On Distributed Communications Series

VI. Mini-Cost Microwave

VI. Implementation of a Mini-Cost Station

In this section, two tentative designs for building the electronics portion of the mini-cost microwave relay station are examined. First to be considered is the version that uses the more complicated, better known klystrons; secondly, the use of all-solid-state varactor multipliers is considered. The latter discussion will examine only the differences inherent in the varactor approach as opposed to the klystrons.

The varactor version should prove to be less expensive (on a long-term basis), simpler, and more reliable than the klystron. The klystron version is superior only in the following respects: the design can be based around a component which has been manufactured for almost 20 years; higher output powers are possible than with varactors; and the initial and maintenance cost estimates are less uncertain.

Figure 15 is a simplified block diagram of the relay or repeater station which will be examined in detail. Separate antennas are required for transmission and for reception, so that two antennas are needed for each two-way relay station. The two antennas are used to receive and transmit simultaneously. Electrical isolation is provided by frequency separation of the transmitter and the receiver and by cross-polarization of their antenna feeds. Band-pass and elimination filters on both receivers and transmitters reject unwanted frequencies. Superhetrodyne receivers which have an intermediate frequency chosen so that both receivers at each site operate with a common. local oscillator supply are used. The entire station is designed for a minimum primary power drain.

Primary power is obtained from a local power line wherever possible, falling back to an L-P gas local thermoelectric generator in emergencies or at remote sites.

Description of the Klystron-Based System

Figure 16 is a detailed block diagram of the entire relay station. Each block is numbered corresponding to the order in which it will be discussed.

1. Primary 117-Volt AC Power Source

In rural areas most primary power is delivered by overhead or "aerial" wires. If such power is available, the aerial line can be tapped and connected directly to a tie-point located on the antenna tower. The vertical tubular members of the antenna tower form protected conduits to deliver the 117-voltage (or higher) current down to the antenna tower base.

2. 117/24-Volt AC Transformer Power Source

If there are no nearby power poles and a long run of cable is required, it is cheaper to transform the 117-volt current to 24 volts AC at the power pole. The low-voltage circuit can be run over inexpensive #10 plastic-sheath buried cable. This cable can be plowed underground for at least several hundred feet and still deliver sufficient power with poor, but adequate, regulation to operate the repeater station.[1] The saving is in the lower cost and lack of a protection requirement for a 24-volt cable. The 24-volt line need not be conveyed within an expensive conduit; direct plowing into the ground is sufficient, avoiding the necessity of meeting the rigid safety regulations of higher-voltage-cable runs.

3. Transformer and Switch

Either of the two described forms of AC power are applied to a multi-tap switch feeding a small, regulated, 60-watt power transformer. A tap on this transformer provides the AC power needed for all klystron heaters and a common AC power supply for all the electronic equipment.

4. Thermoelectric Converter

An intriguing alternative source of primary power is that of thermoelectric conversion. For example, Minnesota Mining and Manufacturing Company will soon formally announce a new 50-watt thermoelectric converter. This device, using butane, propane, natural gas, or similar fossile fuel, produces 50 watts of electricity. The tentative retail unit price, including a transistorized voltage converter regulator is $775. It is anticipated that the cost will drop by some fifty per cent within a year or two with the advent of large-scale production. According to a discussion with a representative of the company who is working on the project, tentative estimates of usable life are on the order of 5-10 years (based on a two-year test); a one-year warranty on their present 15-watt unit in production is being offered.

The overall fuel-to-thermal-to-electric efficiency of the unit is said to be quite low (about two per cent). But the cost of petroleum fuels is sufficiently low as to permit economical operation of relay sites completely independent of a primary power source. It will be shown below that this cost will be less than $0.02 per hour of emergency operation. This figure is so low that a pair of thermoelectric generators may be preferable over electric power lines in remote areas.

5. Transistor Oscillator

Inasmuch as the thermoelectric generator is basically a low-voltage, DC source, it will be necessary to convert current to AC to obtain the high voltages required by the klystrons. Many transistor inverter circuits are mentioned in the literature. Typical values include powers of the range of 60-120 watts with an overall efficiency of 80 per cent using low-cost transistors.

