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TB9300 Installation and Operation Manual Installation 83

4.6.5 Connecting RF

Notice Do not remove the load from the PA while it is transmitting

as this may damage the PA output stage. Before disconnecting any RF

cables, put the base station into Offline mode to prevent any transmis-

sions.

The RF input to the base station is via the marked BNC connector on the

rear panel of the reciter. The RF output is via the N-type connector on the

rear panel of the PA (refer to Figure 4.9 on page 79).

Cables and antennas should be of high quality construction. Solid shield

Heliax type cables are best, but if braided shield cables must be used for

short distances, their braids must be silver-plated.

Recommendations

for Installing the PA

We recommend the following installation procedures, which should

protect the PA from damage under all but the most extreme operating

conditions.

1. Do not connect the PA directly to the antenna. Fit an isolator or

duplexer between the PA and the load. Fit the isolator as close as pos-

sible to the RF output connector on the PA. Do not connect any

switching equipment between the isolator and the PA, unless the

switch cannot operate while there is RF present (i.e. the base station

is transmitting).

2. Fit a surge suppressor to the antenna cabling where it enters the

building.

3. Inspect all cables and equipment connected to the base station for

defects.

Ice on the antenna, or a broken antenna, is unlikely to cause damage to the

PA.

Explanation The circuit design of the PA protects the circuitry from high VSWR. This

makes it difficult to damage the RF power device by keying the PA into a

mismatched load, or if the load deteriorates over even a short period of time

(milliseconds).

However, it is possible to damage the device if all the following conditions

happen at the same time:

■ There is a step change in the PA load (for example, the load is removed)

■ The PA is transmitting

■ The feed line loss between the PA and the mismatch is <1dB

The effect of such conditions is variable: some devices will not be

destroyed, and some may fail after repeated load interruptions.

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