Improve FM Coverage with Dual Polarization
FM stations radiating a horizontally polarized signal experience a definite loss in transmission effectiveness because of the vertically polarized whip or line cord receiving antennas used with many modern FM sets. Transmission of a vertically polarized signal, in combination with a horizontal signal, will considerably improve coverage of the authorized service area. The advantages of a dual polarized FM antenna system are:
1. Increased signal pickup by vertical car whip antennas.
2. More signal into home FM receivers with line cord and built-in antennas. (These antennas are widely used in console FM combination radios.)
3. More signal into transistor portable FM receivers with whip antennas.
4. Increased signal level in the null areas of the horizontal antenna.
5. Improved reception in multipath areas; more listeners in hilly terrain. 6. Improved reception of monaural, stereo, and SCA signals.
This article will provide the FM broadcaster with detailed electrical and performance characteristics for the proper installations of a dual polarized antenna system.
Technical Considerations
The addition of vertical polarization is not a cure-all in providing increased coverage. In
some cases the addition of vertical
antennas will not increase signal
in a deadspot for the horizontal
system. Vertical radiation will not cure the multipath effect, but
used in conjunction with the
horizontal system, improved reception in areas with multipath
problems often results. The dual
system also does not increase signal pickup of a horizontally polarized receiving antenna.
Broadcast engineers should note
that operation of both types of
antenna systems does not degrade the horizontally polarized ERP
when the vertically polarized antenna is installed. Existing FCC
Rules authorize radiation of the
same amount of power in the vertical mode. For example, a Class
B station having a 10-kw transmitter and a 4-bay horizontally
polarized antenna with a gain of
4 will radiate a horizontal ERP of
approximately 30 kw. A vertical
antenna system could therefore
radiate an equal 30 kw in the
vertical mode.
Horizontaly polarized vee, ring,
and circular shaped 'radiating antenna elements have earned an
excellent reputation, and their
technical characteristics are well
known. The vertically polarized
antenna is basically a folded dipole, usually constructed of copper
tubing or transmission line copper .
er (see Fig. 1).
These dipole elements, or bays,
are spaced approximately one
wave-length apart. The bays in
some makes of antenna are then
fed in phase along transmission line that will support from
one to sixteen elements connected in parallel. The impedance
of each dipole is made greater
than the transmission line impedance by the number of elements. Thus, the input impedance
of the antenna must be 50 ohms
to match the transmission line
impedance following the standard
Ohms Law formula for parallel impedance (l/Zin = 1/Z, -F 1/Z
1/Z.).
The standard FM antenna is a
modified half-wave horizontal dipole. Fig. 2 shows the horizontal
radiation pattern, the typical figure 8. According to the position
of the antenna it is possible to
radiate a signal which is either vertically or horizontally polarized. When the dipole is horizontal, the signal is horizontally polarized; when the dipole is in a
vertical plane the radiated signal
is vertically polarized.
To produce a circular horizontal
radiation pattern; the most common antennas in use today are the
circular ring and vee type. These
antennas will radiate a uniform
omnidirectional circular horizontal polarized pattern. The circular
dipole is usually end-loaded to
provide a more uniform current
along its length. The appearance
of the radiation pattern, when
viewed from an end of the dipole,
is shown in Fig. 3. The circular
or ring antenna is simply a folded
dipole bent in a circular shape,
which gives a circular horizontal
field pattern. The vee antenna is
a folded dipole formed into a truncated vee shape. As the number of
horizontal bays is increased, the
vertical radiation beamwidth is decreased or "squeezed down." To
step up the vertical radiation pattern, vertical antenna elementsmust be used in ination with
the horizontal elements.
Using a half-wave dipole in the
vertical mode, the horizontal becomes the vertical and the radiation pattern is circular, like the
doughnut pattern in Fig. 4.
Installation Details
There are three basic configurations to be considered in the installation of dual polarized antenna systems. The first, shown
in Fig. 5, is the stacked arrangement, with the horizontal elements
mounted above the vertical elements. Notice that the center of
vertical radiation is lower than the center of horizontal radiation.
A large tower section must be
used for mounting the complete
antenna system.
The second method, shown in
Fig. 6, is the "back to back"
mounting, which distributes the
weight of the dipoles equally. The
vertical antenna elements are
mounted on one side.of the tower
and the horizontal elements on
the opposite side, at the same
height above ground.
The third method is interposing or interlacing. This system
of mounting places the vertical
antenna in the same plane as the
horizontal antenna with the vertical elements between the horizontal antenna sections (see Fig. 7).
Notice that less tower mounting
space is required than for the
stacked system in Fig. 5.
Interlaced or Interposed System
Of the three described mounting methods, the interlaced or interposed system is the most effective in improving the station's
coverage area. In this system the
pole mounted antenna does not
affect the pattern circularity.
Back-to-Back System
Some engineers prefer the
"back to back" system, since this
arrangement tends to balance the
pole or tower load distribution.
However, because the vertical and
horizontal elements are facing in
opposite directions, the horizontal
pattern distribution of their respective signals may be affected.
Stacked System
Many recent installations are
of the stacked antenna type. These
are popular because advantage is
taken of the existing FM horizontal antenna. The vertical antenna
bays are usually installed directly
below the horizontal bays.
The difference in height of the
antenna elements in the stacked
configuration may affect the line
of sight distance to the horizon.
When tower-side or tower-leg mounted, the antenna pattern will
be somewhat affected by the supporting structure. The extent of
deviation from a circular pattern
will vary with the type and size
of the structure.
Power Distribution
Since normally one transmitter
feeds both_ antennas, the recommended type of installation is a
single transmission line from the
transmitter output to the antenna. Therefore, to operate with
the same horizontal and vertical
ERP, a power divider or splitting
"tee" with a power division ratio
of 50/50, 60/40, or 70/30 can be
used to feed both the horizontal
and vertical assembLes (see Fig.
8). An adjustable transformer
may be used between the power
splitter and the antenna elements
to adjust for proper matching and
power distribution.
As noted previously, the maximum allowable ERP of vertical
polarized radiation is limited to
the licensed horizontal radiated
ERP power. The power available
to the antenna can be determined
by multiplying the transmitter
power output by the transmission
line loss (efficiency). For example,
the total available power of a 10-
kw transmitter is equal to 10 kw
(transmitter output) multiplied by the transmission line efficiency
of 90%, the result is 9 kw of
available power. If the horizontal
polarized antenna is a 3-stacked
array with a gain of 3.0, and the
station's licensed ERP is 24 kw,
then the transmitter will be operating at less than full power
output of approximately 8.0 kw.
Since the total available power is 9 kw and we want to operate
with same horizontal and vertical
power, using one transmission
line, we must use a 50/50 power
split to feed 4.5 kw to each antenna. A 6-bay horzontal polarized antenna with a power gain
of 6.3 would be required to obtain the licensed ERP of 24 kw
with a power input 3f 3.8 kw for
each antenna feed me.
If a 6-bay horizontal polarized antenna is used, a 5-bay vertical
polarized antenna should be interlaced between the hcrizontal elements. One manufacturer's vertically polarized antenna has the
same gain as their horizontally polarized elements; thus, an equal
number of horizontal and vertical
bays may be used. The vertical
polarized ERP for this combination would be 20.2 kw. (5.31 power
gain x 3.8 kw power input = 20.2
kw ERP). Thus the dual polarized
FM antenna combination would
therefore comply with the FCC
regulations. The gain of the horizontal and vertical antennas increases with the number of stacked
bays used; Table I contains the
figures for determining the appropriate number of horizontal and
vertical antenna elements.