6. Burner Fuel Supply

To provide a basis for estimating the fuel supply required by the thermoelectric converter, the Minnesota Mining and Manufacturing Company's Model 3-M-19 unit will be presupposed:

  1. Heat conversion to electricity - 4 per cent.
  2. Available heat from burners used - 50 per cent.
  3. Commercial butane heat value is 21,180 BTU/lb after vaporization; heat of vaporization = 167 BTU/lb.[2]
  4. Overall thermal-to-electric conversion efficiency is calculated as follows: 4 per cent heat-to-electricity times 50 per cent heat-to-usable-heat equals 2 per cent overall efficiency.
  5. Fifty watts of electricity is equivalent to 50 watt-hrs, times 3.413 BTU/watt-hr equals 170.65 BTU/hr.
  6. Thus, the fuel consumption is equal to 0.4 lb/hr.[3]
  7. If a fuel cost of about $0.04 per pound is assumed, the fuel cost will be about $0.016 per relay station per hour.
  8. Fuel consumption per day would be approximately 10 lb (9.6 lb).
  9. 200 gallons is roughly equivalent to 26.6 cu ft.
  10. If the specific gravity of L-P is about 0.8, then a 133-day fuel supply can be anticipated.[4]

As L-P fuel is in common use for cooking and heating in the rural parts of the country, an extensive tank-truck distribution system already exists. Thus, deliveries of fuel can be made by an existing local supplier.

7. DC Power Supply

The DC power supply converts the primary power into regulated +300 volts for the klystron oscillator cavities, -150 volts for the klystron repellers, and a low voltage for the transistorized equipment.

The power supply is of conventional design.

8. Trouble Detector Transmitter

The Trouble Detector Transmitter unit is a simple monostable multivibrator circuit energized by the failure of a received signal from the adjacent relay station which informs the system of the particular link that is defective. This unit creates a filler data stream coded with tower location information.

The circuit is quite elementary: an integrator with a one-second time constant is connected to the monostable multivibrator. Failure to receive any signals (or the reception of a signal of all ones or zeros) in excess of about one second in duration causes the removal of a clamp voltage and allows the multivibrator circuit to run free. The time constants of the multivibrator on-off periods are selected such that a digital pattern is created that will pass through the chain of tandemly connected repeaters, and at the same time be uniquely recognized at the end of the chain. In order to create up to, say, 15 separable signals, one per tandem relay station, the "on" time is chosen to match the width of a single conventional pulse (nominally corresponding to the time of a 4.5-megabit/sec data rate). The "off" time is chosen to be one of 15 integer units of the "on-pulse" value. The length of this adjustable off-time can be selected by the choice of a single resistance. One-per-cent resistors are inexpensive, and a group of four of them strapped together will allow the selection of any desired off time from 1 through 16 pulse intervals, as seen in Fig. 17.

9. Cassegrainian Reflector

The Cassegrainian Reflector, commonly used in optical telescopes, makes a very efficient antenna structure. Efficiencies on the order of 65 per cent with low side-lobe levels have been obtained in large structures (85 ft) as compared to values 2 db lower for conventional parabolic antennas. For solid surfaces with less emphasis on side-lobe levels, 75 per cent should be achievable.[5]

In order to minimize the cost and weight of the antenna and to maintain a precision reflection surface, a rigid plastic foam, such as styrofoam, can be molded around metal stiffening members. The low weight (a few pounds per cubic foot) and low cost permit rigid heavy-wall construction. The plastic can be molded in inexpensive multi-part plaster or wooden molds. For extreme precision, the foam plastic can be cut with a parabolic-shaped aluminum or steel blade, as shown in Fig. 18. (This finish cut may not be required as ~Il6 cm corresponds to an allowed surface tolerance value of 1/16 wavelength at 10 Gcs.) After finishing, the entire antenna surface is coated with adhesive and covered with a reflecting layer of strips of thin ductile aluminum foil. At X- and Ku-band frequencies, the skin depth of electrical penetration is much less than the thickness of thin aluminum foil.

This method of building antennas is not really new, and has been described in the literature.[6]

The foam antenna serves more purposes than solely as an antenna reflector; it forms the housing for the transmitter/receiver unit and feed horn. Figure 19 shows a cross-section of the antenna assembly. A horizontal and a vertical steel shaft intersect at the point at which the antenna is clamped to the tower and the sway and torsion guys are attached. The transmitter/receiver electronics fit into a small removable package that slides into the antenna from the back of the reflector. Three or four steel straps center the secondary reflector and accurately reference it to the antenna feed horn and main reflector.

