Pattern for FM Profits:

5: RECEIVER AND ANTENNA
For The First Time; FM Sets and Atennas are Designed
to Meet Receiving Conditions in the Area Where They Will Be Operated

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FROM the beginning of the RRN project, it was realized that just as a telephone line requires a transmitter and receiver of equal efficiency in order to transmit intelligence, so must successful broadcasting service place as much emphasis on the adequacy of receiving sets as on the transmitting plant.

Specifically, meeting FCC engineering requirements at the broadcast station has no significance in service to listeners who use sets of 400 microvolts sensitivity in an area where a good antenna can deliver only 150 microvolts to the input connections. Also, the station might as well be off-the-air as far as giving service is concerned to listeners who need good antennas but do not have them!

Accordingly, G.L.F. Farm Supplies, a division of Cooperative Grange League Federation Exchange inc., was given the problem of determining first, what type of radio receiving equipment was needed to provide good FM reception in rural areas; next, to obtain equipment that would meet the standards set up; and, finally, to correlate distribution and service for such equipment into our regular system of distributing farm production supplies.

Following a pattern that has been used in the past to supply a wide variety of farm production supplies, the first step was to determine definitely what was needed.

United Cooperatives, at Alliance, Ohio. a cooperative wholesale procurement organization handling farm production supplies for member organizations, maintains a quality control and research laboratory at Ithaca, New York. The laboratory, staffed by trained engineers who have had the benefit of farm training or experience, regularly conducts both field and laboratory tests for member orginazations. United Cooperatives commissioned the laboratory to set up a project to determine specifically what was needed for good radio reception in the areas of New York, New Jersey, and northern Pennsylvania served by the RRN. From these studies, they were instructed to set up product specifications and make comparative tests as a guide to the final selections.

This, briefly, was the strategy mapped out for a broadcast operation that would not end, but actually begin, when the transmitters went on the air.

To carry out the first step of the Project, a mobile laboratory was constructed and equipped with the necessary instruments. A large observation window was provided, with benches for the equipment and shelves for the receivers to be tested. The mobile laboratory was equipped with a collapsible 12-ft. tower for a folded dipole antenna and reflector. This was constructed to permit rotation of the antenna from the inside of the truck. A field-strength meter, to measure signal strength in microvolts-per-meter, was set up to operate a recording meter. The latter had a 5-millampere movement. To synchronize the recorder with the distance travelled by the mobile laboratory, the chart was driven by a cable connected to the speedometer. A compass for accurate direction checks was included, as well as a standard REL FM receiver for monitoring and comparison purposes. A calibrated dipole, adjustable as to length, was used to check reception on the folded dipole. Finally, a generator delivering 110 volts at 60 cycles, rated at 800 watts, was mounted separately in a trailer.

With the equipment ready, the next problem was to work out routes of travel for the mobile laboratory that would provide the reception data needed. Using an FM broadcasting transmitter as the hub, a series of routes were mapped out on approximately 45° radials. Starting each trip 1 mile from the transmitter, the truck travelled at a speed of 15 miles per hour along each radial. Readings were taken all along the route of travel. The engineer in charge noted identifying landmarks on the charts so that the exact location of the truck could be correlated with the chart readings at a later date.

An intercormmunication system was installed between the cab of the truck and the enclosed laboratory to more closely coordinate the work of the driver and the technician.

During these field tests, the strength of signals received front both nearby and distant stations was carefully checked. From the charts and the landmark identifications noted during the tests, the effect of both terrain and elevation could be determined accurately. This method also permitted checking reception on receivers under the many varying conditions of terrain and elevation encountered in the rural areas, as well as at different distances from the transmitter. The truck travelled as far as 75 miles along the radial routes.

After a careful analysis of the test data, it was concluded that the primary area for FM reception should be conservatively considered as 25 to 30 miles from the transmitter used principally during this survey, with the secondary area reaching out 40 to 50 miles. Further, it was concluded that a radio receiver for good FM reception in rural areas should be highly sensitive as well as very selective.

