 

Last avionics I bought for the panel was the GTX345R since it appeared Nav Canada was not going to make up it's mind for GA any time soon and soon after that they announced further delay.
Part of that appeared to be because for GA purposes they could not assure bottom mounted antenna could broadcast sufficiently up to satellites.

So I was curious and maybe the G3Xperts can comment. If you mount the antenna on top will there be sufficient receptions and broadcast for ground based systems. Seems to me the only time you'd have a problem is when directly above a ground based receiver/transmitter, otherwise most of the time that should be fine. Was this ever tested?

Thanks for your comments.

This doesn't answer your question, but I found the potential Nav Canada requirements interesting.

https://www.ainonline.com/aviation-n...ntenna-mandate

Published June 12, 2019:

"Despite objections by U.S. and Canadian general and business aviation user and avionics associations, Nav Canada is not budging on its requirement for ADS-B diversity antenna configurations, meaning antennas mounted on the top and the bottom of aircraft. The mandate is effective beginning Jan. 1, 2021.

Nav Canada, the company that operates the nation’s ATC system, claims these antenna configurations are needed to improve ADS-B reception and performance, as well as support five-nm aircraft separation using ADS-B. However, in a letter to Nav Canada, AEA, AOPA, CBAA, GAMA, NBAA, Garmin and four other organizations contend that the cost to equip small aircraft with diversity is “significant and installations of this type of system are rare.”

In its response, Nav Canada noted that it is not implementing the requirement below 12,500 feet, where it says the majority of small general aviation aircraft fly. Furthermore, the mandate will provide “our affected customers with the efficiency and safety benefits to justify the expense they will incur to upgrade their avionics to meet the performance requirements,” Nav Canada said, adding that the requirement “will harmonize with U.S. and European ADS-B Out mandates.”

The requirement to be equipped with diversity antenna systems will be implemented in two phases. Phase 1 starts Jan. 1, 2021, in Class A airspace between FL180 and FL600, and in Class E Airspace above FL600; Phase 2 starts on Jan. 1, 2022, in Class B airspace between 12,500 feet and FL180.

Nav Canada is also considering a Phase 3 that would broaden the mandate to Class C and D airspace around specific airports. Nav Canada said it plans additional stakeholder consultations and that if there is a Phase 3, it would not be implemented before January 2023."

The reference below states the implementation dates are sliding, with no new dates proposed:

https://www.skiesmag.com/features/di...-requirements/

"However, on Oct. 24, the Canadian Owners and Pilots Association (COPA) reported that Nav Canada will be delaying the implementation of the first and second phases of the ADS-B Out mandate.

Numerous operators have said they will not be able to meet those deadlines, wrote COPA. In addition, Transport Canada has indicated there is still regulatory work to do before implementation. No new dates have been announced so far."

for people like us ...
 

The problem for people like us (that would be builders, in Canada, who want to fly into the US) they have left us in an awkward position for several years.

If the antenna can be put on top problem solved. But for all the rest the only way to comply is antenna diversity. I suspect since 99.9% of the time we are at some offset to towers and other aircraft a top antenna will work very well. And since this will affect Europe and most of the rest of the world our G3Xperts may have already tested for this.

Cheers.

The solution in Canada will be the uAvionix tailBeacon X, that will have diverse antennas.

GTX 345R with top antenna only (Canada)
 

As far as I know (in the middle of this decision myself) the GTX 345R will not satisfy the future Canadian 1090ES requirement. This requires "diversity" antennas, one on top for the satellites, and one on the bottom for ground stations. I believe a diversity transponder alternates transmission between the two antennas which means you can't just have a normal 1090ES and two antennas. I'm planning, like you to have a GTX 345R with a bottom antenna (might install a top antenna so it is pre-wired), and am resigned to having to "upgrade" a near new transponder when NavCanada finally makes up its mind.

Let's look at this in some very real terms...

Nav Canada waited until many of us had already spent serious money complying with the US ADSB mandate before they made any mention of an ADSB mandate for Canada. In fact, when questioned directly on this topic two or more years ago, Nav Canada's answer was that Aerion and its space-based ADSB was only for trans-oceanic airline traffic and that there were no plans to deploy any form of ADSB generally across Canada and definitely not to airspace frequented by GA aircraft. Shame on Nav Canada for having sat on their hands for far too long!

From a technical perspective, we know for a fact that many 1090ES transmissions from aircraft with belly-mounted antennae ARE being received by Aerion satellites. Aerion's ALERT emergency location service has been used successfully in several SAR incidents. I know my own aircraft has been tracked using Aerion's satellites.

With this knowledge in hand the technical question then becomes "do we need a full diversity solution, or will Aerion's ADSB tracking work reliably with something less?"

The financial reality is that Nav Canada wants very badly to ditch their old radars. Yes, they have invested in Aerion, but given their desire to sell Aerion services to other customers, that investment could end up paying good dividends without ever having to saddle Canadian GA aviators with the cost of equipping for Aerion.

From a technical perspective, if we can drive Nav Canada to a solution other than a true diversity transponder then that's our only hope for a reasonable compliance level to an ADSB mandate.

As mentioned in a post above, the uAvionix Tail Beacon X might be an example of one of those "less than a true diversity" solutions. In speaking with the engineers at another 1090ES transponder manufacturer I was left with the impression they would prefer to offer a similar solution. They know their customers who have just shelled out big bucks for a 1090ES solution will be pretty steamed if they have to spend double that much again for a diversity solution.

Everybody will benefit from a solution that is capable of squirting a bit of energy up to the satellites and the majority of it down toward the ground stations.

As for mounting a GTX345 transponder antenna on top of the fuselage of an RV, the answer is a very clear "don't". You will end up being asked constantly by ATC to "recycle your transponder". Same goes for installing provisions, other than perhaps a skin doubler, for a top-mounted antenna. Even at that, I likely wouldn't install that doubler because whatever solution you end up with to comply with a Canadian ADSB mandate will come with some very specific installation requirements so your doubler might not be in the right location.

And speaking of Diversity transponders ...
 

"Garmin Announces Approval of the GTX 335D and GTX 345D Diversity Transponders by FAA, EASA and TCCA":

https://www.aviationpros.com/engines...-easa-and-tcca

https://generalaviationnews.com/2019...s-get-faa-nod/

And, as we saw when they were first announced, those are eye-watering prices.

tracking belly mount
 

To the point you made:
"From a technical perspective, we know for a fact that many 1090ES transmissions from aircraft with belly-mounted antennae ARE being received by Aerion satellites. Aerion's ALERT emergency location service has been used successfully in several SAR incidents. I know my own aircraft has been tracked using Aerion's satellites."

it's the very reason I'm asking about ground stations receiving top mount antenna. Foregtting about SAR for a moment where the bellow could be exposed, you are in good company where others have commented that they can be tracked (albeit it's probably inconsistent) broadcasting up. The only place I could see a problem is when directly above a receiving station. That's why I'm asking if it's been tested by Garmin. The 345 gave me integration and features I'm not going to get on the uVionics solution, so the question is specific to the 345.

Thanks.