Andrew operation and maintenance Specifications download pdf (Page 56)

Chapter 3 - Operation

42S315F 3-4

3. If tilt was not adjusted using theodolite, measure the distance

between the edge of the subreflector and each corner of the feed horn

as shown in Figure 3-1, distance B. Using the subreflector adjustment

mechanism, adjust the subreflector until all four distances are equal;

this removes any tilt with respect to the feed.

4. If the translation off center was not adjusted using theodolite,

measure from the center of the subreflector to each spar pickup point

on the dish surface as shown in Figure 3-1, distance C. Any

convenient and repeatable measurement point is acceptable as long as

it is associated with the machined holes in the spars or the spar tabs.

These three measurements should be equal within 0.1-inch nominal.

If they are not equal, re-evaluation of the antenna geometry and

installation is necessary.

5. If necessary, the subreflector may now be adjusted toward or away

from the main dish with the subreflector adjustment mechanism. By

adjusting the support studs equally, the subreflector alignment is

maintained. Signal level monitoring equipment is necessary to

accurately set the subreflector for the highest gain and the deepest

radiation pattern nulls. The following equipment or its equivalent is

necessary:

• Spectrum Analyzer

• Power Meter

• X-Y Recorder

• Time Base

The radiation pattern (relative power versus off-axis angle) should be

recorded and evaluated for each adjustment of the subreflector.

Typical patterns at various subreflector positions which are identified

in Figure 3-2 show the changes which take place as the subreflector is

moved closer to the optimum focal position.

a. The first nulls to each side of the main beam will become

deeper and eventually bottom out at some level (See Figure 3-

2). This level varies depending on the antenna, but should be

equal to or greater than 10 dB below the level of the first

sidelobe. The deepest null in the H-plane sometimes occurs at

a different subreflector position than in the E-plane. In this

case, the subreflector should be set at a compromise position

between the two planes (i.e., E-plane null depth approximately

equals the H-plane null depth).

b. The signal level of the main beam will also increase as the

subreflector is moved closer to the focal position. The power

meter can be used to monitor this change. The main signal

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