Combined L Band Avionics Systems 2

[kontiki99]

Curiosity Question,

Has anybody seen any TCAS/ATC/DME retrofit products out there that use single multi-tasking L-band Receiver/Transmitters and antennas?

I thought I heard this pitched before, but can't see that anyone's actually building them.


Thanks

 

[Sparweb]

Not sure if I like the idea...

Imagine a blade antenna 18" long, weighs 5 pounds, and has about 4 BNC connectors. The 2" wide base is screwed on with #8 screws directly to the 0.020" skin (in typical repair station fashion) of your favourite little Cessna. Sorry, this is a TCAS: TWO blades 18" long...

And in the process of eliminating the other three antennas, the guys blanked those off and drilled a new set of holes, because this one doesn't match any of the other three's hole patterns...

...or worse, they MADE it match...

I've seen enough hack jobs due to owners wanting the "latest" on their airplanes, that I really don't want to see one of those.


Steven Fahey, CET

 

[kontiki99]

My interest is a large commercial jet application.

I sat in on an A380 briefing and seem to recall it has Honeywell combined TCAS/ATC antennas with a single LRU handling processing for radar, ATC and TCAS.

Most of the L band blade antennas I see ATC, TCAS Omni Directional and DME are 2" high.

The 18" antennas on a large jet are usually VHF and they do give the structures guys fits. I guess it's like a large tuning fork vibrating when there's significant cross wind.

 

[debodine]

The ATC system (which for the purpose of your discussion, includes the TCAS omni-directional function) and the DME system already are interlinked in that the ATC system during transmission must shut down the DME receiver with a suppression pulse, and the DME system during transmission must shut down the ATC receiver with a suppression pulse. This is because the aircraft ATC system transmit frequency of 1090 MHz is near the middle of the DME receiver's passband of 962 MHz to 1213 MHz. Also the DME transmit range is between 1025 MHz and 1150 MHz, which includes the aircraft ATC system receive frequency of 1030 MHz.

The above is why the DME antenna and the ATC antenna can look identical and often are in construction and characteristics. They often ARE identical.

However, although the suppression pulse "shuts down" the appropriate receiver at the proper time, that "shutting down" occurs electronically inside the receiver. Electronic isolation is not quite as effective as physical separation of the transmit energy from the receiver front end. And electronic isolation of lower energy levels is more effective than isolation of higher energy levels.

To protect the receiver front end from the very large transmit energy present in the multi-purpose antenna would require significant (meaning expensive) re-designs of both the DME receivers and the ATC receivers. Or such an isolation requirement would have to be built into the multi-purpose antenna itself, probably requiring the antenna to contain active components versus the current passive antenna design. Requiring active components adds circuits and installation wiring to provide power and control signals to the active components, plus active components typically cause a decrease in reliability over passive components alone.

With the current scheme of physical separation between the DME and ATC antennas keeping the energy levels of the transmissions that find their way to the receiver front ends at a much lower level, the suppression pulse just activates internal circuitry that can effectively block these lower energy levels without having to have high energy isolation circuitry in the receiver front end.

I have no doubt from a laboratory standpoint it can be done, but from a business standpoint I am not aware of any industry groundswell of demand for such an antenna combination. Especially on large commercial aircraft, the real estate is available for excellent physical separation without adding complexity.

I am not saying such an animal does not exist or will not exist, only that the above are some of the design considerations that would have to be addressed by some means to be able to market such a multi-purpose antenna.

debodine

 

[kontiki99]

debodine,

Thanks for the informed post.

 

[BrianR]

I designed an aircraft installation with seven L-band systems (13 antennas) operating simultaneously and support what debodine said. Not only do you need the interference blanking pulses to shut down the other receivers when one of the transmitters operates but you also need a degree of RF isolation to protect the receiver front ends from damage. From memory on the systems I worked on, we targeted 40dB of isolation which was achieved through antenna separation and feedline losses. There was no problem with top/bottom antennas interacting but 1-2m of separtion was needed between co-planar antennas.

There is no difficulty in combining VHF/UHF/L-band or L-band/UHF/C-band but you'll need the help of the equipment OEMs before sharing one L-band antenna between systems.