of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio & Television News articles
I have already posted. As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
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 EZNEC
could 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 management standpoint.
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 radiation
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.
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.
Within 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.
By the 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 1,400 feet.
The towers of Station KNX
after insulators have been installed in order to minimize their
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.
But 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.
"Aha!" exclaimed Mr. Hovgaard. "The plot thickens!"
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 resonance."
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:
(1) Directive and inefficient performance
due to tower resonance.
(2) High attenuation by the Santa
(3) The relatively highly carrier frequency.
(4) The relatively great distance of the station from the
local areas to be served.
A close-up view of one of the massive
insulators which solved the problem.
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
"Well, what are the remedies?" Mr. Rogers wanted
"There are 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 decibels.
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 kilocycles.
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 masts?"
"It can be done," replied Mr. Hovgaard.
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.
February 14, 2014