Although specifically dealing with aircraft systems, the methods described in this article are applicable for other platforms where a number of antennas are in close proximity. The number of antennas in use and in close proximity on mission specific aircraft are as many as 22 on a small search and rescue aircraft.
ANALYSIS METHODS
The methods of coupling analysis include electromagnetic analysis programs, measurements on a 1/10th scale aircraft, a full-scale mockup of a part of the aircraft fuselage and wing, as an example, and provisionally mounting antennas on the actual aircraft.
All these methods mean that the analysis can be performed before installation of antennas on the aircraft and thus the location of antennas can be modified, or mitigation techniques employed if a coupling problem exists. All of the analysis techniques have advantages and disadvantages.
For example, the antennas and antenna drive element would be too small on the 1/10th scale model at 93.75MHz and test equipment for 93.7GHZ would almost certainly not be available.
Ideally the electromagnetic analysis programs alone would be good enough. However, in two articles, reference 1 and 2, and in an upcoming paper comparing the accuracy of the four methods, we see that that is not true.
The 1/10th scale model is shown in Figure 1 and the FEKO program model of the aircraft in Figure 2.
In-Band Coupling
For transmitter receiver pairs that are in band (i.e., transmitter and receiver frequency the same) a simultaneous operations (SIMOPS) red case (non SIMOPS possibility) is obvious. A possibility for acceptable in-band performance at low frequency may be achieved with an in-band cancellation circuit shown in reference 3.
Out of Band Coupling
Most antennas do not function as an effective filter and pass “out of band” frequencies with often little attenuation. When a high-level source of electromagnetic radiation is close to an antenna, and the receiver does not contain a band pass filter at the input, then the signal present at the input of the receiver can result in cross modulation (where the interferer modulates the intentional signal). Also, when the transmitter frequency is close to the receiver IF band- width or the edges of the receiver bandwidth.
With high input levels desensitization/compression of the receiver can occur, which means the gain of the receiver reduces. Alternatively, the high RF level can be demodulated by a semiconductor in the receiver resulting in a dc level which can effectively saturate the front end.
High input levels can result in a spurious response in the receiver which may be in band. If the induced power is too high, a voltage or current can be applied to an input semiconductor, resulting in breakdown or overheating and stressing.
To reduce high levels, a series of band pass, band stop, high pass, and low pass filters have been designed and built from 30MHz to 9.375GHz, described in references 4 and 5.
Passive Intermodulation
A source of in band interference is Passive intermodulation (PIM). Intermodulation products are generated when two or more signals mix in a structure with nonlinear junctions or ferrous metal. When these intermodulation products fall in band for a co-located receiver, a SIMOPS red case may exist.
Passive Intermodulation may occur in any metal structure in proximity to a receiving antenna, such as the antenna structure, railings, towers, or other metallic surfaces. Reference 4 describes PIM in more detail. One common source of a nonlinear junction is either a loose joint or oxidization of metal.
A structure that includes ferrous materials (which has a nonlinear magnetic hysteresis) or carbon fiber (which has a nonlinear resistivity) may also exhibit PIM, and this is, perhaps surprisingly, an order of magnitude higher than the joint generated PIM.
On the aircraft, the ferrous material is typically in the landing gear, flap rods/tracks, and door handles, with the landing gear, flap rods/track the most likely source. Figure 3 shows an example of the incident and PIM fields.
Some of the sources of PIM which have been experienced are:
- Poor alignment of parts
- Moving structures which are not adequately bonded
- Insufficient or incomplete cleaning of parts
- Contaminated plating bath
- Poor plating adhesion
- Dissimilar metals in direct contact
- Plating non uniformly applied and on insufficient thickness (high resistance)
- Material which has not been in the plating bath long enough (high resistance)
- Oxidization
Luckily the reradiated PIM level is usually at a low level.
Another source of in-band noise may be the broad band noise from a high-power transmitter which is in band.
In-Band Coupling Analysis Sheet (Example)
An example of an in-band coupling sheet is provided in table 1 with some of these coupled levels being a clear possibility for a red case of SIMOPS.
The effect on the receivers can best be provided by the receiver manufacturer. However, this may not be provided. Another possibility is that the input circuit of the receiver is available, in which case the effect of the level on the circuit can be modelled using a circuit model program. If neither is possible then the assumption can be made that the 15.5dBm and 18dBm levels will cause a problem.
