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Single-Side-Band Techniques at Microwave Bands, Chapter III
@ OK1AIY
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The X-Band, 3 cm (10 Ghz)
Another frequency band for radio amateur experimenters was since 1960s the 10 Ghz band. Although it is a virtually very high frequency, it has been used a long time by professionals. Various radar and communication systems at X-band have been described in literature of the 1940s through 1950s. Most of the components were developed in that time period and we used the described waveguides, antennas as well as mixer diodes in our creations. Before talking about the first attempts to develop SSB capable designs, let us look back a decade or more to see from the literature what was available.
In 1940s the first book was published with details of radar sysems and components (Fig.81). The details are described with mathematical background. Several successive editions were issued and the book probably served well as a college or university textbook. Description and illustrations are given there on wave propagation in various guiding structures, with mathematical derivations. Even systém designs and installations on warships are presented including pictures and schematics.
For decades this technology was mostly used by the military, so for radio-amateur use it was „taboo“. Since 1940s, the breakthrough component was the reflex klystron, Figs. 82, 83. Its design with the massive contact plugs allowed to replace the tube within seconds, with nothing more to replace in the connected circuit.
One reflex klystron equivalent to Figs. 82 and 83 was the 723A/B made by Raytheon. It was probably used in the first radio-amateur transceiver developed in 1960s in Ostrov nad Ohří. For OK1YN, OK1VMK and OK1LU it was the continuing pioneer development described in PE/AR3-5/2015. The archive of OK7RA includes one part of that equipment(Figs. 84, 85). The klystron is installed in the enclosure connected to R100 waveguide, its coaxial antenna coupled to its end. The mixer utilized Sylvania diode 1N23D, and the IF receiver used a RD12Ta triode as a super-regenerative detector, covering the range from tens to a hundred Mhz. Maybe it was only one version, the IF receiver described in AR9/1961 was a four-stage amplifier with 6AC7 pentodes, with 40 Mhz center frequency. The first QSO was made on the Field Day , 2-7-1961, at Klínovec, using ICW (modulated telegraphy), over 100 m distance, with 20 mW output at X-band.

Fig.81 The book title page from 1942:

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Fig.82 2K25 reflex klystron, (723A/B), (3). A very good design, frequency is varied by a mechanical deformation of the metallic internal resonator using a side screw. The frequency depends on mechanical dimensions of the resonator, so there were also types for lower frequencies. Reflex klystrons were manufactured worldwide till 1970s, and served also in many professional systems made by TESLA :

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Fig.83 A Soviet-made klystron (left), a TESLA VUVET one (right):

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Fig. 84a Mirek OK1VMK and Václav OK1YN, 1960s:

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Fig. 84b Mirek OK1VMK and his workplace, 1990s:

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Fig.84 One section of OK1YN equipment for 3 cm band of 1961, a klystron in a case with R-100 waveguide connected to it:

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Fig.85 A diode mixer with 1N23D diode in the same equipment by OK1YN:

