WMIT'

Equipment Used for the 116 Mile Link
and Review of 4 Year's Experience in its Operation

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IN 1941 when WSJS planned the erection of an FM station atop Clingman’s Peak, a major problem was that of supplying program service to the transmitter. The terrain over a goodly portion of the distance between the studios in Winston-Salem and the transmitting site is extremely rugged. For example, the steep drop to the valley below the transmitter on Clingman’s Peak. Therefore, it was considered impracticable to install a telephone line to the transmitter, even disregarding the fidelity required for FM transmission. These considerations, together with the difficulty of maintaining a wire line under weather conditions which are sometimes extremely adverse, made it advisable to use an FM radio link for program relaying.

Our Winston-Salem studios are 116 miles distant from the WMIT transmitter on Clingman’s Peak at 6,511 ft. elevation. There was much speculation as to the possible success of S-T link operation over this distance. However, after daily use for three and one-half years, the relay link has provided satisfactory service well above 90% of the time, even considering equipment shortages and delays resulting from war conditions.

Of course, to provide optimum program transmission at all times, it is necessary to have two complete transmitting and receiving systems, as well as emergency antennas at both ends of the circuit. Future plans for WMIT include such a scheme as well as an increase in power for the relay transmitter. By increasing the power of the W4XGG ST transmitter, it is felt that the received signal will be more consistent during periods of slight troposheric fading, with which we are troubled occasionally, as well as during those times when a temporarily poor signal results from mistuning of the transmitter or receiver, or at times of partial antenna failure.

RELAY TRANSMITTER: The S-T transmitter is located on the top floor of the Reynolds Building in Winston-Salem, the tallest structure in North Carolina and is remotely controlled from the WMIT studios. The FM studios are in the same building with those of WSJS, the affiliated AM station.

The relay transmitter has been entirely unattended since the installation was made, except for nominal maintenance and inspection. Our transmitting unit was designed by General Electric for broadcast service. It meets all FCC specifications as to distortion, frequency response, and noise level. A modulation and frequency monitor is mounted at the side of he transmitter. This provides a visual means for checking output characteristics, and furnishes an audio signal for program monitoring.

The S-T transmitter employs a type 6J5 frequency-controlling tube, reactance-modulated by an 1858. The 4640 kc. output of this oscillator is multiplied and amplified by means of triplers and a converter stage. The frequency modulated output from one of the triplers is added in the plate circuit of the converter tube to the amplified harmonic output of a crystal oscillator which also controls the center-frequency stabilization circuit. The resultant signal at the output of the converter is again amplified and finally tripled to furnish excitation for the final power amplifier.

Both the final tripler and power amplifier stages of the transmitter utilize G.E. type 8010-AR tubes. These are forced-air cooled triodes, designed for high frequency use. The tubes are constructed with heat-radiating fins attached to the plate connection, and disc type grid and cathode leads which decrease inductance in those circuits. The nominal power output of the push-pull final stage is 25 watts.

The transmitting antenna which has been in use for the greater portion of the time W4XGG has been on the air is a 2-wire, horizontally polarized rhombic with legs equivalent to 4.2 wavelengths. The theoretical signal gain of this antenna is 11.0 db relative to a half-wave dipole. The transmitter output power is fed through a length of 7/8 inch coaxial line to a hall-wave matching section and then to the terminals of the rhombic.

The rhombic is adjusted by means of sliders for the best operating characteristics. As instruments for measuring radio frequency impedances at this frequency have been unavailable, all adjustments have been made by using the S-T receiver on Clingman’s Peak as a field-strength meter, together with indications of proper loading in the power amplifier plate circuit. In the final analysis, a maximum signal input to the receiver is the result to be desired. Thus it seems doubtful if comprehensive measurements would do more, except to determine the power losses in the coupling circuits themselves.