G.L.F. Farm Supplies and the United Cooperative laboratory staff agreed on basic receiver requirements. Next, a field and laboratory program was planned to determine what standard receivers would meet the performance specifications.

A definite test procedure was followed. The standard of testing FM receivers, set by the institute of Radio Engineers, was used to cover the following points:

1. Sensivity (maximum and quieting signal)
2. Selectivity
3. Distortion characteristics
4. Detuning distortion
5. Frequency stability
6. Overall fidelity (audio frequency response)
7. Output and AVC
8. Set noise

A complete characteristic analysis was made for each FM receiver tested, and the data recorded, from which performance curves were drawn.

As the comparative tests continued, it appeared that our requirements for FM reception called for performance that manufacturers generally had not felt justified in providing, at least within our price brackets.

From the mass of accumulated data, specifications were written for a receiver that would meet the standards set up as result of the field testing. In doing this, G.L.F. was working to meet the needs of a known market with which it was familiar and experienced in serving.

The North American Philips Company was commissioned to build engineering test models according to the specifications, and these were included in the series of comparitive laboratory and field tests.

Before this testing program was completed, the performance of 17 different makes of FM receivers had been analyzed.

The outcome of this thorough-going preparation was the design of the G.L.F. model F-770 manufactured on a production basis by North American Philips. Further details of the set are presented in Part 2 of this Section.

The survey of needs for rural radio reception demonstrated clearly that a good outside antenna system is required for satisfactory reception. Because of the erection and service problems involved, a suitable FM antenna system must be easy to install, non-directional in its response pattern, and capable of picking up weak signals.

1. Non-directional reception
2. Horizontal polarization
3. Operation in the 88 to 108 mc band
4. Transmission-line impedance to match the receiver input
5. Lightning protection for both the antenna structure and the receiver.

The G.L.F. design finally adopted, and now manufactured by Technical Appliance Corporation, is supplied as a single turnstile composed of two folded dipoles, to which a second turnstile can be added in a stacked array for increased gain.

Since installation service is not available in some rural areas, a detailed installation instruction sheet is supplied so that any person with minimum of mechanical skill and without technical knowledge can erect and hook up the antenna correctly, the units are preassembled at the factory and identifying marks the final assembly fool proof.

An experienced G. L. F. service organization was already in existence, with completely equipped service shops at seven of the the G.L.F. Farm Supplies Warehouses. Located strategically to serve specific areas in the G.L.F. territory these shops were already providing service for the G.L.F. retail outlets on such items as AM radios and fence controllers.

To prepare these servicemen to handle the new FM equipment properly they were first provided with technical data for detailed study. Then they were sent to the North American Philips plant for a period of intensive training under the supervision of their engineers. Thus they became familiar with the reasons behind our performance specifications, the receiver circuits and components and the most advanced methods for repairing and aligning the sets.

Following the period of factory training, technicians from North American Philips were sent to the seven shops to work with our servicemen. As the first shipments arrived at the warehouse, all circuits and component parts were carefully checked by the servicemen. The purpose was to familiarize our men with the equipment, they were to handle and service, to spot any bugs in the production models, and to enable the manufacturer to make changes or corrections if any were needed.

With the sets in quantity production and service staff trained, the next. consideration was information for the retail personnel in the G.L.F. Service Agencies and time Agent Buyers. A series of nineteen evening meetings were set up, during June, a total of 612 employees attended these four-hour evening sessions. G.L.F.’s interest in FM as an important communication tool to farm families was fully covered. Servicemen explained the characteristics of the new FM farm radio, and what could be expected of it. The proper installation of the antenna and receiver was covered thoroughly, to assure the complete satisfaction of each customer. Each group was given an opportunity to erect one of the antenna systems, so they could answer customer questions. Receivers were demonstrated to sell our own sales staff on the fine tone quality and reception characteristics of FM. As the G.L.F. receiver is built for both FM and AM, comparisons were made between the two types of reception. A question and answer period was held after each meeting, so that specific service questions could be cleared.

