Subsystem RF frontend
The RF part of the entire AMVR serves four subsystems. All of them should not interfere at all or at the minimum level possible. First of all, there are the SDR receivers listening to 868.3 and 1090 MHz position report broadcasts. Third, a GPS receiver is needed, last, some network connectivity is needed. Likely, the latter will be a WLAN hardware because the AMVR probably will not be placed where ethernet connections will be available. On the other hand, a GPS antenna has to be placed outside, no matter where the rest of the hardware will reside. Flarm and ADS-B antenna should be placed outside as well to gain maximum performance. All said, the AMVR seems to need up to four antenna assemblies with RF coax cable and optimized antenna locations. Fortunately, this is not the case. Widely differing situations of operation help in keeping the effort low. Because integration issues vary widely, this article will discuss two use cases only.
WLAN: Tiny USB sticks will do the job as long as access points typically are close in-house. On the other hand an ethernet connection is achievable.
GPS: As the position of the AMVR will never change due to its steady position of operation, the location as well as the GPS height MSL is specified statically as parameters for the core VFR-B. The necessary time feed could be derived from far NTP servers, if an internet connectivity is given. Otherwise, a GPS antenna is to be placed outside.
Flarm: Due to its low transmission power budget, all Flarm antenna should be placed outside.
ADS-B: Depending on the in-house situation, an antenna location indoor might be sufficient because ADS-B transmitter do broadcast with 60 dB more output power and like link power budget than Flarm respectively. However, a dual-band Flarm/ADS-B assembly is advantegeous. If the receiver installation should serve as a long distant virtual radar, a separate optimization of both the Flarm and ADS-B receiving chain is needed, definitely resulting in a two antenna outdoor assembly.
Things become different as every bulky antenna is prone to breaking in a mobile outdoor application.
WLAN: Might get a little bigger if AMVR is not placed directly beneath the airfield manager's workplace. This antenna must keep some distance to the receiving antennas.
GPS: Outdoors, nearly every antenna will gain success, as every GPS antenna is designed for outdoor use. Typically, the ceramic patch antenna with LNA can be placed at the top of the AMVR housing, resulting in a no-antenna-appearance and a not antenna handling as well.
Flarm and ADS-B: A dual band assembly is highly recommended on order to save one antenna mechanically.
Available DVB-T sticks for SDR operation offer decent performance together with almost simple but tuned quarter wave groundplane antennas. A Lambda5/8 antenna probably will give slightly better results. Even far off from their primary frequency of operation they deliver decent performance. Optimized for the 868.3 MHz Flarm band, the antenna will give optimum reception for Flarm broadcasts. Receiving the high power ASD-B system transmits with a handicap of up to 10dB on the 1090MHz band even delivers sufficient reception for AMVR purposes. For use as an airfield manager's tool, a reception range of up to 40km is needed, which definitely is achieved with a Flarm antenna used for the ADS-B frequency. All this most likely does not hold true for highly directive collinear antennas, commonly used in far reception applications. Having this far reception operation in mind, the keen system builder should go back to a very optimized two antenna design. However, the one antenna approach could perform, if the antenna is tuned for the weaker Flarm system. A single antenna signal has to be split into two antenna feeds, feeding two separate receiving chains, because SDR receivers cannot cope with a 2 frequency band operation that far separated. For further optimization, band pass filters might improve the reception. With the splitter and the filters damping the system, the last component, a Low Noise Amplifier - LNA - is needed to compensate for the other components damping. The sketch underneath shows the dual frequency virtual radar receiving chains.
