(source: Electronics World, Aug. 1964)
By LEO G. SANDS
There have been many changes in microwave communication systems the past few years. Besides moving to all solid-state devices, heterodyne techniques are coming to the fore.
MICROWAVE communications equipment is taking on a new look. Conventional vacuum tubes are now being replaced by transistors, klystrons are on the verge of being superseded by solid-state microwave power sources, storage batteries are replacing engine-driven standby power plants, channel capacity has risen to 600 (up from 24 a decade ago), and the trend is away from passive reflectors to directly fed antennas, and to higher operating frequencies.
In the past, microwave systems were purchased principally for oral communications purposes. Today, the need for data, TV, and high-speed facsimile transmission is the prime justification for investing in a microwave system.
Fig. 1. TV microwave link has audio signal on a subcarrier located above
video signal.
The cost of installing a microwave system has not gone down, but has in fact gone up, because users are demanding more sophisticated equipment. However, some manufacturers are considering production of lower-cost equipment for short-haul, lower-density systems.
While there is vigorous competition among microwave equipment manufacturers, they are now jointly competing even more vigorously with the telephone and telegraph companies. The telephone companies now offer Telpak service which, in some cases, costs less to use than to own and operate a private microwave system.
Under Telpak rates, a 12-voice channel facility can be leased for $15 per mile per month, 24 channels for $20, 60 channels for $25, and 240 channels for $45 per month. But, the customer must also pay $5 per month for terminating each end of a channel, whether used for voice, Teletype or slow-speed data, except for the first channel of a group for which the charge is $15 per end. In all cases, the taxes involved and the installation costs are additional.
It is rates like these that have made some prospective microwave users hesitate, and cause equipment manufacturers to offer systems of higher capacity and greater reliability which also cost less to operate than former systems.
Solid-State Equipment
Transistorized microwave equipment, now on the market, uses no tubes. In some so-called hybrid equipment, operable in the 6000-mc. band, a klystron is required and, in most cases, a klystron is still used as the receiver local oscillator. However, the local oscillator klystron is being replaced by a solid-state signal source consisting of a crystal-controlled transistor oscillator and varactor diodes for multiplying the frequency to the 6-gc. region.
Fully solid-state microwave equipment is available at the present time.
Claimed power output of these systems is 100 milliwatts in the 6-gc. band.
Transmitters for the same band, using klystrons, have been available for several years in 100-milliwatt and 1-watt output types.
Solid-state klystrons are coming out of the laboratory and recently Fairchild introduced its version. But they are not yet used in any commercial microwave communications equipment.
Power Sources
Until recently, nearly all microwave equipment was designed for operation from a.c. power lines. Today, most manufacturers offer equipment which is operable from storage batteries floated across a battery charger. When batteries are used, there is less need for permanently installed, engine-driven, standby power generators. Since the batteries are usually capable of several hours' operation after power line failure, there is usually sufficient time to bring in a portable generator to take over the battery replenishing job.
Fig. 2. The old look (A) used back-to-back repeaters where the signal
was demodulated and re-modulated. (B) The new look uses a heterodyne
technique where the FM signal is passed through a system similar to a
double-conversion superhet.
Increased Capacity
When the microwave era began a dozen years ago, 24 voice channels was deemed adequate for private systems.
Channel capacity was later upped to 60, then 120, doubled to 240, and now stands at 600. While most users do not envision ever requiring 600 voice channels, they do expect to need reserve band-space both for high-speed facsimile which requires 250 kc. of the available 3-mc. baseband of a 600-channel system, or 1-mc. baseband of a 240 channel system, or for high-speed data service.
The latest multiplex equipment is capable of expansion to 600 voice channels in increments of 3, 12, and 60 channels. Several manufacturers offer this type of equipment using solid-state, plug-in circuitry resulting in less heat generation and smaller space requirements.
Telegraphy, slow-speed data, telemetering and control signals are transmitted as pulsed tones and as many as 18 tone channels, each operating at an audio frequency below 3400 cps, can be transmitted over a single voice channel.
Each tone channel is usually rated as being capable of handling 75 bits per second. A tone channel consists of an FSK (frequency-shift keyed) or AM ("on-off" keyed) audio oscillator, and a companion receiver at the far end of the circuit.
Microwave equipment now on the market can be used for transmitting TV or up to 600 voice channels, or a combination of audio, video, and data. One of the newer types can accommodate modulating signals from d.c. to 10 mc.
When TV is transmitted, audio program channels or voice carrier channels are operated at frequencies above 6 mc. so that the video signal occupies the lower portion of the baseband. Such a system is illustrated in Fig. 1.
Other Improvements
Some repeater stations, such as shown in Fig. 2A, consist of two transmitter-receiver terminals connected back-to-back. In this system, the FM signal is demodulated upon reception, and used to modulate the companion FM transmitter.
