6Installation TB8100 Installation Guide
© Tait Electronics Limited February 2007
1.2.2 Antenna Load
Important The PA may be damaged if the load is removed or switched
while the PA is transmitting.
Introduction One of the inherent characteristics of 28V LDMOS technology is its lower
breakdown voltage. Tait Electronics, and most other major base station
manufacturers, have adopted this technology to benefit from its superb wide
band performance and high efficiency.
The MRF9060 LDMOS FET is used as the final power device in the
TB8100 50W and 100W PAs. In these PAs, the MRF9060 device is
protected from high VSWR by the design of the circuitry. This circuitry
makes it impossible to damage the device by keying the PA into a
mismatched load, or if the load deteriorates over even a short period of time
(milliseconds). Thus, no PA will fail if it is keyed while connected to a
mismatched load.
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 (i.e. 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.
Note Ice on the antenna, or a broken antenna, is unlikely to cause dam-
age to the PA. There should be enough coaxial cable between the
antenna and PA to protect it from high VSWR.
5W PAs will not fail due to a highly mismatched load. They do not use the
60W board with the MRF9060 device.
Recommendations The procedures recommended below should help to protect the PA from
damage under all but the most extreme operating conditions.
1. Do not remove the load from the PA while it is transmitting.
2. Do not connect the PA directly to the antenna. To protect the PA
output stage from load transients (i.e. switching or removing the load)
or atmospheric disturbances (e.g. rain static, electrical storms, etc.),
we recommend that you fit an isolator or cavity filter (e.g. duplexer)
between the PA and the load.
Fit the isolator as close as possible 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).
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