Peter wrote:
cycling through all the various elements within the duration of a Mode A/C transponder response packet which is, from vague memory, about 100us
100us is an age to the embedded ARM microcontroller and DSP that might be embedded in that antenna (although to be honest if there were digital electronics inside that thing I’d expect the connection to be shielded twisted pair rather than coax)
I did some googling and it turns out that I dug into this matter 5 years ago… it’s a bit sad when the most relevant google hits are something you wrote yourself!
Anyway…
Then someone reported they have one installed in an RV6A but, guess what, that was all – just that completely useless one-liner 
Someone else reported: “Location of the Antenna is crucial”
100us is an age to the embedded ARM microcontroller and DSP that might be embedded in that antenna (although to be honest if there were digital electronics inside that thing I’d expect the connection to be shielded twisted pair rather than coax)
I agree 100% but there is no evidence they are doing anything remotely like that.
The whole underlying impression I get is that this product is really ancient, and so is the designer.
If it actually worked, the Avidyne TAS boxes would not be selling (and, frankly, Avidyne might now be bankrupt without those cash cows) because they cost way more – I paid £12k for mine, and they need a fairly good installer. Especially if installing into a composite roof with a ground plane inside like the TB20GT.
Peter wrote:
Also the ATD-300 antenna is just a dumb microwave antenna. They even say you can use a standard DME antenna in its place. So there is no processing done inside it.
It is not a dumb microwave antenna, it is a directional antenna with antenna switching inside. No processing, just switching. This is a very common direction finding technique. The switching controlling over coax cable is also very common for low end direction finding techniques.
Yes, you can use a standard DME instead of the ATD-300, at that time you will loose the direction sense, thus the system work as other passive traffic systems only by estimating the distance based on signal strength, and working out the altitude difference based on mode C reply of yourself and intruder.
Peter wrote:
What somewhat surprises me is that Monroy appear to claim they are switching different antenna elements, using control signals sent up the single coax cable. And they would be doing this quite quickly – cycling through all the various elements within the duration of a Mode A/C transponder response packet which is, from vague memory, about 100us.
Again this is common, and the reply time v.s. switching time is in a whole different timescale. Whatever processor, will be much faster.
Peter wrote:
It would be quite neat, as it would avoid having multiple cables of closely matched length which the Avidyne (Ryan) TAS boxes need.
It is a different product, where the TAS boxes are MUCH more accurate, if equal electrical length. Lots or TAS issues are due to ignorance on the RF part, or limited or now understanding of RF.
Peter wrote:
The whole underlying impression I get is that this product is really ancient, and so is the designer.
Sure, this is old technology, it does work if you understand the systems limitations. This is equally true for the Ryan / Avidyne TAS boxes or Stormscopes for that matter.
Peter wrote:
If it actually worked, the Avidyne TAS boxes would not be selling
The product is in a different range. It is about 10% of the cost of a TAS box, and it is accuracy might be about 10% of that of a TAS box. It is just not fair to compare these systems from a specifications point only, just ignoring the huge price difference.
It same that that you don’t like Flarm, while a lot of people have good results with it. The systems has due to it’s design / willing to be low cost / entry market, some limitations. If you can live with those, you have a neat system. If you have more budget and want more accuracy, you with want a TAS box instead of Flarm / passive system.
It is about 10% of the cost of a TAS box, and it is accuracy might be about 10% of that of a TAS box
In that case it must be completely useless, because a TAS605 is accurate to say 20 degrees, so the Monroy will be accurate to 200 degrees 
Also, to a first order approximation, I cannot see why a switched-antenna system should be less accurate in azimuth than one which has the antenna elements monitored via separate cables. All one has to do, to achieve the same azimuth accuracy, is to switch all of them around within the duration of a transponder packet. And synchronously run the receiving circuitry of course (need to run an A-D on the amplitude of the L-band carrier).
In fact the switched system should be more accurate for azimuth because you are eliminating the cable length difference.
The Avidyne TAS boxes are active so they will deliver good distance accuracy, but if the Monroy is 1/10 of that, that is also rather useless, because there is a big difference in what one does about a target 3nm away and 1/3 of a nm away.
Have you installed the Monroy 300, Jesse?
It same that that you don’t like Flarm,
I wasn’t aware of that… I merely like to point out issues, and if someone disagrees they are free to say so (with references if possible, otherwise it is just another opinion).
I flew with a borrowed FLARM box for some months, southern UK, and picked up about 1 target in that time. What is wrong with me reporting that ?
I can’t believe that none of the readers here would have some real life experience with the ATD300. PLEASE, don’t be shy and share it, regardless if good or bad. All the theoretical/technical discussion is interesting, but at this price, there must be some units installed in Europe.
Jesse wrote:
It same that that you don’t like Flarm, while a lot of people have good results with it. The systems has due to it’s design / willing to be low cost / entry market, some limitations.
Not to mention low power consumption. It was designed for sailplanes, unless I’m mistaken. And no big, heavy and/ or power hungry device will be installed on one. And you can forget about a couple of external antennas. For that market, it’s not really entry level. Relatively low price was I assume a goal so that as many people buy it as possible. Its usefulness is related to how widely it’s used in any given area. AFAIK powered planes are secondary to sailplane pilots – it’s much more likely you’ll collide with another sailplane.
