March 1953 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
FM broadcasting (frequency
modulation) began in the United States in the late 1930s, it was not until after
World War II and even the Korean War, in the 1950s, that the major shift to
FM took place. It took even longer for FM to get a foothold in Europe mainly due
to the emphasis on rebuilding essential infrastructure and manufacturing destroyed
by the war. As this article points out, the newer FM radio features allowed
it to thwart some of the propaganda efforts of the Soviets in East Germany who would
be stuck in technologies that lag two or more decades behind the free world even
to this day (ain't Communism / Socialism great?). The "medium-wave band" referenced
herein is AM (amplitude modulation), so replacing dominant radio broadcasting with
FM systems would effectively cut off AM propaganda. FM radios were being produced
so inexpensively in the U.S. that they were very affordable in Europe was well.
There is no mention of whether the West German government subsidized the purchase
of FM receivers by citizens, but it would not be surprising if it did so.
Blanketing a nation with high-level signals brings FM benefits
to 50 million.
Fig. 1 - Locations of stations in Western Germany's nationwide
By Professor Werner M. Nestel*
Broadcast-frequency assignments in Europe are based on two international agreements:
1 - The Atlantic City Agreement (1947), which assigned the frequency band 525-1605
kc for "medium-wave" broadcasting. (Europe also has a number of "long-wave" broadcast
stations on various frequencies between 150 and 530 kc.)
2 - The Copenhagen Agreement (1948), which assigned specific channel frequencies
to nearly 600 European stations operating within a relatively small geographic area.
Western Germany, which was not presented at the Copenhagen conferee, was allotted
only a few undesirable frequencies at the high-frequency end of the medium-wave
band. These were all shared channels, with unreasonable power limitations, and had
to compete with very powerful stations in other countries operating on the same
frequencies and on adjacent channels. The fact that Western Germany, with about
50 million inhabitants and some 10 million listeners, would be open to Communist
propaganda by radio as long as its own broadcasting system did not work effectively,
was not taken into account at this conference.
Faced with the difficult situation, Western Germany had to devise a means of
serving all its listeners through a new system of program distribution, if broadcasting
as one of the most effective and important modern instruments of culture and education
was not to be given up entirely. In seeking a satisfactory solution, the following
possibilities were considered, and each method was tested by thorough experiments:
a. Carrier-current distribution of programs over telephone
b. Carrier-current distribution of programs over power lines.
c. A combination of methods a and b.
d. Low-power common-frequency broad- casting (on various frequencies).
e. Short-wave broadcasting.
f. V.h.f. AM broadcasting.
g. V.h.f. FM broadcasting.
System g (FM) proved so superior to all the others that it was decided to make
it available to the public as fast as possible. Much of the experience gained with
FM broadcasting in the United States was utilized, but in many respects new techniques
had to be developed.
The Basic Plan
Fig. 2 - (a) FM slope detector circuit. (b) Peaking the tuned
circuit below the FM carrier converts carrier-frequency deviations to audio voltage
In general, FM receiver circuits developed in the United States have such high
sensitivities that large areas can be served by low-power transmitters. The comparatively
low prices of sets, tubes, and parts in the United States make it possible to manufacture
and sell the elaborate multi-tube receivers needed there. In spite of these favorable
factors, FM broadcasting has not developed as a serious competitor to AM in the
United States. To make sure that it does so in Western Germany - and this was not
merely a matter of preference, but an absolute necessity - it was decided to blanket
the entire country with field strengths of more than 1 mv/m by installing an adequate
number of high-power FM transmitters. To be able to provide this high field strength
over the large area required and to make a complete network of transmitters as quickly
as possible the following basic requirements were established:
The FM transmitting antennas were to be mounted on top of existing 600-foot vertical
transmitting antennas used for medium-wave broadcasting. Assuming that the radius
of the area to be served is 30% greater than the optical distance to the horizon
and that the signal level at the fringes of this area should be 1 mv/m measured
at 30 feet above ground level in accordance with international definition, the effective
radiated power (ERP) of each transmitter must be about 60 kw. Horizontal polarization
is used, as in the United States. Antenna gains of 8 to 1 can be obtained easily.
Line losses between transmitter and antenna can be figured at about 20% if the FM
transmitters are installed in the existing buildings now used for the medium-wave
transmitters, which are generally about 600 feet from the bases of the antenna towers.
This calls for a 10-kw output from the FM transmitter, a power that can be provided
at reasonable cost.
The high field strength available with this system everywhere has two important
advantages from the receiving side:
Very simple and inexpensive receiving antennas - in most cases indoor types -
can be used; Good quality reception is possible without the high-sensitivity FM
circuits developed in the United States.
