The following is a summary of conclusions derived from an investigation into high frequency pack communication units wherein more than one frequency for transmission is considered essential. It was first necessary to consider what standard of frequency tolerance and quality of modulation is desired so as to classify the transmitters into three types, namely:
1. Modulated oscillator transceiver type,
2. Self-excited M.O.P.A. type,
3. Crystal-controlled M.O.P.A. type.
The transceiver type has the disadvantage of transmitting only on the frequency to which the receiver dial is tuned. If communication was to be established on a separate frequency with another set to avoid interference it would then be necessary to reset the dial for each send-receive change-over which is impractical because of the error in resetting the dial accurately. Aside from this, the modulation characteristics are poor resulting in increased audio distortion, and occupies a considerable region of the spectrum causing more interference and allowing easier undesired interception of the signal. All this, plus the inability to scan the dial while transmitting on a fixed point appears to render this type of circuit unfavorable for pack use except for special uses, such as for intentional interference purposes.
SELF-EXCITED M.O.P.A. TRANSMITTER
The flexibility of the self-excited M.O.P.A. transmitter would appear to be the best for dodging interference while still obtaining good engineering practice. However, there are several practical disadvantages to be considered. Assuming special ceramic insulation is used with good circuit design and a reasonably satisfactory temperature coefficient is achieved, there is still the problem of being able to set the dial accurately. If a small tuning range is used so that the dial settings will be broadened then the interference dodging is limited and interception is easier. If a large tuning range is possible then the accuracy is poor enough that the receiving point will experience trouble in locating the signal without scanning the dial. The human error has been doubled in as much as two dials are required to be set and both transmitting and receiving errors occur. These dials must move smoothly and unless look devices are used errors will occur in the transmitter frequency from mechanical jars and knocks during packing operation.
If it is desired to set the transmitter on a given frequency so as to identify that transmitter by its frequency some means of calibration is necessary. This is usually done in field sets b y having a low frequency crystal oscillator built into the transmitter and by operating the receiver the frequency is adjusted according to the received crystal harmonies.
If the transmitter was operated on a selective switch rather than a dial so as to have various calibrated frequencies, the result would be more fool-proof and practical and permits the pack unit to call a stand-by receiver in an emergency. This form of operation is particularly desirable for short periods of operation because of battery economy and renders service similar to a point-to-point break-in system.
The above type of transmission could be duplicated by selective multi-frequency crystal control and to better practical advantage. The necessity of periodical calibration is eliminated and frequency errors from rough handling would not occur. However, the desirability of using crystals from an economic viewpoint must be justified, also the limitations in the number of
frequencies available to dodge interference or undesirable monitoring aunt be considered. Probably the most important practical consideration in the field operation of pack units is the battery life or the weight relation to the intermittent battery life. If a large battery source is needed to maintain communication for a reasonable time-period, the pack units become heavy enough to require one man to pack and operate the radio equipment only. This will reduce the practical use of this equipment as compared to a man being able to carry and operate the peak set on the march along with most of his normal pack complement.
To achieve such small design and low battery drain for a given power output or range, it is necessary to eliminate any battery drain when not actually being used. The best possible battery economy is obtained when the entire transmitter drain including the heaters is off when receiving. This is practical with quick heating filament tubes operated on a push-to-talk, release-to-listen switch providing the transmitter is crystal controlled. With this system applied to self-excited oscillator where the filament emission is a controlling factor of the frequency, the oscillator will swing across a portion of the spectrum before settling down to its calibrated frequency. This same warming-up period merely changes the amplitude of the crystal controlled oscillator output. This advantage definitely favors crystal operation for very small and efficient units.
Because the frequency of the self-excited oscillator depends can battery potential, it will undoubtedly wander or drift when the batteries become low. When the oscillator tube is replaced it is necessary to recalibrate the oscillator when self-excited.
It has become the practice with mobile equipment in some countries to have numerous selectable crystal-controlled transmission channels where it is customary to choose the receiving point by selecting its receiving frequency to establish communication. This system of calling has many advantages; it permits establishing instant communication with the required unit, it reduces the transmitter operating periods and increases the receiver operating periods which reduces interference, and it is extremely economical on the unit batteries.
The spectrum between 28 and 40 megacycles appears to be the most reliable, efficient and practical range to operate a small pack unit with a fixed antenna.
If a lower frequency is used the radiated power is reduced from the output power by the loss in the antenna radiation efficiency. The shorter the antenna (below six feet) the greater the proportional loss and the shorter the operating range. The low frequencies are also undesirable owing to the secondary waves which return from the ionosphere. These may be received over a considerable distance under favorable conditions causing interference to regular communication channels and permitting undesirable monitoring. The reflected wave will likewise return to the receiving point and cause phase fading when operating near the limits of the direct waves.
If higher frequencies than 40 megacycles are used, the increased radiation efficiency is offset by peculiarities inherent in ultra-high frequency operation. These peculiarities are dead spots caused by phase fading in this neighborhood of objects that will reflect and absorb the signal. The terrain effect on the direct wave renders the reception of the signal impractical over a mile or so.
July 10, 1940 D.L. Hings