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The Gunn Diode, a new milestone for microwave designs
During an intensive research in the P/N junction in silicon and mainly the gallium arsenide, new phenomena were discovered which were soon utilized in new components for microwave applications. The Gunn Effect was one of them. Mirek OK2AQ offers the following explanation:
The Gunn Effect was discovered by J.B.Gunn, an American, in 1963. Its structure is not the P/N junction but a small block of a semiconductor, usually N-type . Mostly used is Gallium Arsenide, an AIIIBV type material. The volume effect takes place upon connecting a DC voltage to the device. Electrons are transferred from the lowest minimum of the conducting band to some higher adjacent minimum (energy gaps). When the DC voltage exceeds a threshold value, Up, while the electric field intensity is about 3.5 kV/cm, the DC current decreases and on the A/V plot we can see a negative differential resistance. The electric field in the semiconductor is not homogeneous but has the highest value close to the cathode, named the Gunn domain. For the device voltage U higher than Up, this domain travels to anode where electrons recombine. Close to cathode the field intensity drops under the threshold value. If the conditions are renewed, a new domain is created, thus starting oscillations. The effect is very fast, and the frequency od oscillations are defined by the reverse of the domain flight time through the device, somewhere between 1 and 100 GHz.
Translated to our radio-amateur language, a Gunn „diode“ is a device made to be installed to a resonator. When connected to a DC voltage, 5-10 V depending on type, it generates oscillations like a vacuum tube or a klystron. Various types generate a microwave power level from unis to tens of milliwatts, but also higher-power devices exist. Compared to klystrons, Figs. 82,83, Gunns are smaller in size, also the DC power input is lower, no heater, no high DC voltage. There are many interesting applications, in telecom and small radar systems, also sensors for security, door openers, speed meters, etc. In radio-amateur designs, Gunns brought a new era: „Gunnplexer“ transceivers were born, for a simple communication just for the X-band. Frequency was defined by device type and the resonator. It could be fine-tuned with a Teflon screw protruding into resonator cavity. By varying the DC voltage, frequency modulation was possible. Designs were simple but some precision was needed (back then, many people were quite skilled). By the end of 1970s, the Gunnplexers were popular among the masses: Gunns were available and simple designs were described in magazines. Manufacturing Gunn devices was difficult but fortunately for radio amateurs, those units not meeting strict specifications were sold for a lower price. In the UHF contests the 10 GHz category was established, and even the BBT included the new band in May 197xxx .Josef, DL6MH was an enthusiast, and his design became quite popular, see Figs. 86, 87.

Fig.86 A block diagram of a Gunnplexer by DL6MH:

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Fig.87 An illustrative mechanical design of a Gunnplexer by DL6MH :

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Fig. 87a DJ8VY,DJ3AT and DL6MH (1985 in Straubing) :

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Fig.88 A Gunnplexer by DJ8VY (more sophisticated). The higher-power Gunn feeds the horn antenna via a cross coupler. The cross coupler is directional and feeds the mixer with a suitable power (mixer diode current is around 1 mA). In receive mode, the signal goes from the horn into the mixer along with the LO signal. IF output is in the bottom-side connector:

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Fig.89 A QSL card from 1981 by then active DL0FM shows three different Gunnplexer versions, on a mountain-hotel roof during a contest:

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Gunnplexers
In our country (Czechoslovakia), the first successful Gunnplexers were built by Václav OK1WAB, Bohouš OK1ABO, and Josef OK1AEX (Fig.90). They also made the first QSOs with surrounding neighbor countries on 3 cm, 10 GHz, in 1976-1979. An interesting development in this field was observed in Italy. In 1980s there were outstanding propagation conditions over Medditerranean Sea. Italian stations with their Gunnplexers were making QSOs almost over a thousand kilometers. In one IARU Region I contest the first ten participants were from Italy and Yugoslavia, the first DL station held 11th position. Even when I had no basic material, I received a Gunn from DM2DPL as a gift, so I started building. There was no chance to find a piece of a waveguide, so I had to make everything from a brass sheet including flanges. The result can be seen in Fig.91. My equipment was untested and without any experience I made no QSO from Libín u Prachatic. The first success was with OK1WAB in Václav¨s garden in Tábor on my return from Šumava (Black Forest). I gained invaluable experience in this new band, testing the new antenna (a horn it was, with which nobody could make a mistake), my first home-made milliwattmeter and a provisional wave meter. In the end I was sure that this way I could not make any more progress.

Improved design
Better design followed with modifications by Josef, OK1WFE. Instead of a Gunn oscillator he was already using multipliers from a quartz oscillator (Fig.92). This way a stable frequency was generated and allowed to use a CW mode with all its advantages. A diode mixer in receiver was followed with a 2-meter converter, with EK 10 receiver at the end. The principle was the same, all stations listened to their keyed local oscillators, the „beat“ frequency was 145 Mhz. In receive mode the oscillators were keyed ON, and even the duplex operation was possible. The distance between the stations was gradually growing, the longest QSO was from Sněžka to Klínovec in September 1975.