Martin wrote:
AFAIK powered planes are secondary to sailplane pilots – it’s much more likely you’ll collide with another sailplane.
Yep. Glider pilots carry parachutes for good reason.
This is off topic, but as I wrote before, it is wrong to accuse me of being against FLARM. What I will point out is that it is unlikely to work as is often claimed – simply because almost no powered GA carries it. It would be disingenuous of me to suggest otherwise when my own flying with FLARM picked up almost nothing. One obvious exception might be if you are based near a popular gliding area, which may be relevant to some pilots if they do most of their flying in the local area. But if you generally fly around Europe, you will pick up virtually nothing on it, and you will pick up even less if you take a little bit of care to avoid gliding spots. For example, 3 days ago I was here at about 5000ft, OCAS. Nil wind. There was known heavy gliding below that cumulus, but there was zero chance of anybody gliding above it because there was absolutely zero updraught above the cloud tops (as will always be the case, if there is no terrain, and no wind, around). A glider has no way to get above the cloud in those conditions.
Back to the Monroy product, I have never seen any feedback from anybody in Europe. Also none of the several known avionics installers here on EuroGA have reported anything. The very few US reports from pilots have been very mixed. It is debatable how much one should infer from sparse data, but I have found that in GA one can quite often infer quite a lot from sparse data OTOH the data could be sparse simply because the product is too cheap to be of interest to installers; 25% on say 10k is a lot more than 25% on say 3k. That alone can explain very low sales.
It would be great if it worked because a full active TCAS installation is a big job which involves taking the aircraft interior apart, and very few shops can be trusted to do that. The outfit which did mine stripped the threads on about half the trim screws (power screwdrivers).
By the way, I have had two flarms in my life and I will swear by it. Gliding without them in a number of Alpine countries is now prohibited and even many power planes carry them in the Alpine region these days. I would call them almost a standard in Switzerland (partially, because they also have a very useful internal database of dangerous cables and other hazards). Mine saved the day for me at least a couple of times (that I know of). I recall that the number of mid-air collisions was brought down by a factor of around 10 or so by the arrival of flarm alone in the countries, which made them mandatory. They are now spilling-over across Europe and on an average flight in Germany or Czech Republic, you will see at least a dozen of targets. UK has probably much lower penetration, for various reasons. We are now thinking about adding one to our Centurion, too.
But before this thread is completely hijacked into the flarm topic, here is a reply to my email from the manufacturer and designer of ATD300, which I was authorised to share here… and still begging for PIREPs from any users:
Thanks for your email.
Unlike traditional TCAS antenna systems that involve complex multi cables and channels the ATD-300 uses a single coax cable and a single RF channel for bearing determination. This is possible due to a scanning scheme inside the antenna that conveys the four quadrants signals into a single coax. Unlike TCAS antennas the ANT-300 has a PCB were circuitry and antenna elements are embedded. At the ATD-300 the signals are amplified and decoded for bearing determination. Bearing is determined before the end of the traffic reply by a hardware circuitry and presented to the microcontroller. All the quadrant signals go through the same coax and are affected equally by the coax characteristics. Unlike TCAS systems were the 4 coaxes need to be matched for phase and amplitude. TCAS systems also employ 4 separate RF receivers which add to the complexity and costs.
I was one of the first engineers involved in the first TCAS system at Bendix/King (later Honeywell) development. After several concepts tried we end up with a 4 coax antenna and a 4 RF channels processor. The system price was $100k and weighted over 20 pounds with coaxes. It was intended for the Boeing fleet. No way it would fit in my Mooney.
After I retired I gave it some thought of how can I have something small and within my budget range. Over 20 years has lapsed from the first TCAS to the ATD-300. What made possible the ATD-300 is the new technology found on the new smart cell phones. As an example in the old TCAS systems the ARINC 429 interface PCB was bigger than four cell phones. Today the ATD-300 ARINC 429 interface chip is a mere 0.25” x 0.25”x 0.05”
I have an ATD-300/ANT-300 interfaced to a GNS530W in my Mooney and found to work pretty well. Some of my old colleagues say “why we didn’t think about this”.
Well budget and size was not a limitation since TCAS was mandated for the airlines. And the cell phones of the day would not fit in a pocket but in a briefcase.
Had a lot of fun working on this project. It challenged my old brain.
Peter,
I installed the ATD-300 with the associated directional antenna. It works sufficiently poorly that I need to review the installation to see if there is a fundamental problem. Sometimes it is quite accurate and there is a plane I can actually see at about the relative altitude and bearing presented. Often there is a plane at about the distance and relative altitude on some bearing (+/- 90 degrees, with the potential for 180 degree sense error – i.e. somewhere between 000 and 360!). Frequently it tells my about a target at a relative level and reasonably close (say 1-2 miles) that I can’t see and therefore can’t confirm is real or not. Worryingly, sometimes there is traffic called by a radar unit, or an aircraft I see that the ATD doesn’t report. There is a firmware upgrade to resolve part of this last problem.
Overall, it is not my best investment ever! Annoyingly, it doesn’t pickup 1090-ES data, so in that sense is less useful than say a powerflarm for decting mode-S traffic. I passed on the powerflarm because I hoped to have something ‘installed’ to display on the 530, but the European powerflarm manually says ‘do not install in US aircraft’ which makes it a bit challenging to get an avionics shop to install it anyhow !.
