Be sure to monitor
drawn DC current during installation and compare to the current drawn when operating into
a dummy load.
You haven't done a proper installation if you haven't paid attention to operating
efficiency. Pruning techniques in the length of coax can be effective, but simple matching
circuits installed at an amplifier output are a more consistent approach to improving
amplifier efficiency. These matching circuits have an added benefit of eliminating low
level amplifier oscillations that often appear in some designs when operating into highly
tuned loads such as duplexers. Repeater setup is often simplified.
Reducing output power in a well-designed class C power amplifier
may not be buying that much in improved lifetime.
Reducing the drive power of a class C amplifier can bring it into class B operation, a
much less efficient mode. Turning back the power by 5, 10 or even 15% makes good sense but
reducing power by 50% to achieve better reliability does not. If you need 100 watts,
don't buy a 200 watt amplifier and turn it down. You're only wasting money. While a 200
watt amplifier can be tuned to run at maximum efficiency at 100 watt output, the
reduction in MTBF due to increased component count and complexity negates any improvement
from conservative operation.
Fusing an amplifier attached to a current limited and protected power supply may not even
be necessary. If used, look for sand filled types sold by most electrical fuse
manufacturers as they are less prone to flex failures.
Avoid ferro-resonant power supplies.
Ferro-resonant power supplies may be attractive because they are the ultimate in
simplicity and ruggedness but they are
amplifier killers due to voltage overshooting at turn-off. Several transmitter
manufacturers included ferro-resonant power supplies in their OEM equipment. Their power amplifier decks soon gained a reputation as unreliable.
After market power amplifiers didn't fare much better when attached to them. Use a good
regulated power supply. If you use a linear supply (or even a switcher for that matter),
make sure it has a good OVP circuit..
Regular monitoring of transmitter and amplifier conditions can
warn you of impending problems.
Keep a record of DC current draw and output power. This can alert you to several problems
such as transmitter drive drift before they can cause serious problems. Periodic (after
warranty!) maintenance of solder connections and scheduled fuse replacement will also
extend the lives of amplifiers and power supplies.
Proper care in installation, setup and maintenance will pay off in
many years of trouble free operation. Nothing beats buying from reputable manufacturers,
but there's a lot you can do too!
About the Author
Mr. Glenn has MS and BS degrees in Electrical Engineering and over 18
years experience in RF amplifier design. He was the founder of VoCom Products
Company in 1979 and acted as its President until 1995 when he assumed the role of
Vice President of Engineering upon VoCom's acquisition. He is currently employed at
Motorola's Cell Phone Division