is a story of a real feat of RF engineering where the stakes were
high for determining the cause of the problem and effecting a solution.
In this case Bell Telephone Laboratories was solicited to figure
out why a commercial broadcast station's signal was not being received
as strongly as predicted after the station had relocated its facilities
specifically to address the issue. A lot of power was being pumped
into the antenna, but inexplicably some relatively nearby listeners
were getting lousy reception while reports were coming in of good
signal strength from hundreds - even thousands - of miles away in
other directions. A modern antenna design program like
immediately predict the empirically measured pattern if the entire
system is modeled properly, but it would still require the insight
of a really experienced engineer or technician to ascertain the
cause. It makes you wonder whether, presented with the same challenge,
you would arrive at the correct conclusion. Many options for a solution
were ultimately presented to station management. The one chosen
was the most technically elegant and, had it not been selected,
would have denied the RF broadcast world an important example of
the right way to solve problems - both from an engineering and a
March 1930 Radio News
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Solving a Broadcaster's Dilemma
By C. S. Gleason
Problems That Arise in Connection with Receiving Sets Are Really
Insignificant in Comparison with Those of Broadcast Station Operation.
The country surrounding KNX's transmitter mapped by a Fairchild
ship. The clover-leaf outline indicates the field before
new insulators were installed. The oval shows the result
of the change which redistributed the field to the nearly
perfect circle of theory. The largest curve illustrates
the effect of the Santa Monica Mountains upon the station's
The great majority of listeners have little occasion to
learn about the inside problems which arise in the efficient operation
of a high-power broadcast station, and, providing no interruption
occurs during the enjoyment of a favorite feature, they probably
care even less.
But let a break occur, and their first move
is to telephone the station to know "why." Some even are stirred
to put their thoughts - or at least some of them - on paper in no
uncertain terms, for the benefit of the station manager.
Mr. Gleason, the author, tells here the story of how one station,
out on the west coast, after moving to improve its service, found
that the local area was not being served as well as before the change.
Simple though the trouble appeared to be, it required a good deal
of time and labor to track it down.
Although not intended
particularly for the guidance of station managers, this article
will undoubtedly open up to them new lines of thought. At the same
time, it will give to broadcast listeners some inkling of the tremendous
problems which beset the station manager in his efforts to remain
on the air "on schedule."
Trouble was brewing at KNX, and
wise studio underlings, seeing the storm clouds gathering about
the station manager's brow, were scurrying to cover. True, reports
trickled in constantly from the forty-eight states of the Union,
from Canada, Mexico, Cuba, and Alaska, with a fair sprinkling of
acknowledgments from Australia, New Zealand, Japan, England, and
various South Sea islands. But listeners in parts of the surrounding
area, including the northeastern suburbs of Los Angeles, had reported
that KNX was not coming in as well as before it had moved its transmitter
from Hollywood to a point in San Fernando Valley, seven miles away,
where it is operated by remote control from the studio on the Paramount
Pictures' lot. And when things begin to happen to the local service
area there is bound to be trouble. Broadcasters must fully cover
the local area whether distant listeners are served or not; and
it was the desire of the owners of KNX that their station blanket
Southern California with a bumping signal amply strong to override
static and atmospherics at all times of the year.
scratched their heads. The usual number of amperes were going into
the antenna, and field strength tests showed 15,000 microvolts per
meter at Santa Monica, 15 miles away. But up at Altadena, about
25 miles removed in a straight sweep up the valley, the intensity
was somewhat less than the 10,000-microvolt level set up by engineers
as a standard for "excellent, year-round loudspeaker reception."
And when Naylor Rogers, station manager, called his technicians
into his office and in a few well-chosen words delivered an ultimatum
to the effect that something was wrong and that the technical honor
of the station depended upon their finding out what it was, things
began to move rapidly.
a short time Ole M. Hovgaard, a survey engineer of the Bell Telephone
Laboratories, was summoned. Earle C. Anthony, Packard distributor
and owner of KFI, heard of what KNX was doing, and generously tendered
the use of a seven-passenger Packard straight-eight Phaeton. Into
this vehicle was loaded all the apparatus necessary to determine
the way a station's wave was behaving after being shot out from
the antenna into space.
Upon a geological survey map showing
the contours of the surrounding district, a circle was traced around
the station's location, with a radius such that the circumference
followed, as closely as possible, existing boulevards. This represented
the ideal field distribution. Then, with the transmitter at KNX
running at fixed output, a point on the circumference of this circle
was selected. Here a special shielded superheterodyne was tuned
to KNX's wave and the reading of a milliammeter placed in the plate
circuit of the second detector was noted. Then a local oscillator
was substituted a the source of current, and the intensity of it,
output adjusted until the milliammeter reading was the same as before.
The oscillator output thus measured KNX's field strength. At various
points within the circle other readings were taken.
time the circuit was completed and the readings plotted upon the
map. with the points of equal field strength connected by a smooth
line, the resultant figure was far different from the perfect circle
of ideal distribution. The general trend of the loops and kinks
was roughly toward a figure-eight, with several minor bulges giving
a clover-leaf effect. Evidently the radiation was most effective
in a line running approximately crosswise to the valley and down
to the ocean. But over a sector N. 130° E. and over most of
Hollywood and Beverly Hills, the radiation was weaker - about 67%
of that to the east and south, in which direction lay the Santa
Monica Mountains, a long, low range running north-northeasterly
down to the ocean, and attaining a height, at some points, of approximately
Mr. Hovgaard knew that mountains, as well as skyscrapers and city
areas, have a pronounced screening effect. This effect he set about
to measure. Over the few existing roads through these mountains
- many of them hardly more than trails, rough and crevassed by the
rains - went the field car. Measurements were taken at points on
an ever-widening spiral. terminating at last beyond the mountains.