Fig.90 QSL card from Josef, OK1AEX:

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Fig.91 A Gunnplexer by OK1AIY, the 3-cm transverter of 1st generation. On the right, a Gunn oscillator (with its DC current monitored on a mA meter). On the left a diode mixer with 1N23, with its DC current monitored on a VU meter taken from a tape recorder:

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Fig.92 One of 3-cm transverters by Josef, OK1WFE. The signal path to antenna and mixer is indicated:

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Fig.93 Benecko, Hotel Panorama, 11.5.1975. Honza OK1VAM during a QSO with OK1WFE in Lipany on 10 Ghz band. Then a record QSO (88 km) was his birthday gift :

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Transverter design for 10 Ghz band
The idea of building a transverter for 10 Ghz (3 cm) appeared as usual during a meeting of Semily radio amateur group in 1982, in Tábor near Lomnice nad Popelkou. Jirka OK1MWD had already made some components and some experience existed, so we entered the demanding design after a short discussion. The start was not easy: we had no professional test equipment, the RAFENA wave meter ended at 3 Ghz, a spectrum analyzer was a thing we only heard of. A lot of a precision work was required, we had no waveguide. The best guide were the first descriptions in Dubus and CQ DL. The GaAs transistors were then rare and not available, so we could only generate some power at around 400 Mhz (386 to 426 Mhz) where „ordinary“ transistors still worked, then we could use varactor multipliers we have used before. To obtain tens of milliwatts at multiplier output, we needed at least 5 Watts of input to the multiplier. To determine a suitable IF frequency, we tested both 1296 and 144 Mhz. A block diagram is shown in Fig.94. The varactor multiplier is shown in Fig. 95 (electrical as well as mechanical design). Another improvement introduced Josef, OK1WFE, by using a 3-cm circulator. The ferrite circulator is a passive microwave component, we will describe it in a following section. Its task is to direct microwave energy into only one direction. Josef's design using the circulator leads the signal from antenna directly into the mixer, not back into the multiplier circuits, so loss is reduced in receive mode, see Fig. 96.

Fig.94a A block diagram of a 10368 Mhz transverter - version for 1296 Mhz start frequency:

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Fig.94b A block diagram of a 10368 Mhz transverter - version for 144 Mhz start frequency:

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Fig.95 Multiplier circuit from 378 Mhz to 2268 Mhz, and mechanical design:

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Fig.96 3-cm (10 Ghz) transverter by Josef OK1WFE, with the ferrite circulator:

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Last multiplier and mixer to 10368 Mhz
Using a varactor to multiply the frequency and trying to mix in a SSB signal at some lower frequency (in our case, 144 or 1296 Mhz) was then the only practical way to get a minuscule useful power. (At 47 Ghz and higher this way is still used). Two mechanical designs were tested, both worked. See Figs. 97 and 101. Finding suitable varactors was again a problem, our friends DM2CFL, DJ8VY and DL7QY helped us out. The design took a long time as we had no suitable test instruments. A good device was the filter for 10368 Mhz, which was tuned by Josef OK1WFE. In connection with a waveguide detector and a sensitive indicator the setup allowed to detect a low signal power (tens of micro-Watts.) Thanks to our friend Růžička of Radio Communications facility in Mělník, who was able to borrow suitable test instruments in Prague, we were able in „his“ communications tower at Chloumek measure all what was needed in a calm environment with his assistance. For us it was a wonderful experience to see on a digital frequency meter our signal frequency ofi 10368 Mhz down to units of Hertz. Such action usually took a whole day as time ran fast. We named such visits „control days“ and the term was soon domesticated among amateurs. (Lately we also heard it from our ham mates of America but it obviously is a random event). During Christmas 1982 we made last modifications and 16.1. 1983 we succeeded to make our first QSO by SSB over a distance of a couple of tens of meters. For a longer test it was time to choose locations of both ends. We tossed a coin, my QTH Mrklov was assigned to me, Jirka OK1MWD got Veliš which has a line of sight to Mrklov. We then used our QTHs for more contests. At the first time we had a good QSO, exchanged reports, and packed our SSB equipment back to its cases. Next we operated at lower bands as there were no other stations to communicate at 3-cm band..

