Challenges in Early Automotive Radio Design

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Challenges in Early Automotive Radio Design - RF Cafe

Early automobiles presented significant challenges to mobile radio designers due to a combination of a fledgling understanding of electrical and electronic circuits and quickly evolving automotive materials and configurations. A 1935 issue of Radio-Craft magazine presented eight radio designs that represented break-through techniques for dealing with some of those innovations. All of the technical issues involved here have been pretty much solved in modern radios. Ignition interference is nearly invisible to FM and satellite reception, although audio frequency circuits can still pick up noise if not properly filtered and wired. Only dinosaurs like me still listen to AM broadcasts in the car and still hear occasional interference. Front end sensitivities and antenna design and placement have greatly extended reception range, although multipath, particularly in urban areas, is probably worse now than in the 1930s because there are vastly more sources of signal reflection.

All of the radios, as did the Emerson 5A automotive radio, employed some sort of electromechanical DC-DC converter scheme in order to obtain the couple hundred volts needed to bias vacuum tubes from the car or truck 6-volt electrical system. It was termed a '"vibrator 'B'" unit since it charged the "B" battery used to supply plate voltages. A simple passive voltage multiplier could not supply enough current to power the radios.

Some sort of static suppression was needed to quell electrical noise from ignition systems and, interestingly, static charges built up on the metal body via the triboelectric effect as the vehicle moves through the air. Amplitude modulated radio signals are extremely vulnerable to static interference.

Earth Belt Car Anti Static Strap Earth Belt Ground Wire Strap Reflective - RF Cafe

According to the note published on the General Motors Chevrolet No. 601574 automotive radio, "Master Chevrolet car rear wheels are equipped at the factory with anti-static fittings. Similar fittings for the front wheels are supplied with the radio set." Flexible metal straps connected to the chassis and dragged along the ground to dissipate charges. Even with modern vehicle and electronics design, there are still situations where static build-up is a problem, and you can still buy anti-static grounding straps for your car.

The Motorola 100 automotive radio information details an amazing procedure for cancelling out ignition system interference; to wit: "When balancing out interference by the method used in this receiver, it is advisable to clamp the car hood down tight with the hood hasps, and to sit in the driver's seat; otherwise, in extreme cases, if the noise is balanced out without anyone in the driver's seat it may reappear when the driver is seated." How would you like to do that once a week?

Designers of the RCA Victor M-104 (and M-108) automotive radio addressed the interference as follows: "From antenna to reproducer, this car-radio receiver has been designed for interference-free operation without recourse to suppressors. The ground-end of the antenna coil, for instance, is not grounded directly at the receiver chassis, but is carried to a point on the car frame at which circulating noise currents do not become mutual to the receiver input; the R.F. transmission line favors broadcast signals and greatly attenuates interference frequencies."

Arvin-Ford 17-A automotive radio engineers understood early-on how much more full of electrical interference urban areas contain as compared to rural areas. "A 2-way plug switch provides extreme sensitivity for country driving, and lowered sensitivity and reduced noise level for city driving."

The Zenith 666 automotive radio "...has been designed for operation without recourse to suppressors; an 'A' filter incorporated in the receiver contributes to this performance. Also, the chassis is completely floated in rubber."

The American-Bosch 524A automotive radio made certain the radio listener would receive both noise-free reception and high quality music enjoyment. "The volume control is designed to maintain good bass response at low volume levels. This set is supplied with a kit of spark suppressors, etc.; the installation manual furnishes complete information on the elimination of noises due to: (1) antenna pickup; (2) antenna lead-in pickup; and, (3) noise conduction into the set via the battery cable."

Even the 'reproducer' (or as we now call it: a 'speaker') needed extra filtering because the field coil was an electromagnet - as opposed to a permanent magnet - so it was subject to interference as well. This was not unique to automotive radios because until the 1950s, even household radios used speakers with electromagnet coils. See my Crosley 03CB console radio, for example.

Antenna configuration and placement presented continual challenges as well. Evidently, antennas on early vehicles were located underneath the car rather than being mounted as a vertical whip. Once "all-steel-top cars" began appearing on the scene, the under-car antennas were rendered ineffective. Who in 1929 would've thought that would ever be a problem to be dealt with?

Crosley Roamio 4-A-1 automotive radio designers even planned for the inevitable need for maintenance thusly: "This set mounts under the cowl section of the car, with the dial visible below the instrument panel. Thereby, the lid is easily removable for convenient service."

These schematics, tuning instructions, and other data are reproduced from my collection of vintage radio and electronics magazines. As back in the era, similar schematic and service info was available for purchase from sources such as SAMS Photofacts, but these printings were a no-cost bonus for readers. There are 227 Radio Service Data Sheets as of December 28, 2020.