Figure 20 shows the combination antenna horn feed and receiver/transmitter chassis. The antenna horn is precision-molded aluminum. Stiffening ribs provide electrical shielding, a heat sink, and chassis rigidity. Aluminum sheet side cover plates and a cast back complete the chassis housing. Normal manufacturing tolerances appear adequate, with the possible exception of close control of the surface finish. The center hole should be broached and a nickle flash followed by silver plating of the horn area is desirable, but not mandatory.

A clip-in dust cover composed of a thin sheet of teflon and a retaining ring will keep rain, snow, and dust off the feed elements. A few inches of snow accumulation across the feed is not expected to cause difficulties at these frequencies. The combination of the high thermal resistance of the antenna housing and the low thermal resistance of the horn would permit about half the heat generated to dissipate around the feed horn, providing some ice-melting capability--if ever needed.

Figure 21 is an end-on cross-section of the transmitter/receiver unit. Two small antenna feed probes are shown: one from the transmitter, the other from the receiver. The antenna feed horn is, in effect, a shortened waveguide section terminating into a flared horn. By careful location of the feed points, the transmitter and receiving feeds can be matched with a low standing wave ratio to the antenna without a significant diplexing loss. An identical antenna feed is described by Prechtel in the radio amateur's magazine QST, in connection with an inexpensive microwave transceiver.[7] This article provides a splendid example of the notion that microwave plumbing need not be expensive. Prechtel uses a piece of copper water pipe as a waveguide and a tin horn (literally) as his antenna.

10. Klystron Transmitting Oscillators

Figure 22 shows the transmitter feed assembly in more detail. An inexpensive 2K25 klystron is coupled to a band-pass/elimination filter, which in turn is coupled to the waveguide feeding the antenna horn. Although a wide variety of klystrons are available, the 2K25 has been tentatively chosen merely because it has been manufactured in sufficient volume to establish a true production price. (Microwave tubes are invariably made in very small quantities, and most of their price reflects set-up charges.) The 2K25 is neither an efficient nor a particularly reliable tube, but it is cheap--about $22 in single-unit quantities; this is about one-eighth the price of the small-production klystrons that better fit this application. Of course, it is anticipated that the price of the preferred tubes will be drastically reduced as orders on the envisioned scale mount. However, the design consideration shall continue to be based on the 2K25; expected future improvements will be "gravy."

11. Pulse Shaper

The pulse shaper unit is a simple clipping and gating clamp that regenerates the received pulse for retransmission. Unlike the regenerative repeaters described for use in cables, there is no difficult retiming problem envisioned, because of the low distortion of the channel. (This is not the same problem found in cable regenerative repeaters where the arriving signal is highly distorted so that a precise retiming circuit is necessary.) A single clipper or flipflop will suffice here.

12. Band-Pass and Elimination Tri-Plate Filters

A TEM-mode tri-plate filter removes undesired wideband modulation components that fall outside the assigned frequency space of the transmitter unit. Precision photographic-etching processes appear to permit the reproducible accuracies necessary to mass produce these filters using a form of the stripline technique. (The stripline technique is to microwaves as the printed circuit is to electronics: an inexpensive way of mass producing circuits with tightly controlled dimensions and stray coupling.) Figure 23 shows a sketch of the construction of a filter of the type described by Matthaei.[8] The tri-plate technique is ideal for our application, since the equipment is so compact that losses encountered using a tri-plate, as compared to conventional machined waveguide assemblies, will be unimportant. After much adjustment on a prototype unit, subsequent entire microwave assemblies comprising an entire receiver may be etched on a single small board. The microwave filter described by Matthaei [9] shows less than 0.2-db loss within its pass-band, yet exhibits square-sided rejection curves of at least 50 db. Although Matthaei performed his experimental work at L-band, the results appear valid on X-band.

13. Band-Pass and Elimination Tri-Plate Filters

A second filter, similar in construction to the first, keeps unwanted signals out of the front-door of the receiver. Figure 24 shows the method of mounting the transmit and receive filters, each with their feeds at right angles to one another.

As there may be physical differences between different antenna waveguide feed assemblies, the coupling can be permanently adjusted after assembly by using a variable thickness shim. The shim thickness is chosen to match the best measured depth of penetration of the feed probe.

It will also be necessary to trim the distance between the feeds and the fixed-position waveguide end-piston or shorted termination. A small slot permits the travel required for this adjustment. After jig adjustment, the assembled parts may be permanently bonded together.

