Interesting that a bottom only antenna still sees traffic above, because transponder antennas are usually on the belly. Does than mean the signal is reflected from the ground? Maybe someone with stronger RF skills than I have could comment.
The phase difference should be easily detectable and it’s obviously been thought about, because the phase angle corresponds to the azimuth angle with the GTS 800 antenna. The blades on the GTS 800 antenna are about 67mm apart, which is about 1/4 wavelength at 1090 MHz, so 90 degrees phase difference for a target at 3 o’clock and 30 degrees for a target at 1 o’clock.
I guess waves somewhat diffract around obstacles without losing the phase difference.
Where do you have the antenna mancival? Is it between the wings, in front or behind?
It should also work with λ/4 spacing, in which case AoA = Δφ.
Where do you have the antenna mancival? Is it between the wings, in front or behind?
more or less below the pilot seat, close to the centerline.
You may well be right but I thought that the phase difference will be too tiny to be usable, because the wavelength (~1GHz) is about 30cm and the antennae are much closer than that to each other.
Phase differences are much easier and much more precise to measure than differences in field strength. Even extremely small phase differences smaller 1 degree are not really a challenge….
The best traffic box on the market is the one you can afford.
The best TAS product is an active TAS, of which Garmin and L3 are the current best examples, with the historically most commonly installed Ryan/Avidyne TAS6xx being still popular with installers.
With ADS-B IN you also cover certified (SIL=3) emitting aircraft; very few around today but improving.
The reason is simple: it picks up Mode C transponders which – despite a lot of localised resistance – remains by far most widely adopted conspicuity technology. You get distance, relative altitude and azimuth.
The problem is the cost – over 10k. Mine cost GBP 13k. Accordingly, whenever you mention this on social media (not EuroGA) you get beaten up for being filthy rich and that you should not be expecting others to spend this money, etc, etc 
The various other technologies, with FLARM being the best known historically, is a highly fragmented field. Maybe a leader will emerge one day… many many previous threads on these topics.
The problem is the cost – over 10k. Mine cost GBP 13k.
Which poses a question: does that £13k really represent the best “safety bang for the buck”? The answer to which depends very much on how, where and what we fly.
For someone flying a fast airplane in relatively crowded/constrained Class G above the English Home Counties, perhaps it does.
For someone who flies mainly in Northern Britain, which is statistically immune from risk of airborne collisions, detecting 50 or even 90 percent of (non-existent) conflicting traffic… well, it’s hard to argue that 13 Aviation Monetary Units couldn’t be more effectively invested in some aerobatic training, or a BRS chute, or a decent handbag for “senior domestic management”.
At the other end of the affordability scale are various portable ADSB detectors which cost about 0.15 AMU. These currently detect about a quarter of potentially conflicting traffic. For us northerners 25% of zero is much the same as 90% of zero, but hey, for 150 quid it’s a bit of a no-brainer for the occasional game of dodgems under the London TMA.
Yes, it isn’t obviously great value for money – and it can be a hassle since few avionics shops seem to be able to install it properly.
I am very pleased with the 13k. It has enabled me to avoid a large number of very close encounters, and one can never tell if they would have been worse. And of course you need just one closer one to be dead. Many of these were in the circuit, and probably most were planes which were either unknown traffic (somebody messing around, not related to the airport) or people “misreporting” their position, sometimes by miles. The classic one is someone saying they are on final, I am on downwind, then I turn base, and then I spot the other traffic who was flying a circuit with a 5 mile final and is now in conflict. Especially if it was someone flying at 50kt.
I am however not sure about the % which the portable units will detect if you fly where most UK GA flies i.e. about 1000-2000ft. I reckon the people who fly non transponder (or more likely with the unit turned off, or set to Mode A only) are the same people who won’t be emitting any kind of ADS-B either – because they do it for their own dodgy “civil liberties” reasons.
Anybody with a brain who can read will eventually find out that ADS-B signals are very easily picked up from a huge range; much wider than SSR, and for almost no cost. Throw in the airspace map into the software and you have an instant CAS bust detector You can be sure that ATC authorities know about this; the only Q being whether they can use the information openly. They would probably need SSR corroboration to prosecute, but nothing stops them keeping records on regular infringers.
