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RF-7000E Operations Manual
Chapter 3. Mounting the Antennas
— 47
Measuring Antenna Isolation
– Decoupling
System path calculations by Peninsula Engineering will determine the recommended antenna size
and type plus the feed-horn to feed-horn separation and polarization loss. Our experience has been
that if the recommendations are followed, antenna isolation will be met with some margin and direct
measurement of antenna isolation is not required.
Should there be reason to determine the antenna isolation, proceed with this section.
Measure the actual isolation between the antennas, to ensure that the antennas are sufficiently
isolated from each other and that the system C/E objective is met.
CAUTION:
This is an important consideration in all on-frequency repeater installations.
If the isolation is not sufficient, the repeater might oscillate in the extreme, or the repeater system
might have inadequate carrier-to-echo, C/E, margin, which can lead to bit errors in digital radio
systems or intermodulation noise in analog radio systems.
In some cases, it may be necessary to reduce the maximum repeater gain by adjusting the amplifier’s
GAIN setting potentiometer and thus obtaining the required C/E. Normally isolation is met and
amplifier GAIN setting allows for maximum ALC managed gain.
C/E = [Isolation, dB]
– [Repeater Operating or Maximum Gain, dB]
Example 1: Isolation = 110 dB, Operating Gain (ALC reduced) = 45 dB
C/E = 110
– 45 = 65 dB
Example 2: Isolation = 110 dB, Maximum Gain (full gain) = 60 dB
C/E = 110
– 60 = 50 dB
Table 13 lists the C/E and isolation objectives for various radio modulation types.
Isolation is controlled by the antenna Front-to-Back, F/B, ratio, sidelobe suppression, separation
distance between feed-horns, angle between the antenna centerlines, polarization and feedline
losses.
Measuring antenna isolation requires a signal generator and a spectrum analyzer. The generator
transmits a signal from one antenna, and the spectrum analyzer measures the same signal as the
second antenna receives it. Figure 22 shows the equipment set-up. Losses between 80 and 120 dB
are measured. Be sure the test equipment is capable of adequately measuring such high loss values.
SEPARATIO
N
DISTANCE
BETWEEN
FEED-HOR
NS