On Distributed Communications Series
VI. Mini-Cost Microwave
IV. Determination of Key Design Parameters
Having determined the required system signal-to-noise ratio, signal gains and losses can now be assigned to the components that will form the microwave relay.
Choice of Carrier Frequency
The most heavily occupied microwave frequency bands for common-carrier use lie in the 2-7 Gc range. This portion of the frequency spectrum is already over-crowded near large cities and major communication centers, while the wider-bandwidth higher-frequency bands are relatively unpopulated. Use of these higher frequencies--around 8-13 Gcs--shall be assumed (i.e., the high X-band/low Ku-band). Most of our calculations will be based upon a median frequency of 10 kilomegacycles.
While it would have been preferable to examine the specific frequency bands usable for this type relay service, it was found that there is hardly any experience in the mass production of microwave equipment at the exact frequencies of our prime interest. However, at the radar portion of the X-band, there is sufficient production experience to provide some idea of the anticipated costs for some of the elements of the proposed system.
As the frequency is increased it becomes more difficult to generate power efficiently, there is a slight increase in the crystal mixer noise factor, and there is slightly more rain attenuation. But, these factors are more than compensated for by the gains to be realized through the use of the higher-than-normal frequencies and a fixed-size, parabolic antenna. No technique shall be proposed for use in the X-band that cannot also be satisfactorily performed in the Ku-band.
The use of very-low-power transmitters generating only 32 milliwatts of radio frequency energy are assumed. This is equivalent to 15 dbm (or 15 decibels referred to a 1-milliwatt level; this "dbm" notation shall be used because of its convenience in describing signal gains and losses in microwave systems).
Several different methods of building SHF transmitters were considered, including: 1) reflex-connected, traveling-wave, tube amplifiers; 2) small klystron oscillator tubes; and 3) crystal-controlled varactor multiplier chains.
The receiver-type klystrons showed the lowest initial cost, followed by traveling-wave tubes and the varactor multiplier chain. However, the maintenance and replacement costs of these thermionic devices appear to override the initial savings. On an overall cost basis, the varactor approach appears most attractive--at this time.
The power drain of the klystron version and the varactor version are about the same, and both fit intoabout the same size package. The further development of the klystron design is primarily a matter of insurance. There is only one company making complete Ku-band chains and their maximum power at present in the X-band is about 250 milliwatts, falling off to 50 milliwatts in the Ku-band (13.5 Gcs).
Figure 4 shows the power generating ability of varactor chain multipliers and transistor drivers available inDecember 1962. A somewhat similar chart is to be found in Balakrishnan. RCA is now manufacturing an all-solid-state microwave repeater.
Transmitter/Receiver Losses and Gains
A stripline filter is to be used with the transmitter to prevent the radiation of spurious sidebands and harmonics because of the square-shaped, pulse-modulated signal used. A loss of .4 db due to this filter is assumed.
A 54-inch parabolic dish using a Cassegrainian feed at 10 Gc, operating at 65 per cent overall efficiency, is postulated. As the transmitting and receiving antenna feeds use the same dish, we can assume a similar 41.4-db receiver antenna gain.
A second input radio frequency stripline filter is used to reject extraneous signals within the pass-band of the receiver.
Also assumed is a -145-db free-space loss at 10 kilomegacycles for a nominal 25-mile path length.
As a feederless transmission system will be used between the transmitter/receiver and the antenna, all additional losses can be kept to within 1 db, as is shown below.
On the basis of the above considerations, the maximum received signal from a remote adjacent transmitter will arrive at a -48-dbm level. Figure 5 indicates the change of signal level in dbm; this chart provides a visual indication of the magnitude of the signal losses and gains and the resulting noise threshold.
An effective radio frequency bandwidth of about 36 megacycles is assumed; Figure 6 shows the frequency shift-keying output of a 4.5-megabit/sec digital square wave. The sideband structure generated is shown to occupy a total two-sided bandwidth of about 36 megacycles. Since the bulk of the energy of the modulated signal is concentrated at two frequencies, a square-sided pass-band is slightly pessimistic. But, for ease of calculation, we shall merely assume a rectangular pass-band, giving a thermal, or KTB, noise of about -99 dbm at 2900 K.
A receiver noise figure (NF) of 10 db will be assumed.
Thus, the total available S/N ratio is 40 db, approximately the desired value. This 40 db is divided into a 15-db margin reserved for error rate, with the remaining 25-db margin reserved for Rayleigh fading. Therefore, the minimum, first-cut, overall S/N ratio may be considered to be 15 db.
Safety Margin Allowance
There is adequate margin in the design of the system for improving the S/N ratio, if such should prove necessary. For example, an increase in the diameter of the antenna from 54 to 60 inches will increase the one-way gain by 1 db, or a total of +2 db for the two-way gain. Increasing the operating frequency to 13 Kmc will increase the one-way gain by 1.3 db or a total of 2.6 db on the two-way path. Or, the power output could be increased from 32 milliwats to 0.5 watts, resulting in a 12-db gain as an extreme limit. In combination, these three factors would result in an overall 14.6+ db gain which is designated the Redesign Safety Margin. Even further, the receiver noise factor could be improved by several decibels through the use of a tunnel diode pre-amplifier and a more sophisticated receiving technique.
Figure 7 is an imaginary slide rule composed of six parallel strips, providing a visualization of the scope of the parameter trade-offs necessary to produce the given per-bit-per-link error rate required by the system. For example, increasing transmitted power to 0.1 watts is equivalent to a reduction of bandwidth of 11 megacycles. The figure shows, as an example, that if one wished to space repeaters 42 miles apart instead of the currently postulated 25 miles, the antenna dish would have to be increased to 72 inches in diameter.
 At some of these frequencies, an order for 50 tubes is regarded as big production.
 Such amplifiers are currently available; e.g., that advertised by International Microwave Corporation in Electronic Industries, Vol. 22, September 1963, p. 120. Its specifications are: minimum gain, 25 db; l-Gc band centered at 9 Gc; -30o to +600oC; noise factor, 5.5 db maximum.