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 Zepp.
Fig. 292 - Early converter for short wave bands.
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.
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 and sensitivity.
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 frequency oscillator.

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 as a
Fig. 295 - One of the first pentagid converter, the valve 2 A7.
mandatory approach of improved radio frequency and intermediate frequency circuits.
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.
Fig. 296 - The first autotune transmitter model ART-13 made by Collins Radio Company – USA.
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
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 70’s. 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 World War.
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

Fig. 297 - Schematic of a mechanical filter.

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
Fig 297A - The mechanical filter
the radio operator making it almost impossible for the enemy to monitor or jam the transmissions.
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 – IRE.
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 ranges.
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