DUAL POLARIZATION: A Boon to FM Broadcasters
During the past two years there has been a great deal of interest directed toward achieving more uniform coverage from FM broadcast stations through the use of dual polarized antennas. Tests conducted using facilities at WNHC-FM, New Haven, Conn.
have been reported in CCIR Study Group X, Document USPC-BC 22, dated Dec. 15, 1964. The antenna feed system was modified to provide for radiation of horizontally polarized signals only, vertically polarized signals only, or a combination of the two. Extensive measurements were made in both Hartford and New Haven to determine the effects of dual polarization on reception in the service area, and also upon the interference potential of dual polarized transmissions.
Transmitting Equipment Used
The transmitting antenna consisted of a Jampro 8-bay horizontally polarized section mounted on one leg of a 100-meter self-supporting tower and an 8-bay vertically polarized section mounted on an adjacent leg. The two sections were separated by approximately 4 meters, and fed through a 50 '50 power splitter with individual feed lines originating from separate junction boxes. A coaxial switch was installed between the power splitter and the vertically polarized antenna junction box so that power could be switched into a dummy load. It was therefore possible to make field intensity measurements with horizontally polarized antenna excitation only or both horizontally and vertically polarized antenna excitation. The effective radiated power in the horizontal plane was 10 kw, and in the vertical plane 9.5 kw. (The gain of the vertically polarized antenna was slightly lower, thus accounting for the difference. ) The VSWR of the system was 1.15, and remained the same for all modes of operations.
During the latter portion of the tests, the coaxial switch was moved to permit power to be switched from the horizontally polarized antenna to a dummy load. Thus, it was possible to make measurements resulting from vertically polarized radiation alone.
Receiving Equipment Used
To assure accuracy of the measured fields, it was necessary to design a dual polarized receiving
antenna that would measure the
horizontal and vertical fields simultaneously. The design of this
antenna consisted of a horizontal
balanced dipole mounted on the
bottom skirt of a coaxial vertical
dipole. It was necessary to bring
the coaxial cables from the dipole
down through the bottom skirt of
the vertical antenna to eliminate
radiation from currents flowing
in the coaxial sheath. Tests on
this antenna indicated 37 db decoupling between the horizontal
and vertical sections.
Simultaneous horizontal and
vertical fields were recorded by
using two VHF field intensity
meters to feed two chart recorders. The antenna was raised to a
height of 10 meters during all
measurements. To assure a homogeneous field, measurements were
recorded by making runs varying
in length from 30 to 150 meters,
depending upon available clearances.
Measurements of the service
area field were made with the
same equipment, except that four
spot measurements were made at
each location, instead of a continuous chart recording, because
of limited clearances in populated
areas.
Close-in Measurements
A number of close-in measurements (2 to 5 miles) were made
to determine the extent of vertical component radiation from the
horizontally polarized antenna.
Line-of-sight locations, which had
Fresnel clearance, were chosen for
these measurements. The vertically polarized component measured from 14.9 to 32.0 db below the
horizontally polarized component.The average of these locations
showed the vertical field to be
22.2 db below the horizontal field.
A second set of measurements
was made at these same points to
determine the extent of horizontal
component radiation from the
vertically polarized antenna. It is
interesting .to note that the horizontally polarized component measured from 13.8 to 38.8 db below
the vertically polarized component, and the average of these locations showed the horizontal field
to be 23.0 db below the vertical
field.
Far Field Measurements
Far field measurements were
made starting at the 1-mv/m contour to determine the effect on the
horizontal field when equal
amounts of power were fed to the
horizontally and vertically polarized sections. At each of the eight
locations chosen, chart recordings
were made of the horizontally
and vertically polarized fields being radiated from the dually
polarized antenna, from the horizontally polarized antenna only,
and from the vertically polarized
antenna only. The fields were recorded while the receiving vehicle
was moved over a distance of
from 50 to 150 meters with the
receiving antenna at a height of
10 meters. The most distant recordings were made at a location
where the horizontal field measured approximately 70 microvolts.
