An Experimental 10-Mbit/s Microwave Data Link

Updated October 2, 2000: K3PGP has reported success in using Alpha Gunnplexers without varactor diodes in his 10 Mb/sec link. See Technical Correspondence for more information.





PC Boards

It's ironic that some of the most exciting development work in high-speed data communications within the Amateur Radio community was carried out many years before the birth of the World Wide Web and the popularization of the Internet as a vehicle for mass communication. As a case in point, N6GN's pioneering 2-Mbit/s microwave data link was first described in 1989. A decade later, everyone and his mother-in-law is chatting happily away on the packet-switched, ubiquitously-connected communications network we call the Internet, but little progress has been made towards realizing the benefits of high-speed data networking in the Amateur Radio realm.

My own purpose in developing a 10-megabit radio link was threefold: to explore the design techniques underlying high-bandwidth microwave communications; to do my part to help bring Amateur Radio into the twenty-first century; and, most important of all, to use my company's T-1 Internet service at home for free. :-) This page is intended to describe my work to fellow Amateurs and other experimenters for whom a 9600-bps packet BBS or 31-baud PSK rig just doesn't cut it.

I've tried to describe the circuit and relevant construction techniques in depth, for those who may be interested in reproducing and/or modifying my work, but it should be made clear at the outset that such a project isn't a weekend endeavor for the electronics neophyte. While this isn't the most complex or demanding of projects, it has several unforgiving aspects that will not tolerate sloppy construction practices or inadequate care on the builder's part. You should be conversant with basic RF theory and construction skills before tackling it, or at least have access to one or two Elmers who have "been there and done that." You must have access to some basic electronic test equipment, including at a minimum a good oscilloscope and digital VOM. Finally, you should be prepared to commit the necessary resources of time and money to get the results you want. Underestimating the effort and expense required to get your project up and running is probably the #1 cause of failure.

System Architecture and Limitations

The data radio described here has certain limitations that may make it unsuitable for use in certain applications. It's extremely important to understand the microwave link's limitations and overall architecture before assuming that it will work well in your situation. If your application meets the following criteria, there's a good chance that this is the project you've been looking for!

Design and Construction Notes

In the following discussion, refer to the system block diagram and overhead view to learn how the transceiver modules are interconnected.

Alignment and Testing

The most convenient order of construction and testing of the transceiver subassemblies will be determined by your available test equipment. A basic handheld frequency counter from Radio Shack or Optoelectronics will make it easier to adjust the Gunnplexers' mechanical tuning screws, for example. Similarly, even an inexpensive RF signal generator such as the Elenco SG-9000 or older HP608-series model can be used to put the FSK demodulators on frequency. In general, the less test equipment you have, the more time and patience you'll need.

Component and Data Sources

It's a commonly-heard lament in the ham community: "Nobody sells parts anymore." If you're looking for an excuse to avoid building something, I suppose that's as good a rationalization as any -- but you'd have to be delusional to believe it! It's never been easier to put your hands on some of the most exotic components around. The economics of mass-produced consumer electronics have made it tough to save money by homebrewing gear instead of buying it commercially, but for projects such as this one with no direct commercial competition, the lack of component availability should not be a major obstacle.

Most of these sources have already been mentioned above, but are listed here for ease of reference.

Some of my favorite related links (a few of which I've mentioned elsewhere): A couple of pointers to good books appear below. In particular, if you are about to start building this project and you haven't read Bob Pease's excellent Troubleshooting Analog Circuits, stop reading this and go buy it! I am not eager to hear from thousands of frustrated builders who somehow expect me to do a better job explaining what's wrong with their circuit than Bob can. :-) Finally, some 10-mbit/s experimenters have recently taken over an Internet mailing list formerly devoted to discussion of the N6GN 2-mbit/s link. Anyone interested in discussing either type of project is welcome to sign on here. I will monitor this list on a regular basis.

