(12) It is always advisable to “walk” the coverage area to ensure that there are no areas
of poor coverage. Signal “dropouts” (little or no signal in small areas) are indicated by
the “TX” LED going out or by a brief “fizzing” sound in the audio (which may or may
not be followed by a silent period due to squelched audio). The audio may be
conveniently monitored with a set of headphones plugged into the stereo phone jack on
the receiver. If a “dropout” area occurs, the antenna can usually be repositioned to
eliminate the problem. Frequently, a change of just a few inches in location will solve
the problem completely.
Antennas
The receiver antenna may be the most important single component in a wireless-
microphone system. However, this is the item most often overlooked in setting up a
system, and is frequently the cause of quite unnecessary problems. Proper selection and
placement of the receiving antenna is vitally important in any wireless-microphone
system.
Antennas are a particular issue with miniaturized wireless-microphone equipment such
as the R-27 receiver, in that antenna size tends to seem disproportionately large in
comparison to the equipment itself. This is easy to understand, in that the length of the
antenna is several times the maximum dimension of the receiver. However, while
electronic circuitry can be miniaturized without significant performance compromises,
the same is not true of antennas. That is, while components can be made almost
arbitrarily smaller, physical constants (such as wavelength) cannot. The net effect is
that, while antennas whose dimensions are far less than a wavelength do exist, they are
seriously less efficient than larger antennas. About the smallest antenna with reasonable
efficiency is the 1/4-wavelength design, which must be about 16.5 inches (42 cm) long
at 180 MHz.
Of course, electrically small antennas (such as the “rubber duckie”) are widely
available. These units are normally 7 to 14 inches (19 to 35 cm) long, much shorter
than 1/4 wavelength. However, such designs merely make the best of a bad situation by
providing a good impedance match at the tuned frequency. This at least avoids adding
matching loss to an already low efficiency. In general, even the best such designs are 3
to 6 dB less efficient than a simple 1/4-wave whip. In practical terms, the working
range of a system using this type of antenna will be only 50 to 70% of that of the same
system using a good-quality 1/4-wave whip (available as an option from your Vega
dealer).
The antenna should not be positioned near metallic objects, and especially not parallel
to them. The best arrangement is orienting the antenna vertically and at right angles
with nearby metallic objects. A location relatively high with respect to the person using
the transmitter is also desirable; about 8 or 9 feet (2.5 m) above the floor is usually
best.
A coaxial adapter is provided to convert the “rubber duckie” for right-angle use if this
is necessary. The “rubber duckie” is often used for body-worn applications (with the
receiver slung over the shoulder or on a belt, for example). In this case, efficiency will
be even lower due to absorption of the RF energy by the body. An additional loss of 5
to 10 dB is likely, reducing range by another factor of two or three. If range is a
problem (as evidenced by severe dropouts), using the supplied rubberized whip antenna,
or an optional 1/4-wave whip antenna (Vega Model 121BNC standard or Model 221
two-piece, which screws together), or a limp-wire antenna (Vega Model 224) may be
necessary.
6
R-27 Portable
