MICROWAVE DESIGN NOTES – 1

Phil Lee W6HCC
(12-10-94 rev. a, 3-4-11 rev. b)

This is a collection of design notes. This information is for anyone interested in the design and construction of microwave communication hardware. Much of this data is “old hat” to the experienced microwave experimenter. It is presented as an overview in the hope that there may be some ideas for newcomers and some food for thought for the old-timers.

ANTENNAE: The dish (paraboloid) is the most popular antenna on the higher microwave bands. These range from very small (12-24 in. diameter) to as large as can be mounted by the industrious amateur (10 ft. diameter and up!!??). For use at x-band, the quality of the surface is very important. Mesh dishes and the common TVRO dishes are of marginal value at this frequency due to the size of the mesh. The holes are too large and are poor reflectors. For a modest performance rig (see later table regarding system performance), a 27 in. Macom dish or a 30 in. news-remote (Reuters etc. from the swap meet) will work very well. For a high Performance system, a 48 in. is very desirable (Andrew or other high quality commercial dish).

FEEDS:
The feed system is very important. Most amateur dish feeds produce about 50% efficiency if well designed and adjusted. A poor feed will be much less efficient and system performance will suffer. Commercial feed horns sometimes become available at hamfests or in a surplus dish. These will work, but not at optimum illumination for amateur use. These feeds are designed for a very clean pattern with minimum side-lobes, not maximum gain. Usually, they are 2-3 dB down from an optimum gain feed. The “penny” feed is one of the easiest for home construction. It consists of a 1 wavelength diameter disk (penny??), soldered to the end of a waveguide. The end of the waveguide is slotted on each side to allow energy to be reflected back toward the dish by the disk. There are better feeds, but this is one of the easiest to build. The Macom (bent splasher) feed is widely available from Macom systems which have been repackaged or otherwise used as the basis of microwave systems. These feeds are very efficient in the 30 in. news remote dishes. The circular horn with a scalar ring is widely used in high performance systems. This is usually a piece of copper water pipe which has been flared on one end to accept a WR-90 waveguide flange. The balance of the feed is a waveguide “shepherd’s crook” to place the horn at the proper focus in the dish. For optimum performance, the scalar rings are placed near the end of the horn. These are moved with respect to the end of the horn to match the horn pattern to the f/d of the dish. Circular horn feeds have also been constructed entirely of copper water pipe and fittings. With proper adjustment, these are very successful.

BASES AND SUPPORTS:
The larger the dish, the more important the mount. 16-30 in. dishes can be mounted on large camera tripods or homemade pipe structures. Larger dishes, with their narrower beamwidth, require more accurate pointing. For these, surveyor’s tripods, surplus studio camera tripods, or astronomical telescope piers have been successfully used. The main objective is to point and hold the dish in the desired position. (On mountain top the wind always blows!!!). With lighter mounts, heavy objects, such as car storage batteries, serve well as anchors. A degree scale is very helpful. For DX contacts using large dishes, it is important determine true North, orient the degree scale, and point to a given angle with one degree or better accuracy.

LOCAL OSCILLATOR:
The local oscillator for most amateur microwave rigs is the “Brick”. Bricks come in many forms and from several manufacturers. Some of the most common are by California Microwave and Frequency West. All are similar in principle. A crystal controlled reference is used to phase lock a power oscillator in the 1-2 Ghz range. The output of this oscillator drives a varactor multiplier with a filter at the final output frequency. The typical power output ranges from 10-50 mw for x-band units. Conversion for local oscillator use involves two steps. First, a suitable frequency reference must be provided. For modest performance systems, the internal crystal may be replaced with a suitable amateur frequency crystal. The main problem with this approach is drift. The crystal itself is in a small oven, but the balance of the circuitry is exposed to the unavoidable rigors of the mountain top (wind, sun etc.). A better solution is a separate, oven controlled  reference. This may take the form of a 100 MHz standard or a 5-10 MHz standard oscillator in conjunction with a phase-lock system (see Dave, WA6CGR for boards and components). For a high performance system, stable frequency on the order of 200 Hz at x-band is required. The second step is setting the brick’s output filter to the proper frequency. Most x-band bricks were originally used in the 10-12 Ghz range. Unmodified, some will tune to the high side of 10368 MHz, but without additional conversion stages, inverted tuning and sidebands result. This will work, but at times is very inconvenient. To retune the bricks to the low side of 10368 Mhz usually requires retuning the filter following the varactor multiplier. This is not hard to do, but a power meter and cavity wavemeter are the minimum test equipment needed. A spectrum analyzer is very nice, but not required. Frequency conversions required to get the x-band frequency to the final IF depends on the reference used and the final IF required. Most systems in use have the last IF at 28, 144 or 432 MHz. If multiple conversions are used in the downconverter, the stability required by the conversion oscillators is about 2 orders of magnitude less than that of the reference. Some common schemes used are as follows:

 