RF Safety and Regulatory Issues

It's impossible to cite specific guidelines for regulatory compliance in every national and local jurisdiction where this microwave radio link may be constructed and operated, so I won't spend much time on the topic. In the United States, a holder of a valid Technician-class or higher Amateur Radio license may operate an FSK data link between 10 and 10.5 GHz for noncommercial purposes. Here, "noncommercial" implies a purpose which does not involve direct financial gain on the operator's part. This rule has been the subject of recent liberalization. Using a personal Internet link to telecommute with your office or buy books from would be unlikely to arouse the FCC's ire, but operating an ISP or e-commerce site might be pushing it. In jurisdictions with specific station-identification requirements, a modulated CW subcarrier could easily be applied to the transmitted signal at narrow-band deviations substantially lower than the modulated signal used for data communications. In any event, I do not condone or recommend illegal or unlicensed operation of this or any other radio transmitter in any jurisdiction, for any purpose. This site cannot be considered a source of legal advice on any application of this project. Further, I expressly disclaim all responsibility for criminal or civil liability incurred by anyone who constructs and operates the microwave link in violation of applicable law.

Now that we're past all the mandatory disclaimers, the reality is that as an operator of an X- or K-band data link, you are far more likely to be the victim than the cause of any interference to other radio services. Especially in urban areas, these bands are heavily populated by users ranging from supermarket door-openers, to microwave burglar alarms, to police traffic radar units -- all of which operate at frequencies and power levels which are similar to those used by the link. It is hardly an uncommon occurrence to find these users operating with unstabilized Gunnplexers which have long since drifted out of their assigned frequency slots and into the 10-10.5 GHz ham band. With good directional antennas on your link, operating near the minimum power necessary for reliable communications, the odds that you will cause harmful interference to any other service are essentially nil. However, you may need to be prepared to adjust your Gunnplexer tuning screws as necessary to move your link as far away as possible from any interfering sources. As noted above, if you can solve this type of problem by throwing more antenna gain at it, by all means do so! Like other FM radios, the microwave link will exhibit a "capture effect" which will tend to block interference from other signals arriving at the receiver with substantially less power relative to the desired carrier. A good antenna system can make all the difference when faced with interference from off-axis sources.

With the recent proliferation of personal cellular telephones and other high-frequency consumer devices, RF safety has become a topic of frequent discussion and debate. As with most such unresolved questions, good technical judgment demands that we choose a position somewhere between the Chicken Little crowd ("if it saves just one child, we've all got to freeze to death in the dark!"), and reckless disregard for known biological consequences of exposure to excess RF radiation ("hell, back on the DEW Line in Iceland we used to stand in front of 20-megawatt magnetron feeds to keep warm!"). It's well known that RF power levels high enough to cause thermal (tissue-heating) effects in humans carry a high degree of risk to organs such as the eyes, genitals, and brain. Certainly no sane person would defeat a microwave oven safety interlock and stick his/her head inside to see if the oven's working. However, several recent studies have also suggested a link between long-term exposure to low-level (athermal) RF radiation and certain forms of cancer. These newer studies are far from conclusive, having generally taken the form of epidemiological surveys which are limited to pointing out correlations, rather than causal links, between RF exposure and health risks to humans. Still, given what we know (and perhaps more important, don't know) about the biological effects of RF radiation at varying frequencies and power levels, it's not unwise to minimize long-term exposure to even the low levels of power emitted by the microwave data link's Gunnplexers.

At first glance, the power levels produced by a Gunnplexer transmitter may seem too low for even the most paranoid user to be concerned about. The power radiated from a Gunnplexer with a horn antenna is about the same as that emitted by a cellular telephone -- less than a thousandth of the RF power present in a household microwave oven. However, cell phones use vertical antennas that exhibit little or no gain over an isotropic (directionless) radiator, while microwave-link applications commonly employ horn or dish antennas with 15 to 30 dBi of gain. Such antennas can deliver a surprising amount of power to a human-sized target. By way of analogy, a 1-milliwatt laser appears much brighter than a 100-watt light bulb because the laser's "antenna" exhibits about 50 dB of gain over the relatively isotropic light bulb! Although it's easy to reassure yourself that these combinations of Gunn diode power and antenna gain are still relatively safe, using resources such as the N5XU RF safety calculator hosted by the University of Texas, it simply does not make sense to install a Gunnplexer in an application where it will be pointed directly at someone's head from only a few feet away. Treat any microwave antenna with the same respect you would pay a potentially-loaded firearm. Never peer into an operating feed horn or antenna aperture, no matter how low the power -- and don't install microwave antennas where uninformed parties (children, pets, inquisitive visitors...) are likely to do the same.


