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by J. B. BULLOCK - (Es gibt sehr wenige Artikel über die Möglichkeiten von Richtfunk-Strecken und deren Betrieb.)
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Ihe growing number of privately owned TV microwave installations has focused attention on new techniques for proper installation and maintenance procedures.

One of the most significant of these procedures, particularly in installation, is alignment of the antennas. Proper alignment is somewhat difficult because the radiated energy is concentrated in a very narrow angle, a pencil-like beam which is considerably more confined than the beam of light from an automobile headlamp.

Furthermore, proper alignment requires accurate aiming of both transmitting and receiving antennas since both have equally narrow beam characteristics.

In many systems, passive reflectors are used to permit installation of the electronic equipment at ground level. These passive reflectors introduce added critical elements in the problem of alignment.

Successful initial alignment is not a guarantee of permanent maximum performance. When unusually high winds occur, or ice loads form, it is quite possible that antenna elements mounted on high towers can be shifted or deformed slightly with consequent loss in peak performance. It is, therefore, helpful to maintenance personnel that they be acquainted with effective techniques for checking and adjusting alignment when the need arises.

The most significant indicator of proper antenna alignment is measurement of the video noise level from the microwave receiver with the microwave system in operation. In practical terms this means measuring the signal-to-noise ratio at the output of the receiver. When the predicted signal-to-noise is realized, the full signal power for which the antenna system was designed will be delivered to the receiver.
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The standard method of alignment is to install the antennas (or passive reflectors) as close to previously calculated mounting angles as is possible and then to "fish" for one beam with the other until maximum signal is obtained.

If this signal yields a signal-to-noise ratio within 2db of that calculated value, then the antennas are considered to be aligned properly. This method is quite satisfactory with 4-foot (2.5 degrees beam at 7000 kmc) and with 6-foot (1.7 degrees beam at 7000 kmc) dishes, particularly over flat country with ample clearance provided.

Some of the problems encountered in antenna or passive reflector alignment are illustrated in Figs. 1 through 4. (Once a passive reflector is properly illuminated by its associated antenna, the method of its orientation is exactly the same as that of any antenna, providing the reflector is not swung out of the illumination.)

The narrower the beam of the antenna, the more critical the alignment will be. As larger passive reflectors are used (in order to provide additional gain) the final beam directed along the path becomes narrower.

Typical starting orientation points are shown in Figs. 1 and 2. The difficulty of measuring dish or reflector angles usually prohibits making an initial set-up much closer than that shown.

If a compass is used in the layout, precautions indicated in the section on "Alignment Procedure" should be taken. Azimuths are best obtained by using accurately located landmarks, or by celestial means.

If one of the antennas in Fig. 1 were oriented along the path, only a small signal would show up at the receiver. A stronger signal might be found when moving one antenna only, if the beam should come upon a good reflecting surface already in the path of the other beam (see Fig. 3).

Figure 2 again illustrates that if only one reflector is oriented at a time, there will be no strong indication to show when it lies properly along the path. Also, in both Figs. 1 and 2 the antenna patterns shown are further complicated by the existence of minor lobes which can lead to further false orientations (see Fig. 4).
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Proper orientation requires carefully planned movement of both antennas (or reflectors). To find the proper orientation simply by panning, both ends of a system must be panned simultaneously, and in the panning, the beams must be passed "through" each other, see Fig. 5.

The proper starting point is with both beams horizontal and in the nearest possible "on path" orientation that can be determined from the means at hand.

Surveying and driving marker stakes relative to known landmarks is probably the best beginning. In the absence of accurate landmarks, azimuth can be obtained from the stars, or approximate directions may be obtained from a compass. If a compass is used, precautions must be taken to reduce inherent errors, i.e., magnetic declination, proximity to metal objects, etc.

A stake driven along the line of the path, several hundred feet from the lower base, will permit "aiming" an antenna or reflector in the desired direction. This is done by setting a transit up over the stake and sighting back at the reflector.

When top and bottom edges of the reflector are parallel to the horizontal cross hair of the telescope, the reflector is "aimed" along the line toward the stake. In the case of an antenna, the plane of its feed horn may be compared with the cross hairs in the telescope.

From this starting point, the antenna at one end of the path should be moved in azimuth to a maximum signal indication. This generally assures that if the beams are off, they will be off on the same side of the path (such as shown in Fig. 3).

Simultaneously the antenna at one end of the path is panned right-to-left, while the antenna at the other end is panned left-to-right. In this manner the beams will pass through each other as shown in Fig. 5.