FILTERS:
A filter is required at the operating frequency of a microwave system. This filter serves to remove carrier and/or conversion products and image frequencies from the final signal. There are many forms which may be used. Some of the most common are waveguide (post or iris), interdigital, and comb line. All of these forms may be home-constructed. The main limitation is the available machine shop facilities. One of the easiest to fabricate and tune is the waveguide post filter. The only tools required are a drill press and an accurate measuring scale. The job could be done with hand tools entirely, but accurate alignment of the post holes is difficult. A good design appeared in QEX. Surplus filters (swap meet or surplus commercial equipment) are usually designed for the 6 or 11-12 Ghz range. Most of these can be retuned for 5760, 10368 etc. but the insertion losses are slightly higher then a filter designed for the proper frequency. This may not be a problem if adequate power/signal is available prior to the filter. Tuning of the filters is not difficult, but requires patience and moderately good test equipment.

LNA:
The heart of a good receiver is the Low Noise Amplifier. The best available preamps using PHEMT devices have noise figures of less than 1 db at x-band. These are suitable for both terrestrial and EME applications. If EME is not contemplated, LNAs with a noise figure of 2.5-3.0 db are satisfactory, since the dish is pointed at the horizon where earth noise is visible There are several sources of these LNAs. Good performance preamps are available in kit form from Down East Microwave and other suppliers. There have been some GasFet designs in QST. The devices are not excessively expensive and the performance is good. Some surplus LNAs have been available. The “Utah” amplifier has a noise figure of 2 db and gain of 29 db. These were sold for $50. Small amplifier modules may be found at the swap meet with noise figures of 4-5 db and gains of 18-25 db. These are suitable preamps, depending on the overall system performance desired.

POWER AMPLIFIERS:
The final amplifier in the transverter determines the power output. Both solid state and electron beam devices (TWT) are in use. For a low power system, the surplus preamp modules (noted above) will provide linear power outputs of 20-50 mw. The Macom (white box) units will provide 200 mw. The San Diego group (N6IZW) had boards which will produce about 1 watt after suitable retuning. 2 watt and 8 watt solid state PA’s have been available from Down East microwave. Higher power (10 watts and higher) is currently the domain of TWTs. There are several 15 watt Siemens systems in use. TWTs are where you find them! Some have showed up at the swap meet. The tubes are not too rare, but the power supplies are hard to find. Most of the TWTs require about 1-2 mw of drive from the transverter.

SWITCHING:
All transverters must switch between transmit and receive. This is classically done using relays. There are several variants of this which deserve mention: First, the antenna relay. For microwave use, there are waveguide switches and “transco” type coaxial relays. For best performance, the antenna relay should be placed at the dish. This reduces the feed line losses by allowing the preamp to be placed at the dish, ahead of the feed line to the downconverter. Either coax or waveguide relays may be used, depending on the power of the final amplifier. Power control may be either solid state or by relay. If relays are used, they should be high quality sealed units. This prevents dirty contact problems “out on some desolate mountain top”. Solid state switching is more complex, but allows easier control of TR and VOX delays. In a high power system, delays must be inserted to be sure that the antenna relay is fully switched before transmitter power is applied. Otherwise, preamp burnout is possible.

IF Radio:
Most amateur microwave systems use a commercial all-mode radio as the IF unit. The stability of these radios is typically +- 1 KHz during warmup. For a high performance system, a radio with a high stability reference, such as the Kenwood TS2000 is needed. Typically, these have only one antenna connector unless special modifications are made to separate transmit and receive. When the IF signal is used by a transverter, the transmit and receive paths must be separated. This may be done by a coax relay or a PIN diode switch. If much power is delivered by the IF radio (1-10 watts typical), the TR switching must assure that the path is connected to the transmit attenuator/load before power is applied. Failure of this function can damage IF post amps (and worse, x-band mixer in the upconverter). One simple solution to this problem is to use self-biased PIN diodes. This done by using a quarter wave transmission line shorted by 2 PIN diodes, to couple the receive signal from the transverter to the IF radio. When transmit power is applied, the diodes conduct and protect the receive post amp. No DC control is required. (see W6HCC for details)
POWER SOURCE:
Power to operate a portable system usually comes from batteries or a portable generator. (Some select mountain tops allow access to commercial AC power mains.) With a low power system, a car battery is adequate for many hours of operation. As system power increases, multiple batteries or a generator is required. Even the modest systems often require several power forms. The brick usually needs -20v. Most waveguide switches and relays use +24v. Crystal ovens often use +24v. +12v is required by oscillators, preamps and other parts of the system. One solution to this problem is DC-DC converters. Modern switching converters are very efficient and simple. With proper shielding and decoupling, these are very useful. Another solution is multiple batteries. This is used in larger systems where devices such as TWTs require several amps of -24v or -48v. From a system of 4 batteries (the car battery serving as one) a +24v and -24v buss may be established and all other voltages derived from these using linear regulators.
SYSTEM CONFIGURATIONS:
Here are some guide-lines to system performance. These are not hard-and-fast rules, but definitions to help characterize systems.