Fig. 97 The last section of multiplier-mixer to obtain 10 Ghz signal:

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Fig. 98 A part of 10 Ghz mixer with 1N23 diode in receiver:

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Fig.99 Diode detector with a filter, a simple milliwatt meter for 10 Ghz band:

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Fig.100 Circuit diagram of the last multiplier-mixer for 10 Ghz, with 144 Mhz IF:

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Fig.101 A sketch of multiplier-mixer for 10 GHZ with 1296 Mhz IF version:

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Fig.102 A view of the transverter with a horn antenna on a tripod in the workshop, and a top view of the transverter:

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Fig.103 Another view of the transverter:

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Final multiplier and mixer for 10368 Mhz
Some time had passed before some longer-distance QSO was achieved. The measured output power of 1 mW did not promise a great hope but excellent conditions „arrived“ often. During one good opening a QSO with DK0NA was made by SSB, as he was the best equipped station in DL. This was a big joy that stimulated more improvements.
We planned to improve the transmitter as well as the receiver. To start, two amplifiers were built, one and two stages, Fig.104. The new GaAs FETs , MGF1401 and MGF 1402 became available but expensive. The amplifier section with a mechanical waveguide switch could be installed in the enclosure of the obsolete TESLA MT11 radio link from 1960s, Fig. 105. The enclosure was a robust metal case on a tripod, with a parabolic dish mount (diameter 1m or 1.8 m), with professional prime-focus radiators. This was an important help, some are using these units till today. The waveguide switch had a metal flap controlled by an electromagnet (solenoid) shown in Fig.106. It is a nice example of a skilled mechanical work, by Jirka OK1MWD. At his electro-technology school the students were also taught to rewind electrical motors. The new equipment called for four such switches but then we loaned a waveguide circulator from TESLA Blatná. Their new type was CVX305112. This saved a lot of work. The circulator was attached on transverter side to which it was connected (inside of a car) with an one-meter section of RG214U coax cable (military). This cable offered a low loss at 3 cm , and stayed flexible at a low temperature. We have no picture from that time but the systém was really functional. Another complication with a low temperature occurred at night, the amplifiers became unstable. We looked for a fast and easy solution. In Kablo company at Vrchlabí they manufactured various heaters. A heating strip in a silicone insulation was wrapped over the mentioned switch, and through brass flanges also the amplifiers were heated. The heater was activated from 24 V DC power supply for antenna turning drives. We used such heater also to deice the crossed-dipole antenna of OK0A translator on Sněžka. For more improvements we need waveguide pipes. As the waveguides were not available, we had to make them from brass sheet. Jirka soldered together two bent brass sheet pieces, attached flanges, and we made necessary filters from it. Their tuning was affected also by solar irradiation as we found after a long testing. This was a lesson for us but in other sections this problem did not occur. We will describe the waveguides and filters later.
While the waveguides were not available and expensive, we found them on sale in the factory store in TESLA Rožnov pod Radhoštěm, new and nicely wrapped. Customers purchased them in a quantity and built nice tables under aquariums from them. In 1984 they were sold out but Mr.Sedláček, OK2AJ, gave me all of what he saved to build such a table.
More modifications were done over next years. Using 144 Mhz band was found convenient but 1296 as one-tenth of receive frequency is technically more elegant... One day, 30.9.1986, very good conditions opened the direction to Netherlands. PA0EZ was by then, and for the next 30 years, one of the top stations. After a QSO on 1296 MHz, we made it also on 10 Ghz, Fig.107. Another QSO at 10 Ghz only happened after long 11 years. 18.8.1997 PA0EZ utilized rain scatter (RS) to make Karel, OK1JKT.