The panning procedure will often require that the location of a "maximum" be abandoned by both ends (note Figs. 3 and 4). The location of a maximum should always be noted, however, since it may be significant for one end, as in Fig. 4.
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When a promising signal has been maximized in azimuth, it should be explored in elevation using the same technique as used in panning horizontally. With antennas alone the true maximum will generally be with the dish face vertical.

With reflectors which are directly above their antennas - the maximum will generally be when the rellector face is at 45 degrees with the vertical. This is because the difference in antenna (or reflector) absolute elevations is likely to be very small compared to the path length.

In the case of the rellector, the 45 degree angle will change if its antenna is not located directly below it and on the path line. The new angle will be difficult to calculate only if the antenna is off the line of the path. In panning vertically, the most prominent false maxima will be ground reflections, and they will be below the true maximum in elevation angle.
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Many passive reflectors have a bowing, or curvature adjustment on them. It is recommended that this adjustment be left such that the reflector face is flat until the proper antenna maximum (as verified by signal-to-noise measurement) has been found.

This avoids unpredicted distortion of the antenna pattern caused by too much face curvature. Curvature adjustment may then be made to yield one or two db more signal strength.

A passive reflector must be properly illuminated by its companion antenna if the combination is to yield the predicted gain. An error in reflector illumination which is likely to occur on tall towers is illustrated in Fig. 6. It is evidenced by widely fluctuating received signals when a man climbs through the illuminated portion of the tower.

A nominal 8 to 12 db reduction in predicted receiver signal-to-noise can be expected. Reflections off the surface of other objects (large warehouses, etc.) may also be verified by noting the effect of motion in front of the suspected surface on the received signals.

It is usually possible to optically sight an antenna so that it will very nearly illuminate its reflector properly. One method is to replace the button-hook antenna feed by a straight length of waveguide and sight through the waveguide.

Another method is to attach a long straight edge along the buttonhook, taped to it at the base and at the mouth. It is then possible to sight from various angles on the ground and determine where the dish is pointing.

Before making a signal-to-noise measurement, the illumination of each reflector should be checked by panning its antenna in two planes for maximum signal at the microwave receiver. Do not move the location of the antenna, only its orientation.
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Antenna polarization must be the same at both ends of a link, if a full received signal is to be achieved. This should always be checked prior to a S/N measurement. It is done by rotating the antenna assembly or just the antenna feed at either end of the link.

As a practical mailer, almost no change in signal will be noted unless polarization is off by more than 20 degrees. Where passive reflectors are used, additional polarization rotation is encountered if either antenna is off the path line between reflectors. When polarization is rotated, beam orientation may shift slightly and thus require some touch-up.
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Earlier it was indicated that the difficulty in antenna orientation stems from the narrow beams involved. Where physical arrangement of reflectors to be oriented permit its use, the following method will remove this obstacle, and make it possible to pan one end of a link at a time.

The procedure is presented in step by step form, assuming that the antennas or reflectors have been mounted, properly illuminated, and set as near to proper orientation as possible by optical means (about ±10 degrees).

1. Replace the antenna at one end of the path by a low gain horn. The open end of a piece of waveguide will be an adequate "horn". On a dish to dish path, this can be done by merely removing the antenna feed and leaving the dish undisturbed. This may be performed at either end of the path, see Fig. 7A.
In another method, open waveguide may look out on the path under or over the parabola. Check polarization.

At a location involving a passive reflector, the horn must be oriented so that it radiates along the path with proper polarization, bypassing the passive reflector. Exact azimuth and elevation orientation is not critical due to the broad pattern of the "horn". The horn should be located high enough to give 0.6 Fresnel zone clearance over a 4/3 earth to avoid obstruction losses.

This generally means that the "horn" must be mounted part way up on the tower, but probably not as high as the passive reflector itself. Figures 7B and 7C illustrate two possible ways of locating the "horn". In the method of 7B, the horn is fastened to a convenient point on the tower and fed from the transmitter below via waveguide or coax line. This method is difficult to accomplish in practice if much line length is involved (because of losses if coax is used, and because of unwieldiness and expense if waveguide is used).

If line length makes the method of 7B impractical, then 7C is the preferred alternate. Here the transmitter (or receiver) chassis itself is taken up the tower and fastened so that it looks out on the path. The transmitter output opening itself may then serve as the "horn" directly, or a short length of flexible waveguide may be used.

The latter may be employed to facilitate orienting and by-passing obstructions (such as tower legs, etc.). Standard camera cable is run up to the transmitter from its control unit, and for this purpose there is almost no limit to the length that can be used.

