ADS-B / UAT devices using external stubby transponder antenna
 

Many of the ADS-B / UAT builders have published information that their device will work with any transponder antenna, I believe that may be the case but they are giving up some system performance with that broad statement.
The GDL-39 on the other hand has published antenna performance specifications for use with the device. Looking at it from an engineering point of view, I'm sure the GDL-39 design team was tasked with achieving a certain level of product performance and to insure that level, they had to have some expectation of what the customer may connect to the device. The following from the manual:

The GDL 39 requires a UHF antenna that meets the following specifications:
? Standard 50 ohm vertically polarized antenna with a VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.
? TSO-C66, TSO-C74, or TSO-C112 antennas that also meet the VSWR specification.


I'm aware that many are planning to use the stubby monopole transponder antenna with the new ADS-B / UAT devices on the market, with that in mind, I set out to gather antenna performance data when used on frequencies out of the design parameters.

Since the antenna was designed to operate on transponder frequencies, (1030 and 1090 MHZ) the data indicates it does well with the 1090 requirement but exceeds the limits on the 978 MHZ side. 2.6 with a requirement of <1.7.

The VSWR data points may be difficult to see on the graph and are as follows:

M1 2.623 @ 976.74 MHZ
M2 1.762 @ 1029.17 MHZ
M3 1.195 @ 1087.21 MHZ


In summary, use an antenna that is designed for the task along with good quality coax if you are expecting maximum system performance.

Thanks for the data. This is exactly the question I was asking in our emails last week :)

What coax do you recommend? I have looked at RG58, RG-174, and RG-213.

I installed your ADSB antenna yesterday ! Looking forward to testing in about 3 weeks.

Thanks Don,

It is great to have real data instead of opinions.


In contrast:

Delta Pop UAT antenna results....

Max Performance vs. Required Performance?
 

Good info but it isn't really the wholistic story is it?

Certainly matching the ANT and transmission line to the Rx/Tx with lower losses results in better performance but the question is whether or not maximum performance will ever realistically be necessary? The extra $ spent on the spec ANT may or may not provide a true return on investment as the extra performance isn't required to reliably operate in the ADS-B system.

The ADS-B ground stations ( low, medium, high ) are designed with sufficient power to provide signficant overlap throughout the CONUS when fully deployed by 2020 - feel free to look up the specs. A 2.6 VSWR at the UAT frequency will easily receive ADS-B ground station transmissions well beyond the minimum station range (20nm). In the early phases of implementation when Ground Based Transmitters (GBT) are still being installed it is possible to fly into gaps of coverage - particularly in the mid-West, however a top dollar ANT isn't going to prevent this either. In air-air coverage a few extra miles is meaningless as you'll easily receive traffic at distances measured in tens of miles - do you really want to see all the traffic 50nm away, it is cluttering to say the least.

I installed a $18 transponder ANT with RG-400 coax for my ADS-B (in) UAT receiver and at 700ft AGL receive 14-18 stations routinely in Southern FL where GBT installation is complete (as is most of the East coast) I'm 12 miles from the nearest station and rcv ADS-B while on the ground intermittantly during taxiing.

Just a data point....

We have a few reasons that we are OK with a transponder antenna and are willing to recommend one:

1) Our ADS-B receiver has about 10dB of margin over the MOPS. In simple terms, this means that we are way more sensitive than the TSO would require for a certified device. Thus, even if your antenna isn't perfect, you still have better reception than is required.

By comparison, if an ADS-B receiver that only met the MOPS could receive a station 30 miles away, we would be able to receive that station at about 80 miles.

2) Transponder antennas are not that far off. They have been used for decades as DME antennas, and 978MHz is one of the DME channels (not used in the USA, which is why they could use it here).

A VSWR of 2.6 is really only about 0.8dB of loss vs a perfect 1:1 antenna. That means if a great ADS-B antenna works 32.5 miles away, the transponder antenna will still work 30 miles away. That's a pretty minimal difference. It's also about the same loss you get by using 4' of RG-58 cable, so if you really care about those losses, make sure you keep your cable runs as short as possible.

Also, since the UAT system we are designing isn't a transmitter, just a receiver, total antenna gain is much more interesting than just VSWR. You can build an antenna with perfect VSWR that is also an awful receiver. If you really want a smoking ADS-B antenna, a longer antenna than any 1/4 wave antenna is probably optimal, but it's just overkill for the way the system was designed.

Don makes nice antennas, and we are happy to recommend them for people that want a totally optimized ADS-B antenna in a nice blade package. But the math, design of the system, and actual flight testing all show that a $15 stub transponder antenna works just great as well, and we don't have any issues with anyone using one.

If you want to go really cheap, make your own. A 76.7mm piece of coat hangar or welding rod soldered onto the end of a BNC connector will work even better than a transponder antenna. Just don't poke your eye out. ;)

Also, just as a note, the loss of RG-58 and RG-400 is basically the same per foot, but the shielding is better on RG-400. Shielding is not a huge issue for an RX only device like our ADS-B receiver unless you run the cables in a bundle with other transmitting cables like a transponder.

--Ian Jordan
Dynon Avionics

Now this information is what I would term INFORMATIVE!

Sure the charts and numbers above are great for all you engineer types who want absolutes. However, the truth is our airplanes and the components that make them work DO NOT fly in a world of absolutes. After all, doesn't the old adage that "every airplane is a compromise" apply here also? If the compromise works without spending all that extra money why not?

Oh, and just exactly why does a 978 MHZ antenna cost more than a 76.7 mm piece of coat hangar or welding rod soldered to a BNC connector?

Pay for the machining, materials, labor, mold for the insulator, connector, rubber gasket, lock washer and nut. $17.95 at ACS seems like a good deal.

Don's antenna can't be compared with a stub antenna. His are low drag blade style and cost more to make.

I understand the specific component costs of any particular product we purchase. I guess what I was alluding to in my comment is the old adage that "knowledge is power". Once one knows the specifics of any given idea, mechanism, etc. one should be able to save time, money, effort by going about creating what one needs oneself rather than pay others to do so. Prior to building the RV, I had never experienced, nor knew about, many of the construction techniques used to do so. Now that I have successfully built and flown my own airplane, I believe I could also tackle other things I have never done before, such as build my own antenna(s). It is not only the building experiences that have given me that confidence but also the knowledge I am able to glean from reading posts from all you other guys out here posting your experiences and knowledge about any given topic.

My statement was also meant to say that given what we may have learned about antenna manufacture it seems we should be able to create our own blade style antenna similar to Don's as well, complete with mold for the insulator, connector, rubber gasket, lock washer and nut, and even a pretty paint job as well. All for minimal costs of materials we all may have on hand from our construction of the RV.

Granted many, and I include myself in this statement as well, may decide it is worth it for us to pay someone else to construct such a mechanism for various reasons rather than build our own. That is totally understandable. However, given the abilities we have developed over the span of building our airplanes it should also be understandable that we could very successfully construct our own antenna that could easily be as well built as Don's or anyone else out their selling them. No slight to Don intended. In fact I have one of his antennas myself. They are nice antennas indeed.

Steve,

I agree and the best part is that we can do this under the experimental category legally!

Also, lest anyone mis-interprets Ian's technical explanation as a vote against Don's antennas, note that Delta Pop's is one of the options that we specifically point to in our installation manual (pre-release version for the hardware install available at docs.dynonavionics.com).

Michael Schofield
Marketing Manager
Dynon Avionics