The difference between a difficult project and an impossible one is often the valuable advice and counsel of those who have worked in the field and tackled similar projects before. In developing the microwave link, I was fortunate to benefit from the advice of a number of experts who lent me their knowledge in the fields of microwave radio and data communications, putting up with an entirely-unreasonable stream of dumb questions in the process. :-) I'd especially like to thank Glenn Elmore, N6GN; Alan Rutz, WA9GKA; Rob Frohne, KL7NA; Ralph Stirling; Glen Herrmannsfeldt; and Rich Seifert. Any errors in this document are entirely my own.

Kirkland, Washington
August 9, 1999

Technical Correspondence

August 16, 1999 Alan Rutz, WA9GKA points out a way to squeeze a bit more sensitivity out of the Gunnplexer mixer circuit:

John, may I suggest a small change in ur schematic? Now, you are allowing the 2.2K load on the mixer to be in parallel with the 4:1 transformer. Not bad, but protection can be equal and a bit more signal found if you do it the way I indicate on the attached .pdf drawing, called mixer2.pdf...

Thanks to Alan for this suggestion. There's certainly no reason why the mixer diode's DC load resistor can't be placed in series with the matching-transformer primary.

November 30, 1999 Rob Frohne, KL7NA has reported positive initial results with his own board layout for the link, and he also noticed an error in the FSK modulator schematic. The schematic calls for a Mini-Circuits T2-1 transformer to provide isolation and impedance transformation between the AUI transmit-data pair and the PLP-21.4 filter. This should be a T2-1T transformer -- the T2-1 has one end of its primary and secondary windings internally connected. Use of a T2-1 will degrade isolation and balance characteristics, especially in longer AUI cable runs.

I have corrected the error in the FSK modulator schematic, and will fix the corresponding Orcad file as soon as possible.

December 5, 1999 Rob Frohne has made a minor revision to the AFC circuit on his link which may help reduce an unintentional dependency on transistor beta in the original AFC circuit. Basically, the 510-ohm resistor in the 2N2222's collector circuit is removed, and a 270-ohm resistor inserted in series with its emitter. The 10K resistor from base to emitter is removed. According to Rob:

I had guessed what you were thinking when you put the AFC together pretty well and I think it will work, but not as well as the tweaked circuit. I'm sure neither are really optimum in any sense of the word though. The reason I still like mine a tad better is that I measured the actual varactor voltage as I heated the gunnplexor on one end up with the heat gun. For your circuit I could notice almost no change in the voltage at the collector of the 2N2222 or at the varactor diode either one. With the 2N2222 in the active region I can see it change by several volts in the right direction. It is probably the case that the 2N2222 here happened to have a higher beta than yours did, and so works not so well in your circuit. I like to think of my circuit with the 2N2222 as a voltage to current converter, and think of the current as driving the op amp. At least for me, having the BJT in the active region with its more linear behaviour makes the feedback range much greater than having it most of the time in the saturated mode. Any feedback system should work better with a higher loop gain, as long as it is still stable.

I haven't encountered any trouble of this nature with the prototype, but if you're having problems achieving robust operation of the AFC circuit in your link, give Rob's modification a try!

December 7, 1999 John Cooper reports an error in the tuning meter schematic in which the wiper of the 25K pot was mistakenly tied to ground. The schematic has been updated, with the Orcad fix to follow. Many thanks to John for catching this one.

October 2, 2000 John, K3PGP has reported success in modulating a pair of Gunnplexers via the Gunn diodes' power-supply voltage. Search his home page for the "Gunn" link for details. This technique, previously thought to be impossible at 10 Mb/sec, may obviate the need for the more expensive varactor-equipped Gunn transceivers. John is presently running a 5-mile link using surplus C-band satellite-receiver hardware and Alpha Gunnplexers.

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