Relay Receiver * The 16-tube double super-heterodyne used at Clingman’s Peak has a number of interesting features. By a careful selection of frequencies; a single crystal-controlled channel furnishes the oscillator signal for both the first and second converter tubes. As the crystal is kept at a constant temperature, the oscillator harmonics are sufficiently stable that such a scheme operates very satisfactorily.

In the HF input stage of the receiver, as well as fof the frequency multiplying stages following the crystal oscillator, acorn type tubes are used. These tubes perform very satisfactorily. However, because of failing cathode emission, their normal life does not compmare favorably with standard types of receiving tubes. The intermediate frequency channels, operating at 41.275 mc. and at 4.3 mc, are of conventional design, feeding a dual-stage limiter using type 6SH7 metal tubes. A double limiter in this receiver provides much more effective action than a single stage, and permits full limiting action with 20 microvolts at the antenna terminals. As a result of the careful design of the time constants in the limiter grid circuits, impulse interference has been kept to a low value under normal conditions. A discriminator of the familiar Seeley type is followed by a 2-channel audio amplifier. This makes available a separate output for monitoring purposes and has, in practice. heen used as an emergency program source. A noise-suppressor or squelch circuit silences the audio output in the event of failure of the transmitter, or if the signal becomes abnormaliy low for any other reason during operating hours.

A voltage-regulated power supply has proved particularly advantageous. When adjusting critically tuned circuits, a marked increase in stability is noted when the regulator is in use.

Front panel controls on the S-T receiver provide for adjusting the discriminator transformer secondary trimmer, and for regulating the audio volume to the WMIT speech input equipment.

Receiving Antenna * The antenna originally used with the S-T receiver was a half-wave dipole and square-corner reflector. Using this method of pickup, normal signal input to the receiver was of the order of 100 microvolts. This square-corner, or “Kraus” antenna, worked surprisingly well considering its small physical size, and is still used in emergencies,

About a year after the initial installation, a rhombic receiving antenna was tried. The rhombic proved to be consistently more efficient than the smaller dipole and reflector, and has been in use with minor modifications ever since that time. The rhombic we are now using, is six wavelengths on a side (17.5 feet). It has an angle of tilt of 68 degrees and is 33 1/2 wavelengths above the roof of the WMIT transmitter building.

In order that the entire antenna be kept at direct current ground potential, and to simplify insulation problems, a quarter-wave transformer section,. enclosed in a copper pipe, is used at each end of the antenna instead of conventional insulators. At the feed end of the rhombic, the inner conductor of a 7/8 inch concentric transmission line is connected to the junction of the antenna and the quarter-wave section, while the outer conductor is grounded to the sheath surrounding the quarter-wave transformer. In this manner we are able to obtain an approximate impedance match without any physical difficulty, and in such a manner that the coupling arrangement is not seriously affected by snow or ice during the winter months.

We estimated that the impedances of the rhombic antenna and of the transmisson line are 300 and 75 ohms respectiveley. By connecting the open end of the transformer across the antenna and the transmission line from one side of the antenna to ground, the effect is the same as connecting to one half of a center-tapped transformer winding, or a 4 to 1 impedance ratio. At the receiver end of the concentric line, the inner conductor is connected to a slider on the RF stage grid inductance. This inductance at 337 mc is a silvered 1/4 inch square rod about 5 inches in length.

It has not been possible to conduct as much experimental work with receiving antennas as we have desired, but some work has been done along these lines. At one time, two rhombics were in use, one a full wavelength above the other. This system gave a slightly increased signal RF pick-up over the present antenna, but during a spell of extremely bad weather both rhombics were blown down and have not been rebuilt. It has occured to us that some sort of dual-diversity scheme, possibly using both horizontal and vertical polarization, might be of value in limiting to some extent, the seriousness of tropospheric fading. Then, too, we may find that equivalent or better performance than that we are getting now might be obtained by using another type of antenna such as a long wire V, a colinear, or a stacked array of some sort. When time permits, we plan to test several types of antennas to determine which will deliver the best overall results.

