1、 Rep. ITU-R SM.2056 1 REPORT ITU-R SM.2056 Airborne verification of antenna patterns of broadcasting stations (Question ITU-R 225/1) (2005) 1 Executive summary This Report describes the measurement procedures, the equipment required, and the reporting procedures for antenna radiation pattern measure
2、ments using an aircraft. This Report is independent of the airborne platform chosen and it can be used regardless of the broadcasting system used. However, additional suggestions are given for specific airborne platforms and specific broadcasting systems, so that it can be tailored to anyones specif
3、ic needs. The Report is divided into three Annexes: Annex 1 introduces the different antenna pattern types that can be distinguished, and the measurement procedures to measure those. The equipment needed to perform such measurements is described. This description is sufficiently detailed to assemble
4、 ones own system, without limiting the choice of equipment. The post flight analysis, important for evaluating the measurement accuracy, is described, followed by a reporting standard. Each broadcasting type and each frequency range requires its own settings and has its own points of attention. Anne
5、x 2 is dedicated to these items. Annex 3 describes the specific problems encountered when choosing a specific aircraft type, and proposes solutions when possible. Annex 1 Airborne verification of antenna patterns of broadcasting stations 1 Introduction This Annex describes the measurement procedures
6、, the equipment required, and the reporting procedures for antenna radiation pattern measurements using an aircraft. The structure of the Annex is as follows: Section 2 describes the different antenna pattern types that can be distinguished. Section 3 introduces the measurement method in general. Se
7、ction 4 defines the different measurement flights types. Section 5 describes the equipment needed to perform these measurements. This description is 2 Rep. ITU-R SM.2056 sufficiently detailed to enable assembling ones own system, without limiting the choice of equipment. Section 6 describes the meas
8、urement procedures involved. Sections 7 through 9 deal with the different aspects of data processing, measurement uncertainty calculation and reporting. The recommendations in this Annex are independent of the type of aircraft chosen and it can be used regardless of the broadcasting system used. Ann
9、exes 2 and 3 will give additional recommendations for specific airborne platforms and specific broadcasting systems. 2 Antenna pattern types The radiation pattern of any antenna is three-dimensional. Measured antenna patterns are generally two-dimensional cuts of that three-dimensional pattern. Comm
10、on cuts are the “vertical antenna pattern” and the “horizontal antenna pattern”. The vertical antenna pattern is a vertical cut of the antenna pattern through the antenna and a specific azimuth direction. The horizontal antenna pattern is a horizontal cut of the antenna pattern through the antenna a
11、nd a specific elevation or down tilt angle. See Figs. 1 and 2. The coordinate systems used are described in Recommendation ITU-R BS.705. FIGURE 1 FIGURE 2 Vertical antenna pattern Horizontal antenna pattern In certain cases a lot of emphasis is put on one specific sector of the antenna. For highly d
12、irectional HF broadcasting antennas, the exact form and position of the main lobe, as well as the effective radiated power (ERP) in that main lobe determine the footprint on the targeted area, and are therefore very important. A specific antenna pattern measurement could chart that part of the anten
13、na pattern. An example of such an antenna pattern, the Sanson-Flamsteed projection, is given in Fig. 3. Rep. ITU-R SM.2056 3 FIGURE 3 Main lobe antenna pattern Antenna pattern measurements can be repeated for different azimuths or different elevation angles to obtain more information on the complete
14、 three-dimensional antenna pattern. Those azimuths or elevation angles can be strategically chosen based on the geometry of the antenna, simulations and experience from previous measurement campaigns. Measuring any of these antenna pattern types requires its own set of measurement flights, but the m
15、easurement procedure is very similar if not the same. 3 Method of measurement An antenna pattern measurement is basically a series of field-strength measurements, each taken at an exactly known distance from the antenna to be measured. With these two values the absolute ERP in that point can be calc
16、ulated. If we measure the ERP at a series of points positioned on a circle around the antenna, the horizontal antenna pattern emerges. Other diagram cuts can be measured at will. The formula for calculating absolute ERP is, in linear form: 224. =cfGRPERPRXRXwhere: ERP: power relative to the referenc
17、e antenna (W) PRX:power at the receiver input terminals (W) R: distance (m) between the receive and transmit antennas GRX: gain (linear value) of the receive antenna relative to the reference antenna f: frequency (Hz) c: speed of light (m/s). Care must be taken to measure position and PRXat exactly
