iFly GPS Forum

Not sure what you mean by this.  It's true that for ADSB-out,1090 MHz ("1090ES") is required in the flight levels.  However, 1090ES is also perfectly legal below 18000'--that's the freq that my ADSB-out transponder uses, and my little Cherokee is unlikely to ever make it higher than 15000' even on the coldest of days.  Thus, you can satisfy the ADSB-out requirement in your Cessna 340 with the single 1090 MHz frequency.

But we're only talking about ADSB-in in this thread, and since that capability is entirely optional, neither frequency is legally required.

It appears that the Garmin ADS-B in antenna is a shortened version, either with loading coils aor folded in that plastic case. It will not be as effective at capturing weaker signals as a full length antenna (aerial) like the Stratux kits use.

I use a transmitting blade type antenna mounted on an inspection plate on the belly and a T-connector to feed both receivers in my Stratux and it work's fine. The T-connector is lossy - a bad impedance match - but probably matches the shortened Garmin antenna overall.

Now for some links and less pretty pictures...

r/Stratux is a great resource, and one I've used a lot while supporting clients who have them - https://www.reddit.com/r/stratux/

Here's some commentary from someone who knows more than I do about these antennae, though... 

 

I'm no RF genius, but my understanding is that the stock antenna has higher bandwidth due to its thickness, and of course is significantly bigger than the smaller antenna. I was surprised myself, but I guess it's just physics!

Part of it is the increased bandwidth. As the width of an antenna increases so does the bandwidth, however there are practical limits to increasing the width of the antenna element.

Another consideration is the impedance of the antenna and the input impedance of the SDR dongle. The SDR is advertised to have on input impedance of 75 ohms. However, this holds only when operating in the TV design frequencies of the TV vand. When operating outside those frequencies the impedance can vary widely. The stock antenna allows the user to "tune" the antenna to the SDR.

An antenna can also be thought of as a tuned circuit. The shorter "custom" antennas are loaded to decrease their overall size. They remain tuned to the design frequency but the have a tighter bandwidth. In addition they are physically shorter this having less exposure to the incoming signal.

The ground strap accomplishes two things. One, it helps stablize the input impedance to the SDR and two, it creates a counterpoise this increasing the effective lengthy of the antenna. However, due to the design decision to shorten the antenna the short "custom" won't be as efficient as the "stock" antenna.