One further detail must be considered: that of temperature variation which causes expansion and change of the critical dimensions. Figure 25 demonstrates that a near-zero temperature coefficient invar metal clip may be used to maintain the critical distance between the antenna probes and the reflecting waveguide termination. The invar mounting clip is clamp-attached opposite the fixed reflecting piston. The filters are clamped to the clip in the same planes as the antenna feeds. The invar clip is free to slide to maintain a fixed spacing between the. antenna probe and piston as the waveguide section elongates with temperature increases.

14. Crystal Mixer

Earlier, the use of a tunnel diode pre-amplifier was considered, but its use was not found necessary to meet the target receiver noise figure (NF) value of 10 dbs.

A balanced pair of Sylvania 1N23EM mixer diodes (costing $11.25) has a maximum NF of 7.5 db (measured using a l.5-db IF-amplifier noise figure). As an IF-amplifier input having an NF of 9.5 db and a l0-db gain shall be used, the expected overall receiver NF should be about 8.5 db, or about 1.5 db better than the target NF of 10 db.

The tunnel diode pre-amplifier can be seen to really be gilding. But, it should be kept in mind that it may be another way of buying margin cheaply at a future date, if needed.

15. Intermediate Frequency Amplifier

From the crystal mixer, we enter a transistorized intermediate amplifier strip. Low-cost, high-frequency transistors are now commercially available, making it practical to build excellent broadband IF-amplifier strips requiring a surprisingly small number of components and adjustments. For example, Rheinfelder of Motorola Semi-conductor Products, Inc.,[10] describes RC-coupled amplifiers having a center frequency and bandwidth closely matching our requirements.

Figure 26, from Rheinfelder, shows three iterative IF-amplifier stages having a loaded gain of 36 db, a bandwidth of 35 Mc, and using 2N700 transistors. The new Post Alloy Diffused Transistors (such as the Amperex 2N2084) now sell for about $1.34 apiece in small quantities and exhibit frequency, gain, and noise characteristics roughly equivalent to the 2N700. The 2N2084 has a published maximum NF of 9.5 db.[11]

As has been shown, the high gain, 10+ db of the IFamplifier input stage relative to the IF-noise figure of 9.5 db, is such that IF-amplifier noise has little effect in determining the overall receiver noise figure. Thus, we have gone from 7.5 to an 8.5 overall noise figure. The demands for automatic gain control are not as severe as in the case of extreme fading margin audio modulation systems. Binary FM modulation is used) and the last few IF-amplifier stages preceeding the discriminator can be designed to operate under a saturating condition.

Thus, it is reasonable to believe that a satisfactory IF-strip can be built for a fraction of the cost of those now on the market- -perhaps even cheaper than the all-vacuum-tube IF-strips.

16. Discriminators

The system uses a frequency-shifted signal, spending equal time (on the average) at two fixed frequencies. Thus, a pair of circuits, each tuned to the center of its expected frequency, is suggested in lieu of a conventional discriminator.

17-20. Local Oscillator

Units 17 through 20 together comprise a klystron local oscillator, with stabilizing cavity and an audio amplifier signal source for thermostatically controlling the frequency of the local oscillator. The stabilization method described is that of the Grant version of the Pound stabilizer.[12]

The output of the local oscillator is divided into two branches with resistive decoupling feeding separate coaxial lines to the receivers mounted at the antenna feeds. Low-SWR ratio coaxial connectors are suggested. Decoupling is used to limit the "pulling" effect of the long lengths of coaxial line required. The attenuation and standing-wave ratio using coaxial line appear to be tolerable for our application. The receiver noise figure used is based upon a crystal mixer local oscillator power assumption of one milliwatt.

A Free Order Wire

A convenience aid to the serviceman working at a remote antenna site is an "order wire," or telephone circuit, between the relay site and adjacent sites to the ends of the span. The proposed system already contains all the essential ingredients to provide the order wire facility, with the addition of a few parts. Each serviceman carries a portable telephone, with a built-in transistor pulse-width modulator/demodulator, terminating into a plug. This plug fits a jack in the trouble detector transmitter circuit and energizes the 4.5-megabit monostable multivibrator with the pulse-width voice modulation waveform. This waveform, consisting of an interrupted stream of 4.5-megabits/sec, contains the voice information and is transmitted to adjacent relay stations in lieu of the normal data signal modulation. This signal is converted to audio at the ends of the span by a simple RC-integrator. This two-way circuit should exhibit a good S/N ratio because of the high bit-rate relative to highest audio modulation frequency present.

