|As aforementioned, in the beginning
the developement of amateur radio was related with the early
technology status using “home-brew” equipments.
Starting with a spark gap transmitter, soon the thermionic
valve brought circuitry innovations such as the ones developed
by Hartley, Colpits and even the Ultraaudion with 25 - 100
oscillator transmitting power, using the the most common antennas
available at the moment including the long wire, windom or
Around 1930, both CW and phone modes operation underwent changes
and improvements. Firstly the transmitter stability was made
better with reduced coupling between oscillator and antenna.
In this new circuitry concept, amplifiers were inserted to buffer
the oscillator. In the phone operation mode the early inefficient
class-A modulators were replaced by a more reliable output stage,
using two audio amplifying valves operating in class-B. In the
beginning of the thirties it was launched in the market the
first crystal-controlled oscillators as well as the transmitters
were provided now with adequate filtered plate power supply
on all stages for all the bands except 10 and 5 m.
|Fig. 292 - Early converter for short
In the meantime, with the advent of superheterodyne circuit,
started the replacement of the older receivers technologies
as regenerative, tuned radio frequency and neutrodyne.
In reality the modern communication radio receiver was a compromise
either in the development of components as well as in new
manufacturing concepts that improved considerably its selectivity
Due to the characteristics of the superhet circuit, some experimentation
for suitable intermediate frequency took place before the
standadization at 455 kHz.
| Fig. 293 - Schematic of the beating
However, adaptation of the superhet circuit to the communication
receivers has been a continuing process due to the cost involved.
Firstly the manufacturers launched in the market the “converter”
for use ahead of a standard broadcast receiver. Such type
of circuit used screen grid valves for the local oscillator
and the high frequency mixer as well as plug-in coils.
The disadvantage of the converter was that the intermediate
frequency response of the broadcast receiver determined the
over-all selectivity of the combination. Fig 292
For the reception in continuous wave, with superhet circuit
required the action of a supplementary beating
|Fig. 294 - Ilustração
de um dos primeiros tipos de rádiorreceptores de
comunicação feito em caráter industrial
na década de 1930. Receptor de comunicação
modelo HRO 5, fabricado nos EUA pela National Company.
oscillator - as known as B.F.O. - to generate an audible
signal. Fig 293
In the thirties, considerably effort was concentrated on the
development of superheterodyne communication receivers, emphasizing
the importance of good selectivity ahead of the second detector
mandatory approach of improved radio frequency and intermediate
|Fig. 295 - One of the first pentagid
converter, the valve 2 A7.
The fact that the necessary beat method of continuous wave
reception produced sidebands on both sides of the carrier
was studied by Lamb and so it
gave birth of the superheterodyne radio receiver for “single-side”
reception in continuous wave, now provided with a crystal
filter in the intermediate frequency amplifier. Thus, through
this circuit it was possible tuning the desired sideband in
such way the signal became effectively single-sideband on
the second detector.
Since the selectivity continued to be a relevant goal of
the circuit designers, soon appeared on the market receivers
provided with linear diode detector, automatic volume control
with coverage pushed toward 30 MHz, with the surge interest
in the ten-meter band. Such circuit topology increased the
image problem, which was approached by improving the radio
frequency selectivity by introducing one or more tuned radio
frequency stages ahead of the converter. Fig 294
296 - The first autotune transmitter model ART-13 made
by Collins Radio Company – USA.
Another very important factor in the developmento of the modern
communication receiver was the arrival of the new pentragrid
converter valves, for instance the model 2A7. It worked as
a combined oscillator and detector and was quickly utilized
either in the first and second detector stages as well as
in beat frequency oscillator. Fig 295
In the remaining years of 1930’s the market flourished
with many innovations such as the new six volts metal valves,
new imporved passive components, iron core intermediate frequency
transformer, new circuit topologies, as well as the Lamb’s
IF noiser silencer.
The old wire antenna standby was replaced by bean arrays connected
to the receiver through the new coaxial feed line launched in
the market in 1937.
|Fig. 294A - -Several types
of communication receivers made after WWII0.
|Communication radio receiver model GR.
78, manufactured in U.S.A. by Heath Company in the eraly
||Fig 294B - Communication receiver model
RME 84, made in the USA
During the WWI, the radio amateur activities comprised either
military services or in participation in radio emergency services
similar which had ocurred twenties before during the First
The war effort led to many new developments in the radio communication
technology. Soon after the WWII radio amateur activities started
all over the world employing now many of such improvements.
Among them the most important ones are: the autotune transmitter,
the permeability tuned oscillator (PTO), and the mechanical
filter. Fig 296
|| PARTS DESCRIPTION
||ELECTRIC SIGNAL INPUT OR OUTPUT
||DISC SUPPORT IN EACH EXTREMITY
||SECTION COMPOSED OF 6 RESONANT MECHANIC
||PERMANENT MAGNET FOR POLARIZATION
||MAGNETOSTRICTIVE BAR DRIVE
|| ELECTRIC SIGNAL INPUT OR OUTPUT
|Fig. 297 - Schematic of a mechanical
Certainly such so important technological innovations in
the radio communication were related to the contribution of
radio amateurs like the American Arthur Andrews Collins, founder
of the famous Collins Radio Company.As aforementioned the
autotune transmitter and the PTO were big step forward in
airborne communication during the war. Installed in the American
combat airplanes, those new radio innovations offered any
combination of various automatially tuned frequencies, which
could be selected quickly by
the radio operator making it almost impossible for the enemy
to monitor or jam the transmissions.
|Fig 297A - The mechanical filter
By other hand the mechanical filter is an electromechanical
device providing an extremely high selectivity, allowing the
rejection of adjacent channels in the region of 500 cicles/second.
In Reality the mechanical filter uses the principle of magnetostricition.
Thus it actuates as a tranducer to convert the oscillating
electrical energy into mechanical vibrations and back again
to electrical energy. Through the researches developed in
the Collins Radio Company, engineers discovered that this
principle could be used for application with electric pulses.
By late 1952 the mechanical filter would make its official
appearance during a meeting at the American Radio Institute
Basically it consists of a set of coils connected with a serie
of nickel alloy discs inside a metal housing. In this way,
the income electrical signal is converted to mechanical vibrations
by the magnetostriction input coil that is transmitted to
the set of nickel alloy discs and finally by the reverse principle
it is converted back into electrical energy. In order to improving
the mechanical coupling efficiency a set of biasing magnets
are fixed in either end of the coils.
Production of mechanical filters was extremely complex and
initially plagued with many technical problems for the Collins’s
engineering staff. Around 1950 finally they were overcome
and the company invested in a new manufacturing plant for
the production of a complete line of mechanical filters that
could be supplied for the operation in several frequencies
As aforementioned, the mechanical filter was a step ahead
in radio communication technology. It had several applications
in the electronic industry: in the development of multi-channel
microwave system, as well as it was the backbone of the single
side band radio. Fig 297