Store banners, posters, booklets, and other promotional material were prepared and made available to all G.L.F. Retail Service outlets.

Thus, during a two year period of development a new product was ready for market with quality control, manufacturing, service, and distribution, specifically geared to provide complete radio service in our particular segment of the radio market.

Service on this new equipment will be handled at the seven warehouse points. With the G.L.F. Farm Supplies trucks making regular weekly deliveries to all retail points, receivers can be brought to the service centers, checked and adjusted, and returned to the purchaser in a week's time. This assures our customers of service by technicians who are thoroughly familiar with the equipment. The engineered precision originally built into the receivers will be maintained because only replacement parts specified for the purpose are used in the service work.

Periodically, sets are picked out at random from new shipments, and thoroughly checked. This provided the manufacturers, as well as G.L.F. with constant quality control. In addition, there is a return flow of information from the owners' reports and service experience that enables G.L.F. and the manufacturer to be constantly alert for opportunities to improve production methods and equipment design.

This is another example of the original thinking that has been done in connection with the RRN. It is the first time that receiver specifications have been determined in conjunction with the specific network, and the sets manufactured, sold, and serviced under one control.

The receiver employs the Armstrong limiter-discriminator system for FM, and a conventional superheterodyne circuit for AM. The input is a tuned RF stage, necessary to meet the requirement of better than 20 microvolts sensitivity, it also improves image rejection and reduces radiation from the oscillator. One of the new miniature 6AU6’s is used because of its very high mutual conductance. This stage is impedance-coupled to the double-purpose 7F8 which acts as a mixer and oscillator. This is a triode mixer, selected to meet the requirement of extremely good signal-to-noise ratio. In order to achieve high oscillator stability, the oscillator frequency is on the low side of the RF.

Two IF stages are used for FM, and one on AM. The first cornmon stage, has a 6BA6, while the second, for FM only, has a 6AU6 to secure high gain. A combination of fixed silver-mica condensers and adjustable iron cores is used to tune all the IF transformers. Changes of temperature and humidity, or vibration have a negligible effect on the tuning of these circuits.

A conventional Armstrong circuit is employed for the limiter and discriminator. Since sharp cut-off is required for the limiter, this is a 6AU6 tube. Two of the diodes in the 6S8GT are used as discriminator rectifiers, this tube is also the AM detector and first AF amplifier. The triode section is resistance-coupled to the grid of the 6V6 output stage which feeds a 7 inch, loudspeaker of excellent quality.

Feedback is taken from the output transformer and applied to the grid of the 6S8GT for degeneration. This provides an audio system of high quality and low distortion.

A separate converter is used for the AM section. This is to avoid losses in the RF circuits from the band switch. As a result, the only switching is done on the audio input, plate supply, and tuning indicator. The latter is controlled on FM by the voltage developed in the limiter grid circuit, and on AM by the AVC voltage developed by the detector and AVC diode.

In practically any area today, the geographical distribution of FM stations is such that only a part of those within the normal range of a good receiver can be picked up on a single dipole.

Further improvement is achieved by the 2-bay turnstile. First, there is the obvious gain in signal pickup. Second, and in some areas of greater advantage, is the fact that the field around the antenna is flattened out laterally as is the case with a transmitting antenna. Thus the response to interference sources on the ground near the antenna is reduced substantially. The net result is that the signal to-noise ratio is improved both by an increase in signal strength and a drop in the noise level.

This antenna is supplied in two parts. The single turnstile is finished with a 5-ft. aluminum mast, lead-in, mounting hardware, and lightning arrestors. The second turnstile and 5 ft. mast extension are furnished separately, as an addition to the first.

FM and Television/ September 1948 William C. Black/Information Service Cooperative G.L.F. Exchange Inc., Ithica, NY