THE FCC has long permitted AM broadcasters
to use directional antennas, but prior to the FM
freeze a few years ago, very few FM stations were
allowed this privilege. FM directional antennas were
beginning to appear in applications for new FM
stations and improved FM facilities just as the freeze
was imposed. Today, there is a resurgence of interest
in FM directional antennas.
FM directional antennas.
As activities in FM construction become more
settled, the FCC is stabilizing its outlook on directional FM antennas and spelling out their requirements more clearly. The major purpose of the directional system is to enable short-spaced FM stations
to increase power to the maximum now allowed their
classification under new FCC Rules. Directional antennas may not be used, however, as a means of
reducing minimum mileage separation requirements
in order to fit in a new station.
Before a station manager decides to use a directional antenna, he should become familiar with the
pertinent parts of the FCC Rules. Two deal specifically with such antennas and their uses (see box).
A typical application under the classification of improving service might be a situation where the
proposed principal city is close to a mountain or
similar shadowed area and there is no advantage
in radiating toward the blank mountain side. The
other approved application is for the purpose of
using a specific antenna site. If an applicant owns
an existing tower or high building, or even another
class of broadcast station, and wishes to use this
as the supporting structure, a directional antenna
might be required in order to limit the combination
of height and power in a specific direction to conform
with the Rule regarding power and height combinations.
Technical Requirements
Technical requirements are spelled out in detail
in FCC Rule 73.316 (c), (d) subparagraphs 1 through
3. Certain portions are particularly noteworthy.
In most cases where a directional FM antenna is
used, the engineering portion of FCC Form 301 will
be completed by the applicant's consulting engineer.
However, sometimes a well qualified chief engineer
can handle this work. (See BM/E-June, 1965.) If
this is the case, these points should be observed. The
application must completely describe the antenna and
explain the method of obtaining directivity. You must
provide a radiation pattern showing free space field
intensity at one mile in mv/m for the horizontal
plane' and data on vertical radiation between plus
The FCC prefers to have all data in dbk, rather than kw. See Fig. 6.
Proof of Performance
Measurements are required in the horizontal as well
as the vertical plane. The 360° horizontal radiation
pattern must be shown. There is no formal way in
which the FCC requires the proof to be measured or
submitted, provided the material is there and is correct. Ed Hackman. current Head of the FM engineering group, is a reasonable man, and is always
willing to explain things to an applicant's engineer.
The Rules call for a proof to be made in the field,
or by the manufacturer. The latter is far easier and
less expensive for the applicant, although such activities do tend to remove the bread from the mouths
of consulting engineers. However, the Commission
requires that the manufacturer make his measurements with the antenna mounted on the actual tower,
or a replica thereof, together with all lines, ladders,
lights, etc., that will be used in the final installation.
The reasoning behind this is obvious. However, the
word tower should not be taken to mean the 300-foot
high mounting structure, but to a section of tower
or pole on which the antenna is secured prior to
mounting on the actual tall tower. The old rule which required measurements along eight radials with a pen
recorder is out! Today, directional antennas for FM
use are as simple to specify and use as non-directional
antennas.
Management Considerations
FM directional antenna costs should not be more
than 10% of general equipment costs. Their construction is simple and, in many cases, undetectable from
non-directional antennas. DA arrays are as simple to
install as non-directionals, except for proper orientation. Unlike AM DAs only one tower or supporting
structure is needed.
The average FM station now operating at full
power, will not have to consider directional operation.
One application of the FM directional is in the case
of of a short-spaced station which wants to increase
power. Then it may be necessary to use a directional
antenna to control radiation in the short-spaced direction. Occasionally an operating FM station will discover that coverage in a given direction is not what
it might be for various reasons such as terrain. In
this case a directional antenna will achieve the desired coverage. FCC Rules governing the proposed
operation must be adhered to. Often an existing FM
antenna can be modified, depending on the design,
to give a directional pattern by means of fairly simple
phasing alternations. When this is done, field pattern
measurements will have to be made with the antenna
mounted on the station's tower, and this can run into
several thousand dollars, depending on the complexity
of the measurements. Generally, if the antenna in use
has been amortized, and the station is about ready
for a new one it would be better and cost less in the
long run to install a new one that has been factorytuned, adjusted, and furnished with a proof of performance.
Condensation of FCC Rules Governing FM DAs
Rule 72.213 (c): In the case of short-spaced stations,
maximum radiation may be used—provided that the
maximum power radiated in the short-spaced direction
is not in excess of the amount allowed non-directionally. No more than the maximum permissible power
for the class of station concerned may be radiated in
any direction, and the power increase off the radial
separating the two stations must not be greater than
2 db for every 10° of change in azimuth. (See Fig. 1.)
Rule 73.316 (c): A directional antenna is considered
to be any antenna that obtains a deliberate noncircular pattern for the purpose of improving coverage
or using a particular site. It may not be used to
circumvent the minimum mileage separation requirements. A ratio of 15 db maximum to minimum radiation will not be accepted. The hypothetical patterns
in Fig. 1 comply with this rule.
FM Antenna Manufacturers
Since almost any FM antenna can be converted into a
directional antenna, most manufacturers are able to
meet any requirement. It probably can be safely said
that every antenna manufacturer is able to offer directional antennas.
Alford Mfg. Boston Mass.
Andrew Corp. Co.dicago Ill.
Canadian GE, Toronto,'Ont., Can.
Canadian Marconi, Montreal,Que.,Can.
CO-EL, Westfield, N.J.
Collins Radio Cedar Rapids, Ia.
Gates Radio Co„ Co.,uincy, Ill.
General Electric Co. Syracuse, N.Y.
Jampro Antenna Co., sacramento, Cal.
Radio Corp. of America, Camden, N.J.