To reduce receiver selectivity requirements - helping to keep receiver costs
as low as possible - the minimum frequency separation between channels was set at
0.4 mc, twice the separation used in the United States. (This does not mean that
each station utilizes the full 400-kc bandwidth provided by this arrangement. Deviation
is limited to ±75 kc, as in the United States, and the unused separating
channels may be used eventually for additional stations.)
Sixty-two FM transmitters operating between 87.5 and 100 me have been installed
in Western Germany in the two years since FM was introduced. The map (Fig. 1) shows
how completely they cover the territory to be served. Relatively few transmitters
are needed to cover the flat northern part of the country with transmitting antennas
on top of existing 600-foot towers. The southern part of the country has mountains
of 6,000 feet high available for the stations, but there are deep valleys as well,
so that more transmitters are needed to cover a given area in the south. One of
the most important facts that made this network of transmitters possible is that
the "shadow" areas cast by mountains, houses, and other obstacles are only gray
- never quite black. Reception is possible not only with roof antennas, but also
with indoor antennas on all floors of buildings, the ground floor included. Even
indoors the electrical shadows are only gray and not black, and the high field strength
assures adequate receiver-input voltages for good reception.
For the future it is intended to install many more transmitters in order to offer
a bigger choice of programs in the FM range.
One of the many interesting special problems that had to be solved for the development
of the FM technique in Western Germany should be mentioned.
Fig. 3 - Low-cost superregenerative FM adapter developed for
the NWDR national network. The GU8 and 6X8 are approximate American equivalents
of the ECF12.
In many cases a single tower is used simultaneously for radiating two different
medium-wave programs with up to 100 kw power each; for one or two FM programs; and
will soon also be used for television with picture and sound. The same tower also
carries an FM-relay receiving antenna besides power circuits for aircraft warning
lights and de-icing heaters.
FM Receiver Design
The ability to provide listeners with adequate receivers is the real key to success
or failure of FM broadcasting. Although at first the German radio industry was against
FM - partly on the grounds of the high cost of adapters and complete receivers -
it finally, after several heated conferences, changed its attitude. Once the industry
started developing FM receivers, such simple and inexpensive solutions were found
as to start a positive race on the part of the industry in their eagerness to promote
The decisive move was a complete break with accepted standards for FM receivers.
The strong signals provided by the transmitters in Western Germany nearly always
make limiters unnecessary. According to American ideas, FM reception without limiters
and discriminators is unthinkable. Of course, these features allow good reception
with inputs of only a few microvolts, but they also entail a great outlay in tubes
and components. Abandoning the idea that these circuits alone are feasible, German
manufacturers then were able to use two very simple receiving circuits which do
not require one single component more than similar AM circuits. These are the super-regenerator,
and the slope detector (Fig. 2).
The cheapest receivers and adapters are super-regenerators. In this connection
the broadcasting authorities and the Post Office (which has the same authority over
broadcasting here that the FCC has in the United States) made very strict regulations
requiring that the interference produced in the superregenerative circuit must not
get into the antenna. Out of this were developed 2-tube adapter circuits, with both
tubes - a pentode and a triode in a single envelope.
The circuit of a typical adapter is given in Fig. 3. One of the tubes has a fixed-tuned
grid circuit and merely serves to isolate the antenna from the actual receiving
tube. It blocks oscillator radiation and reduces the effects of variations in the
antenna circuit on receiver tuning. The actual FM detector is the triode section,
in a super-regenerative circuit. The production of the auxiliary (quench) frequency
is completely reliable, even during line-voltage fluctuations. Following the superregenerative
stage is a simple RC h.f. de-emphasis filter. The output is adequate for feeding
the audio sections of existing receivers.
Fig. 4 - Schematic of a German 4-band AM-FM receiver. American
tube types in parentheses are approximate equivalents. On this and many other German
diagrams, switch blades are not shown. Instead, letters are inserted between contact
points which are opened or closed. All contacts marked with the same letter operate
simultaneously. The FM antenna circuit is untuned with bandpass input.
Fig. 5 - Block diagram of the unique relay-receiver-transmitter
which eliminates wire lines and special point-to-point links in the West German
The sensitivity of these simple FM adapters is about 0.1 mv - that is, at this
input voltage the "no-signal" noise is suppressed satisfactorily.
Adapters of this type, which cost between 27 and 50 German marks ($6 - $12) can
be added to any existing receiver, and the availability of such inexpensive units
probably is the underlying reason for the popularity of FM broadcasting. The obsession
that expensive receivers are absolutely essential no longer exists, but - curiously
enough - the cheapest adapters are by no means the most popular. Most listeners
prefer a slightly higher quality signal, but it was imperative psychologically to
make such very inexpensive adapters available.