Once the "horn" has been mounted, the orientation situation will generally be like that indicated in Fig. 8.
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2. The high-gain antenna end of the system should now be oriented in azimuth and elevation for maximum signal. The signal-to-noise at the receiver output may be noted, and should come within about 4 db of the value calculated (unless clearance is not adequate, or the "horn" orientation is radically off). The calculated signal-to-noise is determined in the usual manner, *1) using 10 db for the gain of the open waveguide end (the "horn") in place of that of the standard antenna (usually about 40 db). Thus a signal-to-noise ratio about 30 db below the final should be expected. Although noisy, it will be readily discernible. (A measurement is usually not necessary since generally only one maximum will be found).

*1) Information on the calculation of expected S/N ratios for TVM-1A and IB equipment may be found in any of the several editions of the equipment instruction books. IB-36757.

Initial panning may require watching the receiver's video output on a CRO instead of observing the signal level meter. A weak signal may sometimes be observed in the noise before it produces sufficient AGC to register on the meter.
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If a passive reflector is being bypassed, as in the connections indicated by Fig. 7B, then the losses in the transmission line will further decrease the signal-to-noise obtained.

If the transmission line loss is 5 db, then the signal-to-noise expected would be reduced by 5 db. An obstruction loss of 5 db due to inadequate clearance would have a similar effect, but would vary with time and weather changes.

Transmission line loss or inadequate clearance losses will render this alignment technique useless if they amount to much over 10 db.

Comparison of Fig. 8 with Figs. 1 through 4 shows the advantage of this method in readily detecting "on path" orientation of the sharp beam since

a. When the high gain antenna is panned through the path line, it receives near maximum signal unless the "horn" is off path line by many-degrees. Such inaccuracy is unlikely.

b. If a signal reflected from something in the beam of the horn is encountered in panning the sharp beam, this reflection signal will be reduced by loss; and thus barely noticeable.
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3. Remove the "horn" and reconnect the high gain antenna at the horn location. Orient this high gain antena for maximum signal in azimuth and elevation. This should place it also in the "on path" position.

4. Both antennas should now be "on path". Check polarization and measure the signal-to-noise at the receiver end of the link. It should be within 2 db of calculated. If passive reflector movement was involved and was greater than a few degrees, it may be advisable to recheck the reflector illumination.
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The achievement of a predicted S/N ratio over a microwave link depends on several other factors including:

Proper transmitter power output, proper transmitter modulation level, no excess waveguide losses, proper receiver noise figure, and mechanical perfection of antenna elements (parabola curvature, etc.).

If all these items are known to be in order, the following technique is recommended for measurement of video S/N on TVM-1A/B microwave links.

1. Turn off sound diplexing equipment; also turn off any standby transmitters or receivers.
2. Disconnect any video input from the transmitter, and terminate the VID INPUT jack.
3. Switch transmitter modulation to the TEST 1 position. This sets transmitter deviation at 6.0 mc p/p. It may be advisable to check this. Remove restoration network if any from the receiver.
4. Set the receiver OUTPUT LEVEL control to give 1.5 V p/p2 signal, measured on a CRO across a terminated output.
5. Switch the transmitter MOD SELECTOR switch to PIX.
6. Measure noise level at receiver output. Use either a wideband (10 mc) CRO or an average responding wideband VTVM, calibrated in rms of a sine wave. (This meter is nominally down 3 db at 8 mc).

FIG. 1. Antennas less than 6 degrees off path, yet virtually no signal is obtained at receiver.

FIG. 2. Reflectors less than 6 degrees off path, yet virtually no signal is obtained at receiver. The same thing can happen in azimuth as is shown here in elevation.

FIG. 3. Antennas approximately 20 degrees off path, yet reflection yields greater signal at receiver than was received in either Fig. 1 or Fig. 2. If either antenna "alone" is oriented along path, signal received will be weaker than reflection shown.

FIG. 4. Major lobe oi one anlenna and minor lobe of other nearly on path. Receiver signal strength is comparable to that of Fig. 3. Minor lobes in sketch are exaggerated.

FIG. 5. To tind maximum signal, beams must be panned "through" each other. Here beams from a passive reflector and antenna are off path in azimulh.

FIG. 6. Reflector illuminated by tower reflection. This is to be avoided.

FIG. 7A. Horn located at parabolic antenna site.
FIG. 7B. Horn located at passive reflector site.
FIG. 7C. Horn located at paBBive reflector Bite.

FTG. 8. Possible initial positions of horn and high gain antenna prior to orientation.

FIG. 9. Arrangement for measuring random noise showing R-C filter connection for blocking hum from measurement.
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