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Wireless Systems Guide for
ANTENNA SETUP
ANTENNA COMBINING
The converse of antenna distribution, antenna
combining, can be employed in one of two ways. With
wireless microphone systems, multiple antennas can be
combined together to feed a single receiver (or multiple
receivers with antenna distribution) to provide coverage
across multiple rooms or in extremely large spaces. For
wireless personal monitor systems, which usually consist of
rack-mounted transmitters, antenna combining is used to
reduce the number of transmitting antennas, i.e. the
antenna combiner allows all the transmitters to share a
common antenna.
Multi-room Antenna Setups
For multiple room coverage, use passive combiners.
Since they do not require power and are typically compact,
they can be located wherever necessary. A passive
combiner will typically result in at least 3 dB of loss, so be
sure to include this figure when calculating cable loss.
Multiple combiners can be used in series, if more than
two locations need to be covered, so long as enough
amplification is provided to make up for whatever additional
losses are incurred. For situations where more antenna
amplifiers are needed than can be effectively powered by
the receiver or antenna distribution system, additional bias
"Tee" power adapters must be used. These adapters allow
a bias voltage to be "injected" into the antenna cable.
It is important to keep multiple antennas feeding a
common receiver input as isolated from each other as
possible in order to minimize potential phase cancellation
that could result in signal dropout. Certain receiver designs
will be better equipped to deal with this situation than
others, but it is a worthwhile precaution nonetheless.
Room C
UA830WB (RF amplifier)
UA221 (passive combiner)
bias T (necessary for more than two amplifiers per line)
Multi-room coverage: 3 separate rooms – 6 antennas
10
Room B
Room A
Equipment
Room C
= UA830WB
(RF amplifier)
= UA221
(passive combiner)
= UA820
(1/2 wave antenna)
Multi-room coverage: 1 room divided by airwalls – 4 antennas
Antenna Combining for
Personal Monitor Transmitters
Antenna combining is crucial to obtaining optimal
RF performance from personal monitor transmitters.
Several closely-spaced, high-power transmitters suffer from
excessive intermodulation (a transmitter interaction that
produces additional frequencies) problems. In this case, a
passive combiner should be used for combining two
transmitters. For more than two, though, an active combiner
is recommended. An active antenna combiner will typically
accept between 4 to 8 transmitters. Unlike active antenna
distribution systems, which can be cascaded together for
larger setups, active antenna combiners should never be
"actively" cascaded. If
more than one combiner
is needed to combine all
the transmitters together,
a
passive
should be used to
connect
two
combiners together. As
always, be aware of any
extra losses incurred with
the passive combiners.
Rack
Similar to active
antenna
distribution
systems, active combin-
DC
ers also have a specified
supply
frequency bandwidth.
Be sure to select the
proper bandwidth for
the given transmitter
frequencies.
Airwalls
Room B
Room A
combiner
active
Active antenna combining
Equipment
Rack
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