WMIT Power Supply * In connection with the ST receiver it might be mentioned that all electric power required by the WMIT main transmitter, as well as auxiliary receivers and other apparatus. is generated at the mountain-top location. Three 75- kva diesel-electric Caterpillar sets have been in use from the beginning. It is as impractical to run a line to Clingman’s Peak for the transmission of electric power as it is to use a line for programming. In the former case, the problem resolves itself into simply hauling the necessary diesel fuel oil. The WMJT power plant is complete with a specially designed switchboard which includes provision for starting and controlling the machines from the transmitter room. A synchroscope is provided for adjusting the speed of generators so that they can he connected in parallel across the main power bus.

Mountain-top Studio * We have an emergency studio at Clinginan’s Peak which can be put into use at a moment's notice. Of course, live talent in time Mount Mitchell area is seldom available, so the program transmissions originating at the mountain are usually limited to transcriptions.

Because of the remoteness of the transmitting plant, the Federal Communications Commission has authorized limited use of the WMIT and W4XGG facilities for studio-transmitter communications during non-broadcasting hours. These communication periods are permitted only with reference to station operation or other matters of an emergency nature.

Signal Variations * A continuous half-hourly record of the signal strength received from W4XGG has been kept over a period of three years. The signal has proven to be surprisingly consistent. However the records show a number of interesting facts concerning propagation at these frequencies.

Of particular interest are the effects of tropospheric fading, and the variations which appear to occur in more or less daily cycles. The usual trend of the signal over a daylight cycle is that of being above normal in the early morning, gradually dropping to what might be called a median value shortly before noon. The field strength usually remains at approximately this level until several hours after sunset when it rises to a value comparable with that recorded in the early morning. This does not mean that every day follows the same pattern, but over a long period of time, time average signal strength curve does have that general characteristic. Tropospheric disturbances are noticed frequently but occur with no particular regularity. It has been almost invariably true, however, that when the received signals reach the highest values, they are more subject to severe fades than at normal intensity.

From our experience it would seem that when the signal is unusually strong, possibly as a result of reflection from high, billowy clouds, the chances of fading, brought about by air-masses or cloud formations blocking the signal path or causing other reflections which cancel out the original signal, are much more probable than under ordinary circumstances. That is, the strength is highest in the early morning and late evening, and varies about an average value during the middle of the day. This same change in refraction capability would bring about stronger reflections than normal and would consequently result in signal strengths either higher or lower than might be expected had the same reflections taken place at some other time of the day.

Conclusions * From the experience gained in nearly four years of S-T program relaying, it is our opinion that, for any similar relay system, the required amount of maintenance will increase greatly as the, distance between relay points is increased. That is, even though the normal signal is perfectly usable, it would appear to be good engineering practice to keep the relay path as short as possible. Even the reduction of the transmission distance by a mile or two would be of value. For example, when a circuit such as this is operated over its maximum range, all RF and IF stages in the receiver must be kept in perfect alignment so as to pass the full 100-kc. deviation. If this is not done, the noise level may be objectionably high. Over short distances, on the other hand, it would be entirely permissible to decrease the audio input to the transmitter. during emergencies, to eliminate distortion resulting front receiver mistuning. In the way, the noise level could be kept low over the entire system. Sub-standard results over long distances might result from weak tubes at either the transmitter or the receiver, or from incorrect adjustment of either of the antennas. In our particular installation, for example, certain tubes in both the transmitter and receiver must be replaced at intervals of approximately three months. If the relay distance were shortened, it is entirely possible that these tubes could be kept in operation for a much longer period.

One comment should be added concerning interference from natural or man-made static with program relaying. It can be stated as a definite fact that no trouble from static pick-up of any sort has been experienced with this system. It is quite common in summer months for the thunder and lightning of severe electrical storms to become most annoying, yet cause no disturbances whatsoever in the relay receiver. It is a strange but familiar experience to see lightning flashes without hearing an accompaniment of static on the program.