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Aircraft to Aircraft Antenna Coupling

Aircraft to Aircraft Antenna Coupling: Analysis, Measurements and Mitigation

Three modes of antenna to antenna coupling

The analysis of coupling between aircraft mounted transmitters and receivers is required to apply mitigation techniques or to decide if SIMultaneous Operation of Systems (SIMOPS) is possible The coupling can be from the intentional transmitter frequencies which can result in in-band reception, or out of band cross modulation, or receiver de-sensitization. In addition EMI can be caused by transmitter broadband noise, harmonics, spurious emissions or Passive Intermodulation (PIM) generated in band emissions.

In extreme cases damage can result to the receiver when the received power is too high. Unless cross modulation or de-sensitization occurs the transmitter tuned frequency, harmonics, and spurious transmitter emissions may possibly be excluded by tuning the receiver to a different frequency. In the case of broadband noise covering the complete receiver frequency range this is not possible. In one case where 22 antennas are located on a small aircraft the probability of some level of antenna-to-antenna interference is 100%.

Once a potential EMI problem has been identified mitigation techniques such as moving antenna location, RF switches, blanking, filters, phase shifting and summation and the application of localized absorber may be applied.

2MHz to 9.375GHz coaxial in line signal filters

These filters are contained in small enclosure with either BNC or TNC connectors and so are ideal for connection between the antenna cable and the receiver.

Low pass filters available with an upper band pass frequency from 2MHz to 406MHz

On an aircraft the TCAS , DME, ATC, IFF ,Weather radar, Maritime Search Radar, SLAR , and other transmitters all operate at very high frequency, typically 1030 – 9500MHz. Although this is much higher than the HF, VHF and UHF receivers on the aircraft, because of the high transmit power interference can occur. Band stop filters are available at these frequencies but they are of the tuned cavity type and usually very bulky (sometimes as large as a receiver and very costly). EMC Consulting has designed and built a simple, physically small filter which is unaffected by temperature change and shock and vibration and is available from 700MHz to 1GHz and at 9.375GHz with other frequencies available on request.

In band signal cancellation

When the transmit and receive frequency bands between two antenna overlap then filters are clearly not applicable.

When the interferer received level is extremely high and may result in damage to the receiver a limiter may be used. However even with a limiter in circuit the level after limiting may be high enough to result in receiver de-sensitization, cross modulation or spurious generation.

To cancel or reduce the interferer level a propagation delay, 180 degree phase shift and summation circuit has been developed. The output of the transmitter is tapped off with no reduction in power level between the transmitter and antenna. This can be achieved anywhere on the transmit cable from the transmitter end to the transmit antenna end. The ideal location depends on the total propagation delay of the transmit cable, the coupling path in air between the antennas and the receiver cable, as described later.

The output of the transmitter is attenuated and matched to the impedance of the cable .

The interferer signal is then passed through a bulk delay. The bulk delay can be in the tens of microseconds the only limit is the attenuation inherent in the delay elements and the physical size of the enclosure.

The total transmitter to summation circuit cable delay, bulk, and variable delays are chosen to be exactly equal to the transmitter to receiver total path propagation delay. This means that the input signal to the receiver from the receiving antenna and the signal in the summation circuit are in phase. The internal signal then undergoes a 180 phase shift which is virtually frequency independent.

The phase shifted signal is then adjusted in level and summed with the receiver input signal. This achieves an almost complete cancellation at a specific frequency. At either side of this frequency the amplitude at the output of the delay circuit is different from the receiving antenna level. This is because the attenuation with frequency of the transmitter to receiver path is different from the phase shifter cancellation circuit

. The circuit was adjusted and tested from 1MHz to 30MHz, with the maximum attenuation adjusted at 30MHz. The attenuation achieved is shown in table 1 . With the circuit adjusted for 108MHz the attenuation from 60MHz to 108MHz is shown in table 2. At a single frequency such as 152MHz the cancellation can result in an attenuation of 28dB.

Table 1 Cancellation attenuation 1MHz to 30MHz

Frequency (MHz)

Attenuation of transmitter signal (dB)

1

19

2

19

5

20

10

35

15

26

20

33

25

35

30

>48

 

Table 2 Cancellation attenuation from 60MHz to 108MHz

Frequency (MHz)

Attenuation of transmitter signal (dB)

60

26

70

26

80

33

90

26

100

35

108

49