Fig.104 Amplifiers for 10 Ghz band with MGF 1401:

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Fig.105 MT11 microwave link on the front page of Sdělovací technika magazine, 1/1962. This radio link was used to transmit directly TV signal from Prague TV studio to Cukrák TV transmitter:

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Fig.106 Waveguide switch for 10 Ghz. The heater is attached to it as described in the text. At the output there is a directional coupler and detector, the connected MP120 mA meter indicated output power:

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Fig.107 PA0EZ QSL card. His equipment used a MGF2102 FET to transmit, 800 mW output, receiver with MGF1402, parabolic antenna , 75 cm diameter, 40 m above sea:

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More design improvements: 3rd generation transceiver for 3 cm band, 10 Ghz
The equipment described in the last section of our series, was operational and during contests allowed to make one-two QSOs with nearby stations. It was, however, quite bulky and needed to be car-transported in Škoda Š 120. The heavy tripod and one-meter dish cannot pass through car door, gradually broken seat covers and other damages forced us to make a new equipment, the 3rd generation. In CQ DL 11/86 through 1/1987, Juergen¨'s paper appeared. DC0DA clearly described and a good mechanical design was a challenge for many designers around Europe. This stimulated a new development, and in Europe, many followed. The design allowed for additions and improvements according to one's material or financial limits. Individual functional modules were inter-connected with a thin Teflon cable, VB PAM 50-1,5 (old marking from 1986) , a product of Kablo Vrchlabí, with SMC connectors, sold back then even in the old GDR. This concept was important during activation work.
In the 1990s, was (mainly in DL) a welcome new improvement- using a suitable satellite down converter, either into the amplifier chain before filter, or using the complete converter with LO signal injected to a suitable point in the mixer. RF input was R120, so it was necessary to build a waveguide-coaxial transition. Antenna switch was also made in a waveguide section, see Fig.108. During several years, the original output power 5...10 mW gradually increased. At the summer BBT by the end of May, it was an opportunity to test the new equipment. In 1994 I took to Klínovec also a 24 Ghz transverter for an opportunity. Everything was designed to use one parabolic dish , 75 cm diameter but band-switching took some time. Finally the dish was used at 24 Ghz to make five QSOs, while 20 QSOs were made at 3 cm by only an open waveguide. A good prospect to the future. By the end of 1990 a new thermostabilized oscillatorwas added after DF9LN, designed by Míla OK1UFL, and a 4W amplifier by DB6NT. This equipment is still in use today, Fig.109 shows it with the 75 cm dish during BBT2002 at Zadní Žalý. Readers may find the history of 3 cm band too extensive but it was worth of it. At the beginning nobody would dream about making QSOs over hundreds of kilometers,, by „tropo“ as well as „rain scatter“ during storms and rain. With output power of a fractions or units of Watts. All operation modes came to use after many experts claimed that SSB operation would never be possible. The detailed description only illustrates the long and difficult the development was, and other designers may compare their own experience with ours.

Fig.108 Antenna switch for 3 cm band after DL1RQ. It also serves as a waveguide-coaxial transition (coaxial relays were then quite expensive):

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Fig.109 A 3 cm band transverter on a tripod with the dish. On the table there are other transverters for 13,9 and 6 cm with a band-selector switch and IC-202:

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Fig.110 A satellite converter of an early version (still very good) suitable as a 3-cm preamplifier (since 1990 everybody used it). I brought one as a great hit from St.Engelmar Hamfest:

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Fig.111 A view on 3rd generation transverter for 3 cm band (right):

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This paper was also published in print in Practical Eectronics magazine, with permissione (PE/AR Magazine – Practical Electronic and Amateur Radio, Czech amateur magazine, in Czech).


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