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N6GN INEXPENSIVE MULTI-MEGABAUD MICROWAVE DATA LINK



INEXPENSIVE MULTI-MEGABAUD MICROWAVE
DATA LINK


By Glenn Elmore, N6GN, 550 Willowside Road,


Santa Rosa, California 95401 and Kevin Rowett,



N6RCE, 1134 Steeplechase Lane, Cupertino,



California 95014

We'd like to tell you about some inexpensive antenna,
radio, and computer interface hardware which allows communication of digital
data at rates up to 2 megabaud (1 megabaud = 1 million bits per second)
on an Amateur Radio band. In addition to the data link, an analog voice
channel is provided. It requires only an external microphone and speaker
for simultaneous full duplex audio communication. The link operates in
the 10-GHz Amateur band and uses an inexpensive commercial parabolic antenna
along with a Doppler radar transceiver module to provide medium range communications
at low cost. We'll discuss modifications to surplus networking interface
cards that let you use this high speed data in Amateur Radio service with
IBM-style personal computers.

The Amateur accustomed to conventional AX.25 packet
operation might wonder why anyone would want to go to the trouble of building
a digital radio approximately 1000 times as fast as the 1200-baud systems
prevalent on the VHF bands. Although many metropolitan areas are experiencing
severe congestion on some of the packet channels, it's also true that many
keyboard-to-keyboard QSOs are taking place. A great deal of traffic is
also being handled on the worldwide bulletin board systems using today's
equipment. The success of AX.25 packet radio has suggested the need for
faster systems to improve current performance and has spawned some fundamentally
new ideas for Amateur Radio.

A whole spectrum of new user applications and
the possibility of a nationwide or even worldwide digital Amateur network
are two major areas made possible by faster hardware.


New applications


Packet has been regarded as a way for two stations'
computers to communicate, allowing keyboard-to-keyboard QSOs, but the potential
for far greater applications exists. Almost any information which can be
transmitted by analog means can also be transmitted digitally, making digital
audio, facsimile, graphics, and even digital TV feasible on the Amateur
bands once sufficient data speed is available. The concept of repeaters
for a variety of modes is conceivable, when combined with the ability of
each Amateur station to serve as a relay of data to and from other stations.

Amateurs will also be able to share resources.
A station with an interesting database will be able to make it available
to others. On-line call directories, QSL information, and technical data
-- not to mention computer programs and even the computers themselves --
can be shared. It's possible for one Amateur to actually run programs and
applications on someone else's computer as though it were located in his
own shack. Remote control of equipment and remote sensing are other possibilities.
Remote digital control of repeaters or even complete stations, including
audio or video uplinks and downlinks, can be supported. Conventional voice
repeaters (analog) may be replaced by digital hardware for completely digital
round tables. Since this data can be transmitted anywhere the network permits,
there can be multistate, national, or even worldwide voice nets. If the
data rate permits, all of these different applications could conceivably
be going on at the same time.


An Amateur Radio network


The possibility of an Amateur network is just as
exciting as the variety of applications that high data rates can support.
To date, groups of Amateurs have used limited networks for traffic handling
and sharing information among members. A high speed digital network can
provide these same services, as well as new applications, over very broad
geographical areas on a full- time basis. A nationwide network capable
of transmitting data quickly and with little delay could be beneficial
to Amateur Radio public service and emergency operations. Data and resource
sharing on a nationwide or worldwide network offers great potential in
ushering the information age into Amateur Radio. The diversity of Amateur
interests -- DX, ragchewing, technical, and public service -- could all
be greatly enhanced by such a network. A network might also entice a great
many potential and computer literate candidates into getting their tickets.


Goals


The link we'll describe was designed to help further
the applications and networking made possible by high speed data exchange.
It was built as an initial step in providing a moderate speed digital Amateur
network in northern California to be used with a fledgling TCP/IP radio
network.

This was necessary to support some of the applications
previously described. It was also built to help advance Amateur use of
the microwave spectrum.

Fortunately, microwaves and high speed communication
fit together very well. In fact, if the data rate is increased significantly
it is absolutely necessary that wider and wider bands be used. As frequency
is increased, antennas of reasonable physical size are better able to focus
the transmitted beam without wasting signal in different directions. The
Amateur microwave bands, through 24 GHz, offer the best available performance
and cost for such communication. In order to be widely useful our link
needed several attributes:



  • To be inexpensive -- competitive with present TNC/radio
    combinations


  • Moderate speed -- significantly faster than current
    alternatives of 1200 to 56,000 baud

  • Medium range -- at least 20 miles to be effective

  • To use readily available parts

  • To be simple to build and maintain

  • To be reliable -- a variety of applications may depend
    upon it


  • Problems


    In addition to the problem of building radio hardware
    which the average Amateur could feel comfortable installing and maintaining,
    there are problems with the digital interface hardware portion of such
    a link.