18、the same time. If this condition is not met, the resulting ERP-value is not correct. In this formula ERP and GRXare expressed relative to a reference antenna. This reference antenna can be an isotropic radiator, a half wave dipole or a 4 Rep. ITU-R SM.2056 monopole, depending on the broadcast band u
19、sed. Additional losses such as cable losses, antenna alignment loss or polarization loss should be included in the value for GRX. Generally, using a logarithmic version of the same formula is more practical: )/4(log20)(log20)(log20 cfGRPERPRXRX+= PERPand PRXare now expressed in dBW, GRXin dBref. 4 M
20、easurement flight types The type of measurement flights conducted depend fully on the antenna situation and the aircraft used. For example, for the measurement of the diagram of a VHF broadcast antenna with a helicopter, a different approach is needed than for the measurement of a medium-wave array
21、with an aeroplane. The different measurement flight types and their application are described in this Section. 4.1 Propagation flight To determine the optimal measurement distance, a propagation flight can be performed. This is a flight in a straight line towards the transmit antenna, at exactly the
22、 height of the transmit antenna. That way the angular position of the measurement antenna as seen from the transmit antenna is constant, and therefore the transmitted ERP in that direction is constant. If no reflections are present, the measured ERP during the propagation flight will be constant too
23、. If ground reflections or scattering off buildings is present, their influence will show as deviations from that straight line, as shown in Fig. 4. FIGURE 4 Propagation flight The suggested measurement direction for a propagation flight is in the direction of the main lobe in the antenna pattern. M
24、ultiple propagation flights are recommended on antennas with multiple radiation directions and in cases where ground conditions and thus ground reflections differ. In addition to the theoretical graph of Fig. 4, an actual measurement result is given in Fig. 5. This graph is made of a 50 kW VHF FM br
25、oadcast transmitter. The transmit antenna consisted of an array of vertically polarized log-periodic dipole antennae mounted on tower approximately 150 m above the ground. The circle indicates the distance that was selected for a subsequent circular flight. Rep. ITU-R SM.2056 5 FIGURE 5 Actual propa
26、gation flight result From the result of the propagation flight an optimum distance is selected for subsequent measurements. The optimum distance is the distance where: the amplitude of the reflections is least, and the minima and maxima are closest together. The first criteria is obvious, the second
27、 may require explanation. If the minima and maxima caused by ground reflections lie far apart, and the ground is flat and homogenous, e.g. a complete circle flight could be conducted at a distance where the minimum or maximum occurs. This would result in the biggest measurement error achievable, whi
28、le the problem would show the least as variations in the measurement result. So this situation should be avoided. With the example shown, the optimum measurement distance would lay around 1 300 m. This distance is marked with a circle in Fig. 5. If the height at which the propagation flight is perfo
29、rmed differs from the actual height of the antenna, the graph will drop when the aircraft comes close to the antenna. When flying too low and measuring a transmit antenna with downtilt, the graphs may show a temporary rise before this drop in value occurs. This effect is illustrated in Fig. 6. FIGUR
30、E 6 Effect of incorrect height during the propagation flight 6 Rep. ITU-R SM.2056 Prior to the propagation flight, the pilot display assists the pilot by showing the actual position of the aircraft relative to the transmit antenna, as well as the desired start-position of the propagation flight. Thi
31、s position can be described with the desired azimuth angle with respect to the transmit antenna and the desired height. During the propagation flight, the pilot display assists the pilot by showing the offset in metres from the desired flight path. A propagation flight is easier to perform with an a
32、ircraft that maintains good control and manoeuvrability at low speed, such as a helicopter. One can fly in a straight line up to 200 m of the tower, then stop and fly away. This is not possible with all other aircrafts. A minimum distance to the transmitting antenna(s) should be kept at all times, t
33、o avoid excessive electromagnetic exposure. If the transmit antenna is mounted directly on the ground, as in the case with most long-wave, medium-wave and short-wave antennas, a propagation flight is not possible. 4.2 Vertical flight To obtain the vertical antenna pattern of a broadcast antenna in a