And when the results were plotted upon the map, it was found that
on the far side of the mountains the signal strength dropped sharply,
the shadow effect in places reaching as high as 60%. The outlook
for improving reception was not bright, since it was not considered
practicable to change the physical features of the topography so
as to straighten out the clover-leaf into a nice. symmetrical circle.
Boundless as was Mr. Rogers' faith in the ability of the Bell engineers,
it was not sufficient to move these mountains.
The towers of Station KNX after insulators have been installed
in order to minimize their absorption effects.
Mr. Hovgaard did not believe that this told the whole story. The
intensity on the side away from the mountains was also low. Why
should there be a shadow where there was nothing to cast one? So
he set out on another series of tests. The frequency was lowered
to 700 k.c., and a new curve was plotted.
Mr. Hovgaard. "The plot thickens!"
The distribution of the
field, while not perfectly uniform, was at least ovular in shape.
And the reason, deduced Mr. Hovgaard, was what he termed "tower
Now the antenna at KNX, as will be seen from
the photo, is a single vertical, six-wire cage 179 feet long, swung
between two graceful steel towers 225 feet high and 550 feet apart.
It so happened that the natural period of the tall slim masts coincided
very closely with the frequency assignment of KNX: namely, 1,050
k.c. The towers therefore acted as resonant circuits, and a large
current was easily induced in them by the passing wave, with the
result that the field was greatly distorted in that direction.
In his report, Mr. Hovgaard summed up he situation briefly
as follows - that the trouble was due to four causes:
Directive and inefficient performance due to tower resonance.
(2) High attenuation by the Santa Monica Mountains.
(3) The relatively highly carrier frequency.
relatively great distance of the station from the local areas to
Although it is true, he said, that greater distance of transmission
had been obtained more easily on the shorter waves, it is also true
that for local service, which depends upon the "ground wave," the
lower frequencies are attenuated less with distance and so can serve
larger areas dependably. Given equally powerful stations and equally
efficient antenna systems, the signals at fifteen miles' distance
from a station on 1,050 k.c. will be 15% lower than from a transmitter
on 700 k.c. At greater distances, this effect is even more pronounced.
The intensity of the ground wave falls off rapidly with distance:
for example, on 1,050 k.c. it will take only one kilowatt of power
to produce the same signal as will be heard at 15 miles with five
kilowatts output. The one-kilowatt signal at three miles is four
times as strong as that from a five-kilowatt station at fifteen
miles. Therefore it is a good thing to get as close to the area
to be served as possible, bearing in mind, however, that too close
proximity to business districts may result in high attenuation and
"shadows" cast by tall, steel-frame buildings.
A close-up view of one of the massive insulators which solved
are the remedies?" Mr. Rogers wanted to know.
a number of possible solutions," replied Mr. Hovgaard. "First, you
can move closer to the city. That is expensive. Second, you can
increase your power, provided the Radio Commission will let you.
That is cheaper and has the advantage that at the same time your
distant service range is increased. Third, you can change your carrier
frequency - with the consent of the Commission, of course - from
1,050 k.c. to 720 k.c. or less. This will avoid resonance with the
towers, and will improve the field strength in the more important
areas by about 100% - an audio frequency improvement of about twelve
Fourth, you can erect a new and larger antenna
with two 350-foot steel towers 700 feet apart and mounted on insulated
footings, which will break up the electrical circuit of the masts
and prevent resonance with the carrier. If you do this, you may
expect an improvement of 180% in the Beverly Hills region and about
40% in other directions. Or you can reduce the height of your present
towers, in order to reduce their natural period until it no longer
approaches your transmitting wavelength. By lowering the towers
to 150 feet the resonant frequency would be shifted to about 1,500
Sixth, you can doctor up the masts and change
their resonant frequency by loading them electrically with inductances;
this is not a sure cure nor entirely effective. A multiple-tuned
antenna would get around the difficulty, but it would be expensive
for the amount of improvement it would give. Lastly, you can insulate
the footings of the present antenna by installing insulators, thus
breaking up the electrical circuit. This would give you an improvement
of 50% in Hollywood and Beverly Hills - an audio gain of seven decibels."
"Well," said Mr. Rogers, "we know we don't want to move.
We have had trouble enough finding our present location without
breaking up housekeeping and moving again. Even if we should increase
power, we would still be resonating with the towers. We want to
make the most of the wave we radiate. To get a new frequency assignment
would at least take time, if it were possible. We don't want to
wait, nor do we want to rebuild our antenna if it can be fixed up
as it is. It would be too bad to spoil our beautiful towers by cutting
them off at the top. Your sixth and seventh solutions you yourself
do not regard as very satisfactory. What about insulating the present
"It can be done," replied Mr. Hovgaard.
"Do it." said Mr. Rogers.
The Ohio Brass Company set to
work upon the design of a new insulator to meet the specifications
of the technicians. They brought forth a pattern consisting of four
round brown porcelain columns grouped together between heavy steel
plates, with a large shield at the top to keep out the weather.
At the base was provided a chamber with an outlet normally sealed
by a steel plug, which can be unscrewed, so that in case moisture
should cause the insulators to leak, compressed air forced in at
this opening would soon dry them out. Eight of these sturdy units
were shipped out to KNX and a local steel company sent over a crew
of men to install them.
The results of the change were gratifying.
Not only did testimonials indicate that local listeners were getting
better service, but measurements showed that field distribution
much more nearly approached the ideal circular form. A large, densely
populated area of Los Angeles and its extensive environs was thus,
in effect, moved a number of miles closer to the transmitting station.
The work of broadcasting bigger, better and brighter bedtime
stories once more went on apace.
Posted February 14, 2014