Country Home Products Wireless Microphone Systems - Manual (Page 18)

In all of these designs, the radio wave pattern emitted
by the 1/4 wave antenna is omnidirectional in the plane
perpendicular to the axis of the antenna. For a vertically
oriented 1/4 wave antenna the radiation pattern is
omnidirectional in the horizontal plane, which is the typical
case for a trailing wire antenna. There is very little output
along the axis of the antenna. A three-dimensional
representation of the field strength from a vertical antenna
would resemble a horizontal doughnut shape with the
antenna passing through the center of the hole.
Recall that a radio wave has both an electric field
component and a magnetic field component. A vertically
oriented 1/4 wave transmitter antenna radiates an electric
field component that is also vertical (while the magnetic
field component is horizontal). This is said to be a
"vertically polarized" wave. Horizontal orientation of the
antenna produces a "horizontally polarized" wave.
In receiver applications, the antenna must pick up the
desired radio signal as efficiently as possible. Since the
strength of the received signal is always far less than that
of the transmitted signal this requires that the antenna be
very sensitive to the desired signal and in the desired
direction. However, since the size and location of the
receiver are less restrictive, and since directional pickup
may be useful, a much greater selection of antenna types
is generally available for receivers.
Again, the minimum size for adequate reception is 1/4
wavelength. A whip or telescoping antenna of this size is
supplied with most receivers, and it too is omnidirectional
in the horizontal plane when it is vertically oriented. An
important consideration in the performance of a 1/4 wave
receiving antenna is that its efficiency depends to some
extent on the presence of a "ground plane," that is, a metal
surface at least 1/4 wave long in one or both dimensions
and electrically connected to the receiver ground at the
base of the antenna. Typically, the receiver chassis or
receiver PC board to which the antenna is attached acts as
a sufficient ground plane. (See Figure 2-26.)
If more sensitivity is desired, or if it is necessary to mount
an omnidirectional antenna remotely from the receiver, 1/2
wave or 5/8 wave antennas are often used. These antennas
have a theoretical "gain" (increase of sensitivity) up to 3 dB
greater than the 1/4 wave antenna in some configurations.
This can translate into increased range for the system.
However, the 5/8 wave antenna, like the 1/4 wave type, only
achieves its performance with an appropriate ground plane.
Without a ground plane unpredictable effects may occur
resulting in asymmetric pickup patterns and potential signal
loss due to the non-ideal cable/antenna interface. A
properly designed 1/2 wave antenna does not require a
ground plane, allowing it to be remotely mounted with
relative ease. It can also maintain proper impedance at the
cable/antenna interface or can be directly attached to a
receiver or antenna distribution system. In addition, it is
resistant to the effects of electrical noise that might otherwise
be picked up at the interface.
When antenna size is an issue, such as for portable
receivers, the previously mentioned 1/4 wave rubber
duckie is an option. UHF designs can use 1/4 wave rubber
duckies because of the shorter wavelengths. Another
relatively small size remote antenna can be found in the form
of a 1/4 wave antenna with an attached array of radial
elements that function as an integral ground plane. Both of
these types are omnidirectional in the horizontal plane when
mounted vertically. For maximum efficiency, receiving
antennas should be oriented in the same direction as the
transmitting antenna. In the same way that a transmitter
antenna produces a radio wave that is "polarized" in the
direction of its orientation, a receiver antenna is most
sensitive to radio waves that are polarized in its direction of
orientation. For example, the receiving antenna should be
vertical if the transmitting antenna is vertical. If the
orientation of the transmitting antenna is unpredictable
(ie. handheld use), or if the polarization of the received wave
is unknown (due to multipath reflections) a diversity receiver
can have even greater benefit. In this case it is often
effective to orient the two receiving antennas at different
angles, up to perhaps 45 degrees from vertical.
Unidirectional antennas are also available for wireless
microphone systems. These designs are comprised of a
horizontal boom with multiple transverse elements and are
of the same general type as long range antennas for
television reception. They can achieve high gain (up to 10
dB compared to the 1/4 wave type) in one direction and
can also reject interfering sources coming from other
directions by as much as 30 dB. (See Figure 2-27.)
Two common types are the Yagi and the log-periodic.
The Yagi consists of a dipole element and one or more
additional elements: those located at the rear of the boom
are larger than the dipole element and reflect the signal
back to the dipole while those located at the front are
smaller than the dipole and act to direct the signal on to the
dipole. The Yagi has excellent directivity but has a fairly
narrow bandwidth and is usually tuned to cover just one
TV channel (6 MHz). The log-periodic achieves greater
bandwidth than the Yagi by using multiple dipole elements
in its array. The size and spacing between the dipoles
19
Selection
and Operation
of W
ireless Microphone Systems
C
H A P T E R
2
Basic Radio Systems
Figure 2-26: 1/4 wave and 1/2 wave antennas UHF range