34、 specific azimuth direction, a vertical flight can be performed. Measuring the vertical antenna pattern can be necessary to determine the optimum flying height for measuring the horizontal antenna pattern, as indicated in Fig. 7. FIGURE 7 Vertical flight To perform a vertical flight, the pilot first
35、 moves to the desired azimuth direction, then descends to the desired start height. The pilot display assists the pilot by showing the actual position of the aircraft relative to the transmit antenna, as well as the desired start position for the vertical flight. The pilot then starts ascending in a
36、 straight vertical line, trying to retain his horizontal position as good as possible. If a helicopter is used, maximum stability is obtained when the flight is performed flying from a low altitude to a higher altitude at full throttle. During the vertical flight, the pilot display assists the pilot
37、 by showing the offset in metres from the desired flight path. This could be done representing the aircraft as a dot on a circular display. The centre of the circle represents the desired horizontal position, the circle itself shows the maximum allowed horizontal offset. The pilot should keep the do
38、t within the circle while flying upward. The circular display can be connected to a compass to align its orientation with that of the aircraft. This makes steering easier, as the wind dictates where the nose of the aircraft is pointing. When no aircraft is available for vertical flights, the vertica
39、l diagram cannot be obtained this way. It has then to be estimated by interpolating measurement points of subsequent horizontal flights. Rep. ITU-R SM.2056 7 During the vertical flight the two correction factors need to be applied. A compensation for the difference in gain in the vertical antenna di
40、agram of the measurement antenna and a compensation for the difference in distance (r1and r2in Fig. 4). 4.3 Circular flight To obtain the horizontal antenna pattern of a broadcast antenna, the pilot starts flying a circle around the transmit antenna while correcting his altitude and distance to the
41、transmit antenna until the target values are obtained. The measurement is then started, and the pilot continues flying along a circle around the tower, until the measurements are completed. During this process, the pilot is assisted with information on the pilot display. It shows the actual position
42、 of the aircraft relative to the ideal path around the transmit antenna in real-time. During the circular flight, the pilot display assists the pilot by showing the offset in metres from the desired flight path (distance and altitude). Generally it is necessary to fly part of a circle to enter the r
43、equired flight path so the definition of a predetermined start azimuth is not practical. In most cases the pilot likes to see the object he or she is flying around so the layout of the aircrafts cockpit dictates if the circle is flown clockwise or counter clockwise. The software and antenna system s
44、hould be adapted to this. Best stability is obtained when the aircraft flies with a steady and not too slow speed. As the aircraft flies around the antenna the relative wind direction changes with the azimuth angle, as a result of this the part of the aircraft pointing to the antenna changes during
45、the flight. It is therefore in most cases necessary to steer the antenna during flight. FIGURE 8 Circular flight Rap 2056-084.4 Other flight types Antenna radiation pattern measurements around ground-based antennas like, e.g. HF curtain arrays and medium-wave towers or arrays require a different app
46、roach than tower-based TV or FM broadcasting transmitters. For example: circular flights at other heights than the height of the main lobe can give the measurement points needed to construct a 3-dimensional picture of the radiation pattern, straight flights at low heights in the azimuth of the main
47、lobe can give an impression of the vertical radiation pattern. As long as the 3-dimensional position of the measurement point is known exactly, and the ERP is calculated in that measurement point, there are no limits to the actual flight path used, as long as the engineer that interprets the measure
48、ment data has a profound knowledge of the matter. 8 Rep. ITU-R SM.2056 5 Measurement equipment As shown in 3, ERP can be measured by exactly measuring position and field strength. Position can be measured using any positioning device that gives fast and accurate 3D position information. Field streng
49、th can be measured with a free space calibrated antenna and a calibrated measurement receiver. The position and field-strength values are recorded and processed by a computer. It calculates ERP and the position of the measurement point relative to the antenna under test, and displays the results in an appropriate form to the technician. The technician controls the measurement system and takes decisions based on the results shown on the screen. The software also generates information for the pilot, to assist his navigation around the antenna site. The pilot is respons