Technician Test Point

All the circuits comprising the relay station are essentially free of field adjustments. If any unit becomes defective or shows symptoms of becoming inoperative under marginal testing, it is removed and replaced by a complete alternate unit. Complex servicing is performed only at central depots where full test instruments and the previous history of each unit are available. As each unit weighs but a few pounds, a plastic-foam shipping container allows safe shipment by mail to the central depot. (Tentatively, certain Multiplexing Stations may serve as maintenance depots.)

A few tests are performed during routine maintenance. One is the testing of voltages and waveforms within each of the few major assemblies. Test meters have not been built-in as a part of each circuit. As a lower-cost alternative, key test points can be led from a single connector. The serviceman then carries his own connector-matching test meter and tests are made by taking a few meter readings.

Part of the routine maintenance cycle includes the measurement and adjustment of guy wires. Simple wire tension meters are available that apply a fixed strain to a guy wire and measure the resulting deflection. Loose guy wires affect the accuracy of the antenna pointing. This subject will be discussed in more detail below.

Frequency Stabilization

The frequencies of the voltage-stabilized local oscillator and the transmitters have a tendency to drift, primarily caused by thermal expansion of the resonant circuits. Since the system must operate over a wide range of temperatures (perhaps from -40

The convention will then be employed that each station will modify its own transmitter frequency to compensate for the measured frequency shift (DC level out of the circuit). As both stations comprising the link are both following the same rule, each then is compensating for the drift of the other or both; hence, the name for this method of compensation: "You are fine, how am I?"

Alternatively, the bandwidth could be broadened several megacycles. This would throw out the temperature compensation, and the penalty would only be a 1- or 2-db increase in KTB noise.

Modifications for the All-Solid-State System

To this point, the building of the relay station has been considered using only small klystrons as the RF power source. In this section will be briefly indicated the changes necessary to build an all-solid-state version having performance specifications comparable to the previously described system.

Figure 28 is a block diagram of the all-solid-state version of the repeater. While closely resembling its predecessor, it is much simpler. The crystal-controlled varactor chain does not require the precise frequency control circuits and cavity as in the klystron version. Secondly, there is no longer the requirement for the high vacuum-tube voltages of the klystron. This means that the power supply arrangement may be greatly simplified, eliminating the need for transformer conversion within the power supply. The power supply to the relay station can be made to be the same low DC voltage, whether from thermoelectric converter or from the AC power line. Since only low-output voltages are required, the power supply serves only as a DC filter and voltage regulator.

All the frequency regulating devices needed for the klystrons can be eliminated. The rest of the system remains identical.


[1] Assume the 50-watt relay station draws 64 watts of primary power. Starting with 24 volts and losing 12 volts in the transmission line would mean that the 4-amperes current has encountered a series resistance of 3 ohms per 3000 ft of wire. Thus, a two-wire #10 cable of up to 1500 ft in length will be acceptable, provided a regulating transformer or supply is used at the antenna tower.

[2] Marks, Lionel S. (ed.), Mechanical Engineers Handbook, :4th Ed., McGraw-Hill, New York, 1941, p.821. Most hydrocarbons have fuel values approximating this value.

[3]

[4]

[5] Attaway, L. D., L. B. Early, N. E. Feldman, and E. E. Reinhart, Design and Use of an Early Communication Satellite System, The RAND Corporation, RM-3514-NASA, July 1963.

[6] Papas, C. A., and E. B. Murphy, "Encapsulating Precision Antennas in Plastic," Electronics, March 8, 1963, pp. 68-73.

[7] Prechtel, C. J., W8DRR, "Experimental~Transceiver for 5650 Mc," QST, Vol. 44, August 1960, pp. 11-15.

[8] Matthaei, G. L., "Interdigital Band-Pass Filters," IRE Transactions on Microwave Theory, Vol. MTT-l0, No. 6, November 1962, pp.479-491. Also see: L. Young, G. L. Matthaei, and E.M.T. Jones, "Microwave Band-Stop Filters with Narrow Stop Bands," IRE Transactions on Microwave Theory, Vol. MTT-l0, No. 6, November 1962, pp. 416-427.

[9] Ibid.

[10] Rheinfelder, W. W., "RC-Coupled High-Frequency Amplifiers," Electronics World, September 1962, pp. 61-62.

[11] Rudich, I., "The PADT Technique," IRE Transactions, Vol. PEP-6, No. 2, September 1962, pp. 37-38.

[12] Grant, E. F., "An Analysis of the Sensing Method of Automatic Frequency Control for Microwave Oscillators," Proceedings IRE, Vol. 37, No. 8, August 1949, pp. 943-951.


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