    At these speeds the data is too fast for normal
    serial ports on most computers, for the internal bus operations of many
    computers, and for TNCs. Similarly, the software to process data at these
    speeds can no longer operate on a character by-character basis. Any solution
    we developed for these problems also needed to work with commonly available
    hardware, most notably the IBM PC and its clones.

    Microwave hardware, propagation, and high speed
    data are new ground for many Amateurs. This means that any high speed link
    hardware needs to be relatively easy to work with.


    What we built


    Previous successes using 10-GHz Gunn diode oscillators
    as local oscillators and transceivers for narrowband weak signal work,
    brought to mind the possibility of using these inexpensive units for higher
    speed digital data transmission. In addition to being inexpensive, these
    units -- which are commonly used for motion detection (door openers and
    burglar alarms), speed measurement (police radar guns), and microwave receivers
    (radar detectors) - have all of the microwave circuitry self-contained.
    This is important because it makes the equipment more attractive to nonmicrowave
    users. The system block diagram in Figure 1
    shows
    the operating principles.

    The two ends of a link operate "split." One transceiver
    oscillator typically operates on 10,450 MHz while the other end is 105
    MHz lower, on 10,345 MHz. The difference between the two transmitter frequencies
    corresponds to the receiver first IF frequency. The receiver first IF on
    each end is generated when the remotely transmitted signal (frequency modulated
    by the data to be transmitted) is mixed with the local transmitter. Each
    end uses its own transmitter as a receiver local oscillator, and each unit
    transmits continuously. Therefore, each receiver sees the same IF. This
    is the same full duplex arrangement used for many years by Amateur microwave
    enthusiasts. The transmitters run 5 to 10 mW of output power. The transmitter
    is frequency modulated as its bias supply is varied and the frequency voltage
    dependency of the transceivers is used for tuning. This same technique
    was used previously to phase lock such oscillators.1 The 2-foot dish shown
    here has a gain of about 33 d8, or 2000 times at 10.5 GHz. When driven
    by the microwave transceiver, the effective radiated power (ERP) is about
    the same as that of a 10-watt 2-meter radio driving a quarter-wave whip.
    We selected 105 MHz for the receiver first IF, with provision for tuning
    +/- 10 MHz to accommodate differential frequency drift with time or temperature
    of the free-running microwave transceivers. Using an IF in this range also
    lets you do some simple troubleshooting and testing with commonly available
    commercial FM broadcast receivers. No correction is necessary if both ends
    drift in the same direction because the IF doesn't change. Automatic Frequency
    Control (AFC), implemented by tuning the second LO nominally at 150 MHz,
    is provided to keep the receiver tuned correctly. This conversion produces
    the second IF at the point where detection takes place at 45 MHz in a Motorola
    MC13055 FSK receiver chip. This chip is specified to operate at data rates
    up to 2 megabytes per second (Mbps), but has actually been used as high
    as 10 Mbps.

    Automatic frequency control circuits keep each
    receiver correctly tuned, even when the first IF deviates from 105 MHz.
    A search oscillator is also provided to allow the receiver to "find" the
    incoming signal when the link is first powered up, or if you lose signals
    temporarily. The searching is controlled by the Data Carrier Detect (DCD)
    circuitry. Once the signal is found, the oscillator is shut off and the
    AFC tunes the receiver correctly. Because the data is digital, an appropriate
    offset is introduced to the tuning depending on whether the data is a "0"
    or a "1" We added the audio channel as an afterthought. It provides for
    human communication, particularly while debugging the link and operating
    it with digital data the first time. An electret microphone produces the
    transmit audio signal. This is amplified and limited by high and low pass
    filters before modulating the transmitter. Levels were selected to provide
    only small deviation compared with that of the digital channel. This allows
    the audio channel to operate without significantly interfering or degrading
    the digital data. A volume control and speaker amplifier sufficient for
    driving headphones or a small speaker are provided on receive.

    An analog meter displays strength as carrier-to-noise
    (C/N) ratio or discriminator output to aid when manual frequency control
    (MFC) is used. The MC13055 FSK receiver hip has a built in logarithmic
    amplifier which can give a pretty accurate measure of C/N. A switch lets
    you select manual or automatic frequency control.

    The microwave transceiver, modulator, and receive
    Preamplifier are mounted together in a box located at the Prime focus of
    the parabolic antenna. A horn antenna was designed to illuminate the dish
    antenna efficiently so that ear maximum gain could be obtained. The rest
    of the receiver, as well as circuits for the audio channel, are located
    ,a separate enclosure. This lets you place the antenna and microwave hardware
    a considerable distance from the receiver for tower or mast mounting.