The second type of receiving circuit mentioned above (slope detection) is used
to provide 3-band (short-, medium-, and long-wave) receivers with an additional
range for FM broadcasting. Compared with the usual superhet circuit, this type requires
only few additional components: a band switch with four positions instead of three;
and 10.7-mc i.f. transformers in addition to the 470-kc transformers already in
the receiver. The price differential between a 3-band AM superhet and one with the
added FM range is only about 10 marks ($3).
Fig. 4 is the circuit of a typical receiver of this type. Whereas during the
first year of FM broadcasting in Western Germany only one-third of the receivers
made had an FM band, the current public demand is so great that now over 90% of
all sets produced are equipped for FM.
In addition to these inexpensive sets, a group of first-class receivers selling
at higher prices was developed. The FM sections of these receivers have r.f.-amplifier
stages, and limiter-discriminator-type detection, and give very high-grade reception
even at great distances.
It is now an accepted fact in many parts of Western Germany that long-distance
FM reception is practicable, and there are many districts where the owner of one
of these FM receivers can pick up several - often as many as seven - different stations.
Another circumstance has helped in the very quick swing toward FM broadcasting
in Germany. During the war and post-war years (from 1938 to 1948) German listeners
could not buy receivers. When FM started, home receivers were completely out of
date. Everyone was in the market for a new receiver, and it was possible to get
one incorporating FM immediately.
FM broadcasting fulfills the two main requirements which led to its adoption.
a. Coverage of areas not reached by medium-wave stations.
b. Giving all listeners a choice of programs not available on medium waves under
the Copenhagen plan.
Beyond this, however, it has become very evident that FM broadcasting gives the
public a very welcome improvement in audio quality, and has helped to overcome certain
misconceptions. According to popular belief, standard AM stations have an audio
limit of about 5 kc, while FM transmits everything up to 15 kc. But since very few
loudspeakers will reproduce such high frequencies satisfactorily, FM was held to
be of no value from the point of view of improved quality.
The true picture is quite different. Most European medium-wave receivers, to
provide adequate adjacent-channel selectivity, have a restricted r.f. bandwidth,
which limits the sideband response to about 3 or at most 4 kc. The audio stages
of such sets, including the loudspeakers, generally have reasonably good response
to 8 kc, Therefore - even the simplest and cheapest adapter, selling for only about
27 marks ($6), extends the audio response from an upper limit of 3 kc to 8 kc, which
is such a very great improvement that every listener is impressed.
The reduction in harmonic distortion - from 4% at 100% modulation with AM to
only 1/2 of 1 % with FM - means a corresponding improvement in audio quality at
the receiving end.
Another advantage is the reduction in interference. Even receivers without noise
limiters do not suffer the interference - from electrical devices or natural static
- which frequently ruins medium-wave reception.
Transmission up to 15 kc. over long-distance phone lines is still not practicable
except at prohibitive cost. High audio quality is maintained over the entire network
by a novel method of rebroadcasting from station to station. Each station is equipped
with a special FM receiver and directional pickup antenna. (The usual 8-kc long-distance
wire lines are provided as a standby.) The relay receiver contains the r.f. and
i.f. sections of a normal receiver, but the i.f. is transposed by means of an 0.8-
or 1.2-mc crystal oscillator to a frequency 0.8 or 1.2 mc higher or lower than the
original r.f. carrier. See-Fig. 5. This new frequency is amplified and then radiated.
Receiver and transmitter thus have very simple design.
In planning the network, the channel separation and the geographic distances
between transmitters were coordinated with the designs for very cheap and simple
The great success of FM broadcasting in Western Germany has led many other European
countries to start or at least to plan FM broadcasting, and a European conference
on FM and TV frequencies was held at Stockholm in June, 1952. The frequency plan
decided upon at Stockholm provides for 2,000 FM stations in Europe. /p>
In summing up, it may be said that: All the individual technical problems of
FM broadcasting from the transmission, propagation, and reception points of view,
have been solved in every way. FM broadcasting is economically and technically beyond
reproach. In many ways it is cheaper than other systems.
If the deterioration in medium-wave reception throughout Europe since the adoption
of the Copenhagen Plan is to be overcome, then thought must be given right away
to the outcome of the next Wavelength Conference. Only one path will lead to improved
broadcasting. Only if as many countries as possible adopt FM for all local and regional
programs will it be possible to reduce the number of medium-wave transmitters.
** Technical Director, Nordwestdeutscher Rundfunk (Northwest German Radio System).
Posted March 13, 2020