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RADIO AGE for February, 1927
The Magazine of the Hour
No shield is necessary on the first radio frequency stage although the receiver is designed so that a shield can also be used for this stage if desired.
The output of the second radio frequency stage which is a highly amplified copy of the original signal picked up by the antenna, is then fed to the non-regenerative detector where it is demodulated or converted into audible frequencies. These audio currents, or electric sound waves are then further increased in strength by the two stages of transformer coupled audio frequency amplification and passed on to the loud speaker.
R F Amplifier
TWO stages of radio frequency amplification used in this receiver present some rather new and novel features in the design of the antenna coupling coil and the interstage radio frequency transformers. The design of these coils is based on two fundamental laws of radio engineering that are as old as radio itself. The first of these laws is this: Up to a certain point an increase in the coupling between two coils
LIST OF PARTS
Samson transformers, type
ifd. midd
3 Hammarlund .00035
line condensers 3 Hammarlund auto-c o u p 1
coils (set of 3 coils)
1 Hammarlund Jr. condenser, 9 plates, 32 mmfd.
2 Mar-co No. 192 Vernier Dials.
3 Benjamin No. 9040 sockets (with bases)
2 Benjamin No. 9049 sockets (without bases)
2 A m pet-it es No. 1A
1 Amperite No. 112
1 Carter No. M-10-S combined rheostat and filament switch 10 ohm
1 Carter No. 1 short jack
1 Carter No. 12 Imp aerial switch
1 Sangamo .00025 mfd. fixed condenser
1 Sangamo .001 mfd. fixed condenser
1 pr. Sangamo grid-leak clips
1 Durkam metallized resistor, 3 megohms
1 Hammarlund-Roberts foundation unit (containing drilled and engraved Westinghouse Bakelite-Micarta panel, drilled Bakelite, Micarta sub-panel, two complete shields, two equalizers, extension shaft, resistance unit, wire, screws, nuts and all special hardware required to complete receiver.
affords an increase in energy transfer and a decrease in selectivity. The second law is this: The energy transfer between two coils such as the primary and secondary of an ordinary radio frequency transformer increases rapidly as the frequency increases. In other words, the energy transfer is much greater at high frequencies (short wave lengths) than at low frequencies (long wave lengths), and the relative selectivity is less at high frequencies and greater at low frequencies. Conversely, a constant transfer of energy and constant selectivity can be maintained by loosening the coupling as the frequency is increased.
Successful broadcast receivers must be capable of receiving wavelengths from 200 meters (1500 k c) up to 545 meters (550 k c). This represents two extremes in frequency corresponding to a range of about three to one. These requirements together with the two laws stated above make it evident that some means of variable coupling must be provided if we are to obtain equal energy transfer and selec(Please turn to page 51)