Because of the many advantages
of wireless microphones, simultaneous use of four, six or even
more systems at a location is quite common. Often, one or two
wireless microphones are used initially, but as the benefits
of wireless operation become more apparent the need for additional
systems develops. The freedom, flexibility and enhanced presentation
made possible by wireless microphones are almost irresistible,
and it is natural for their use to increase over time.
However, as the number of wireless systems in operation at a
location increases, so does the potential for problems. Unless
some simple preventative measures are taken, it may eventually
become difficult to achieve reliable and trouble-free operation.
In particular, interference is a common problem when several
systems are used unless the wireless frequencies are carefully
chosen. Other problems such as reduced range, unexpected signal
losses and erratic squelch operation can also occur.
The most common problem with using multiple wireless systems
is interference and the most common cause of interference is
the use of wireless frequencies, which are not compatible with
each other. Wireless frequencies must be selected to work with
local TV stations and with all other wireless systems in the
area. Otherwise serious interference problems are highly likely,
especially when several wireless systems are used simultaneously.
Other types of interference can also occur when using multiple
wireless systems. One is caused by using too many wireless systems
in a particular frequency range or using wireless frequencies
that are too close to each other. Another interference problem
can occur when the antennas for two receivers touch or are parallel
and close together. In these situations, a small amount of radio
energy can leak from one receiver to the next, causing harmful
interference under certain conditions. A similar problem can
occur when receivers are stacked directly on top of each other.
As a practical matter, the need to keep receiver antennas well
separated makes it very difficult to use more than a few units
on a tabletop. Audio cables and power cords also tend to compound
the problem, especially if more than one row of receivers is
needed.
As the number of wireless systems at a location increases, so
does the amount of effort required to ensure reliable and trouble-free
operation. It is also necessary to begin taking into account
the arrangement and mounting of equipment, the positioning of
antennas and other factors that can safely be ignored when only
one or two systems are being used. Although there is no hard
and fast dividing line, once more than four or five wireless
systems are involved a new approach is probably appropriate.
Frequency selection is the critical first step in achieving
success with multiple wireless systems. Unless all the frequencies
used are compatible, the chances of satisfactory operation are
very low. The TV channels in the area of use are also highly
important and must be considered when selecting frequencies.
It is common for a system to work well in one city and perform
very poorly in another city because the local TV channels are
different. For this reason, it is essential that TV channel
data be available for all locations where the system will be
used.
As the number of wireless systems used increases, the technique
of simply spreading the receivers out over a tabletop (or some
similar arrangement) eventually becomes impractical. The need
to locate the receivers at some distance from the preferred
antenna location or to arrange them so that they are easier
to monitor may also arise. Mounting the receivers in travel
cases for easy movement might be desirable in some instances.
Once it becomes necessary to permanently mount the receivers
in an equipment rack or travel case, several new issues arise.
The first question is what to do about the antennas. If the
receiver antenna connectors are on its rear panel and the rack
or case is metal or has a metal frame, the radio signal will
be at least partially blocked and performance will almost certainly
suffer greatly. Even if the antenna connectors are on the receiver
front panel, they will be close to the metal structure and each
other, resulting in at least some loss in performance. This
arrangement also results in poor reception when the metal rack
or case is between the transmitters and the receiving antennas.
The best solution to this problem is to use remote antennas;
that is, antennas other than the whip antennas attached directly
to the connectors on the receiver. This can be as simple as
remounting the whip antennas on the top of the metal cabinet
or using accessory antennas offered by the manufacturer. It
may also be desirable to have the antennas at some distance
from the receivers. This allows the receivers to be positioned
conveniently next to the mixer while keeping the antennas in
a favourable location nearer the transmitters.
Remote antennas must be connected to the receivers with coaxial
cables, which are specifically designed to carry radio frequency
(RF) signals. Even with good-quality coaxial cable, a significant
amount of the RF energy is lost in the cable. This is especially
true for small-diameter cables at UHF frequencies, where only
25 feet (8 m) of economy coaxial cable can reduce operating
range by as much as 40%. Cables with better performance are
available and should be used when cable lengths exceed about
15 feet (5 m) at UHF or 40 feet (12 m) at VHF.
Many types of remote antennas are available for use with wireless
receivers. These can be divided into two basic kinds: ones that
work equally well in all directions (omnidirectional) and those
that work best in one direction (directional). Directional antennas
offer increased range in the preferred direction but reduced
range in other directions, as compared to omnidirectional antennas.
Directional antennas are useful when increased operating range
is needed and the wireless transmitters will always be in one
particular direction. They can also be used to make up for the
losses in coaxial cables. Because of their large size, directional
antennas are not usually practical in the VHF band.
In situations where the use of long coaxial cables cannot be
avoided, RF preamplifiers can be used to overcome the inherent
signal loss in the cable and the resulting decrease in operating
range. Also known as boosters and RF line amplifiers, these
devices must be located near the antenna to be effective. Because
of the nature of RF, additional amplification at the receiver
end of the cable will rarely improve range. Unfortunately, wireless
systems using RF preamplifiers are usually somewhat less resistant
to interference than systems with the antennas connected directly
to the receivers. For this reason, preamplifiers should be avoided
when possible by rearranging the system, installing a lower
loss coaxial cable, or using directional antennas.
When a number of diversity receivers are used in a system with
remote antennas, the amount of RF cabling and antennas required
can become a problem. For example, four diversity receivers
will need a total of eight antenna cables and eight remote antennas.
Not only is this an installation problem, it can represent considerable
additional expense. In this situation, an antenna distribution
amplifier is usually a good solution. These devices divide the
RF signal from the antenna into four separate signals that can
feed four receivers. This allows use of only two antenna cables
and two antennas for four diversity receivers.
When mounting wireless receivers in equipment racks or cases,
it is a good idea to separate them from digital devices such
as delays and processors. This is because digital equipment
almost always has spurious outputs that can interfere with sensitive
wireless receivers. Usually, just 12 inches (30 cm) or so of
separation will prevent a problem. It is also a good idea not
to mount wireless receivers directly above power amplifiers
and other units that run hot. The extra heat will unnecessarily
reduce the stability of the wireless equipment, reduce its performance
and possibly shorten its life.
When feasible, it is good practice to leave space between wireless
receivers when rack mounting them. This allows the receivers
to run cooler and reduces the chances of spurious outputs from
one receiver affecting another. This arrangement also increases
antenna separation when front-mounted antennas are used. If
there is not enough rack space available to allow for a blank
panel between each unit, mounting in pairs so that there will
be space either above or below each unit is suggested.
Large wireless systems can become moderately complicated, especially
when remote antennas and ADA's are used. Frequency selection
can also present some challenges when a large number of wireless
microphones are involved, the system will be used in several
different cities, or wireless equipment from other manufacturers
must be incorporated. If you are unsure how to proceed, contact
us for assistance. |