Lately, there has been a growing demand for the heterodyne-type of repeater, where the FM signal is not demodulated at each repeater. As shown in Fig. 2B, the incoming frequency f1 is passed to a mixer where it is combined with a crystal-controlled local oscillator to form a 70 mc. i.f. This signal is passed to another mixer where it is combined with another crystal-controlled local oscillator. In this case, the higher beat frequency is chosen (f3), amplified and transmitted as the relayed signal. The other channel works the same way.
Diplexers in each antenna system keep each of the different r.f. frequencies from entering the wrong channel.
The heterodyne principle is also being used in microwave terminals as shown in Fig. 3. Here, the input signal is amplified and used to modulate a 70-mc. FM transmitter. The output from the transmitter is then mixed with a crystal-controlled local oscillator and the 6-gc. beat is picked out (upconversion), amplified, and used as the output signal (6825 mc.) . Incoming signals pass through a conventional high-quality superheterodyne receiver having a broad bandwidth.
Another improvement in microwave system receivers is the use of phase-lock detectors, parametric amplifiers and tunnel diodes to improve effective signal-to-noise ratio.
Most prospective users of low-cost microwave systems, those who are eligible for licenses only in the Business Radio Service, must operate at frequencies above 10,000 mc. Equipment is now available for the 12,000-mc. Business Radio band that costs about the same as 6000-mc. band equipment.
It is expected that low-cost 12,000-mc. band equipment will be available that will make short-haul private microwave systems more competitive with leased-circuit facilities. This new equipment will probably use vacuum tubes to keep cost within reason.
Antennas and Radiation
Most 6000-mc.-band microwave stations use passive reflectors at the top of a tower to reflect the signal from vertically oriented antennas at the base of the tower. This use of reflectors eliminates the need for long waveguide runs.
However, when antennas are aimed skyward, it is possible that in the future, interference to, or from, space communications systems might result. Antenna systems for the 6000-mc. band, using special coaxial cable and permitting top-of-tower mounting of antennas, have been introduced. Microwave systems operating in the 960and 2000-mc. bands ordinarily use top-of-tower mounted antennas fed through conventional coaxial cable.
Longer distances can be spanned by scatter systems (see Fig. 4) but their use within the United States is not permitted because of their capability of interfering with other stations over a wide area. However, there are no restrictions against the use of trans-horizon transmission as long as all pertinent FCC requirements are otherwise met.
To get microwave signals over mountain tops without installation of hard-to-get-at repeater stations, large passive billboard reflectors are sometimes used.
A pair of parabolic antennas, connected back-to-back, can also be used as a passive repeater.
Fig. 3. In a microwave terminal setup, the input signal modulates an
FM transmitter whose signal beats with a local oscillator in the up converter.
The 6-gc. beat is amplified as the output. The receiver is a high-quality,
broadband type.
Fig. 4. Three techniques for transmitting over mountains. (A) Shows
the scatter method, (B) the trans-horizon (knife-edge) system, and (C)
a line-of-sight system that uses a pair of parabolic reflectors.
Propagation tests conducted in California revealed that trans-horizon transmission is superior to the use of a back-to-back parabolic repeater in an over-mountain test link about 50 miles long. As a result of these tests, more trans-horizon links are expected to be installed for the longer hops.
While telephone and telegraph companies can employ frequency diversity to ensure more reliable communications, other private microwave system users cannot. They are permitted to use space diversity which requires installation of a second antenna system above the primary antenna. To add 50 feet of height to a tower, it must be done at the bottom end to obtain the required strength--and this costs money.
When a hop is relatively long or crosses water, and the use of diversity would ensure greater reliability, there is a new wrinkle. Frequency-diversity operation, achieved by operating a 6000-mc.-band and a 12,000-mc.-band system in parallel, is permitted, according to an FCC official. New antennas have been developed which can be utilized on both the bands simultaneously.
In a frequency-diversity system, the transmitters and receivers operate simultaneously. The receivers are linked to a diversity combiner that selects the output of the receiver producing the highest usable signal level.
ETV and Other Markets
One of the big new markets for microwave is in ETV (educational television). A recent survey indicates that about 82% of colleges responding to queries expect to have a requirement for transmission of closed-circuit TV between on-campus buildings and off-campus points, some at considerable distances.
Under recently proposed FCC rule changes, educational television (ETV) microwave systems not only can be operated at frequencies above 10,000 mc., but also in portions of the 2000-mc. microwave band.
Another new market for microwave equipment is in the remote control and data gathering field. The FCC recently permitted use of omnidirectional transmission on specified frequencies in the 952-960 mc. band. It is now possible to use an omnidirectional base station for one-way or two-way transmission of coded signals to street corner traffic signal controllers. Another application for omnidirectional microwave is transmission of burglar alarm signals to a central point.
Citizens Band
The FCC is considering opening up new microwave bands for Citizens Radio type operation at frequencies above 16,000 mc. While most citizens will not be able to afford a microwave system, there will be a growing need for frequency space for short-haul microwave systems. Since present microwave bands are already congested, the only direction in which expansion is possible is higher in frequency.