Service Field Measurements
To determine the effect of the
addition of the vertically polarized field in the service areas of
WNHC-FM, a number of measurements were made in the Hartford and New Haven areas. The Hartford area is approximately
25 miles from the transmitting
site. The transmission path is
over terrain which produces varying degrees of shadowing starting
with practically line-of-sight in
West Hartford to moderate shadowing in East Hartford. The dual
polarized antenna is mounted on
the north face of the tower, toward the city of Hartford.
A grid system was laid over a
city map of the Hartford area so
that most of the 49 grid points
fell in the populated area shown on Map No. 1. Measurements were
recorded at 29 of these grid
points, with the greatest concentration in the downtown area. The
locations were chosen by going
to the grid point and then finding
the nearest site where the measuring antenna could be raised to 10
meters with sufficient clearance to
move the vehicle approximately
20 meters. At each of these locations, four spot measurements
were recorded with the vehicle
being moved about 5 meters between spots. Measurements were recorded while transmitting with
the horizontal and vertical antenna, the horizontal antenna only,
and vertical antenna only. The
four spot measurements at each
location were averaged and tabulated in Table IV.
Similar measurements were
made in the New Haven area, approximately 9 miles from the
transmitting site, as shown on
Map No. 2. New Haven is located
at the base of a number of mountains which end abruptly and almost immediately before entering
Long Island Sound; thus, the
transmission path from WNHC is
over very rough terrain. Severe
shadowing is evident in some
areas, while in some parts of the
city farther south, line-of-sight
paths were obtained. The transmitting ant2nna, however, is
mounted on the opposite side of the tower from New Haven. A
map system with 30 grid points
was used. Measurements recorded
at 20 points in populated areas
appear in Table V.
Summary of Results
As shown in Table III, measurements made at distances from 27
to 61.5 miles from the transmitter, using alternately dual polarization and horizontal polarization,
show very little improvement in
the horizontally polarized component received at the eight monitoring points. At distances greater
than 55 miles, very little change
is observed. Up to 50 miles, on the
other hand, a vertically polarized
component on the order of 100
microvolts or better is established.
This signal would be of considerable advantage to listeners employing automobile FM receivers
with a whip antenna.
The measurements given in
Table IV, while not made in areas
which are line of sight from the
transmitter, were not substantially
affected by shadowing. Conse quently, the plane of polarization
of the received signal should be
substantially the same as transmitted. This, apparently, is the
case. Although the relative magnitudes of the dually transmitted
vertical and horizontal components appear to be about equal,
there was little increase in the
horizontally polarized field over
that measured when the horizontal antenna provides a horizontally
polarized field measurably improved over that received when
using the horizontal transmitting
antenna alone. This improvement
is most noticeable in those areas
where the magnitude of the fields
indicates substantial shadowing—
for example, points D2, Dl and
C2 on map No. 2.
Reference to the New Haven
measurements shows that at 18 out
of the 20 locations measured, the
horizontal component of the field
increased when vertical polarization was added. At 7 of the 20 locations, the vertical field increased
with the addition of horizontally
polarized radiation. It should also
be noted that at 10 of the 20 locations. the horizontally polarized
field measured with only the vertical transmitting antenna operating, was of higher intensity
than the horizontally polarized
field measured when only the horizontal antenna was excited. At 16
of the 20 locations, the vertical
component of the measured field
antenna alone was in use. This,
of course, would be the case if
no rotation of the plane of polarization were to occur. At the same
time, there is a substantial vertically polarized field throughout the
Hartford area. which would indicate that car radios employing
whip antennas or home radios
that employ so-called "rabbit
ear" antennas, should receive a substantially better signal due to
the presence of the vertical component.
Table III presents a different
picture, however. The measurements, in general, were made in
areas shaded to a greater or lesser
degree by the rocky formations
which lie between the transmitting antenna and New Haven.
In 90% of the locations the dual was of higher intensity than thE
horizontal component when radiating with both vertical and horizontal antenna sections. In New
Haven, as in Hartford, the presence of the strong vertically polarized field would substantially
improve the service rendered to
FM receivers with indoor, or
"rabbit ear," antennas.