An Example of Antenna Coupling
The aircraft has a Side Looking Airborne Radar (SLAR) antenna mounted on the sides operating at 9375MHz. Underneath the fuselage is a Maritime Search Radar also operating at 9375MHz. The antennas transmit and receive a vertically polarized wave.
| Receiver and Frequency (MHz) | Transmitter and Frequency (MHz) | Frequency Overlap | Received Level (dBm) |
| #1 Cockpit V/UHF 30-88 18-174 225-400 400-600 |
#2 Cockpit HF 2-30 |
At 30 MHz | -11 |
| #1 Cockpit V/UHF 30-88 18-174 225-400 400-600 |
#3 Mission HF R&S 1.5 – 30 |
At 30 MHz | 15.5 |
| #1 Cockpit V/UHF 30-88 18-174 225-400 400-600 |
#3 Mission V/HF R&S V/UHF 100-512 |
225-400 | -13 |
| #1 Cockpit V/UHF 30-88 18-174 225-400 400-600 |
#5 Cockpit VHF Comm. VHF#2 118- 137 |
118-137 | 18 |
| #6 Acoustics VHF Sonobuoy VHF 136-173.5 |
#3 Mission V/HF R&S V/UHF 100-512 |
136-173.5 | 1.1 |
| #6 Acoustics VHF Sonobuoy VHF 136-173.5 |
#5 Cockpit VHF Comm. VHF#2 136 -173.5 |
136-173.5 | 28 |
Table 1: In-band coupling example
A creeping wave will be generated from the SLAR antenna to the Maritime Search Radar and vice versa, but due to the high frequency the power will be at a low level. The use of a 1/10th scale model is not practical, nor is the use of one of two analysis computer programs, again because of the high frequency.
Neither the SLAR antenna nor the Search Radar Antenna were available. Instead, an E plane sectional horn antenna was built and calibrated.
Figure 4 shows the gain plot of the SLAR and the sectional horn, and it can be seen that they have a good correlation.
A parabolic dish antenna was used in place of the maritime search radar. It was angled 5.6 degrees in the H plane and 60 degrees in the E plane to the side of the fuselage. The sidelobe of the radar is minimum 36dB down on the main lobe, and so the sidelobe is 31-36 = -5dB. The parabolic dish gain is 28dB and at 90 degrees it is 27dB. So 27dB-28 = -1dB, and that is the gain used in the analysis.
Figure 5 shows the coupling path from the SLAR to the radar.
The SLAR output power is 25,000W. The power into the sectional horn is 10W and in the analysis the power received by the parabolic dish was corrected accordingly, along with the gain correction.
A full-scale section of the fuselage, wing and nacelle were built with a copper foil covering and the horn and parabolic dish antennas were mounted at the appropriate location. The ground under the mockup was covered in absorber with high absorption at 9375MHz.
The predicted level induced into the Maritime Search Receiver is 42.3dBm.
The SLAR generates a 50nS wide pulse at a repetition rate of 50Hz. This means that the Maritime Search Radar will only see an interfering signal for a short time at a low repletion rate, and may be able to identify it and ignore this level. Because the level is so high (16W) damage to the receiver may be possible. If the SLAR generates a blanking pulse, the Maritime Search Radar may be able to use this to protect the receiver input.
CONCLUSIONS
Lack of SIMOPS between transmitters and receivers on a platform can have many causes, including in-band coupling; out of band coupling with high levels at the receiver; and PIM.
The mitigation of lack of SIMOPS may be achieved by locating transmitting and receiving antennas on opposite sides of the fuselage (the higher the frequency the more effective this is); moving antennas down the aircraft to minimize reflections from structures such as engines and wings and reduce PIM; signal filters at the antenna end of the receiver cable; in-band cancellation at lower frequencies (See reference 5).
Blanking receivers when a transmitter operates may reduce cross modulation, generation of spurious emissions, and receiver damage.
REFERENCES
- Antenna to Antenna Coupling on an Aircraft Using a 1/10th Scale Model with Results Compared to the FEKO Electromagnetic Analysis Program. IEEE EMC Europe 2010. D.A. Weston
- Electromagnetic ambient inside an aircraft from transmitting antennas mounted on the outside compared to safety levels and radiated susceptibility test levels. D.A. Weston. IEEE International Symposium 2013.
- Website: https://emcconsultinginc.com/
- Electromagnetic Compatibility Methods, Analysis, Circuits, and measurement. 1,150 pages. Third Edition. D. A. Weston CRC press 2017.
- Antenna to Antenna coupling on an aircraft: Mitigation techniques. Interference Technology Magazine. EMC Directory and Design Guide 2012.