    We used Emitter Coupled Logic (ECL) for incoming
    and outgoing data. These are differential lines and can be used even when
    there is considerable line length -- for example, when the microwave portion
    is high on a tower or the receiver is located some distance from the host
    computer. A standard 15-pin connector is used as the interface to the radio
    hardware. Those familiar with Local Area Networks (LANs) may recognize
    this connector and pinout as identical to that of a Media Access Unit (MAU),
    the device used to link a computer to a coaxial cable connecting a building
    or area wide network. You need just 12 volts DC at approximately 350- mA
    data input and data output to operate all the radio link hardware.


    Antenna


    The antenna is mounted to a mast with a rear mounting
    bracket. This plate is cut from sheet aluminum and folded to produce four
    "feet" which attach to the dish. Mount the plate to the mast with U clamps.
    For minor elevation steering of the antenna, add two extra sets of nuts
    to the clamps. This lets you adjust the spacing between the mast and plate.
    Less spacing on the top than on the bottom clamp points the dish upward;
    more spacing points it downward. Because the antenna has less than a 4-degree
    half-power beamwidth, you may need to make this adjustment -- particularly
    when the two ends of the link are at different elevations and not very
    far apart.

    The horn feed and microwave assembly attach to
    a plate and are held at the dish focal point by four 1/4-inch diameter
    struts made from soft aluminum rod. We found this material at a local home
    supply store. First cut the aluminum rod to length; then drill and tap
    it at each end. Use a tubing bender to shape the rod properly. Figures
    2A
    , 2B and Figure
    3
    show the mounting bracket and feed support struts, respectively.


    Feedhorn


    To construct the feedhorn, first cut the sides and
    flange from copper or brass shim stock as shown in Figure
    4
    . Because the material is so thin, you may want to begin by tacking
    the whole assembly together using a medium sized soldering iron and complete
    your soldering after everything is in place.

    The feed/microwave assembly is shown in Photo
    A
    . The mounting plate has a short section of waveguide at its center.
    The feedhorn and transceiver mount on opposite sides and a small Bud box
    encloses the electronics. Make the waveguide section by milling (or drilling)
    and fitting a 0.40 by 0.90-inch rectangle in the plate. Great precision
    of construction for either the feed or the mounting plate isn't necessary
    to obtain good performance. Figure 5 shows
    the feed mounting plate.


    Microwave assembly


    Because the microwave transceiver is self-contained,
    the RF electronics aren't particularly critical or difficult to assemble.
    The receive preamplifier is probably the only sensitive circuitry, and
    because this uses MMICs, short lead length and good grounding are the only
    prerequisites. If you use pc board material, you can make the entire board
    using a file and small hobby knife. Fifty-ohm transmission line is used
    to connect to and from the MMICs. Try making this line by filing 0.005
    to 0.010-inch slots 0.1 inch apart. The line can then be cut into short
    sections and the components soldered directly to it. Holes drilled in the
    board allow the MMIC packages to sit flush with the lines. Holes are also
    drilled for all component ground leads, and the leads are soldered on the
    top and bottom of the board. The regulator and modulator circuits aren't
    critical and the ECL IC may be "dead bug" mounted on top of the board.
    You can mount the board to the aluminum box with short spacers. Use a twisted
    pair made from hookup wire to connect to the mixer diode on the transceiver. Figure
    6
    shows an approximate board layout. (See Figure
    7
    for enclosure dimensions.)


    Receiver assembly


    Receiver assembly construction also isn't critical.
    See Figures 8A, 8B,
    and
    8C for details. Use ground plane
    as much as possible on the component side of the board and keep traces
    to the 45-MHz filter and discriminator reasonably short. Otherwise, no
    special precautions need be taken. All ICs may be socketed for convenience.
    (PC layout or circuit boards may be available by press time. Contact the
    authors for further information. Ed.)


    Computer interface


    We chose the IBM PC as the initial platform for developing
    and testing faster packet hardware. The IBM PC is generally available and
    affordable, with sufficient capability and adaptability. The IBM PC is
    the defacto standard for people pursuing higher speed packet. The bus architecture
    is well known, and there are a large number of experts to consult should
    you encounter problems.

    The original system design called for standard
    off-the shelf EthernetTM adapter cards. Normally Ethernet lets a number
    of computers intercommunicate within a local area at a data rate of 10
    Mbps. N3EUA suggested we use the same widely available cards and the associated
    IEEE 802.3 protocol with the adapter card clocks slowed to 1 Mbps. Using
    a standard Ethernet adapter gives you access to an existing range of networking
    software, including NetBios/PC Network and TCP/IP implementations. It's
    an interface familiar to the general ham community, one that the advanced
    packeteer probably already knows and works with.

    Stock Ethernet cards can't be slowed from 10 to
    1 Mbps without extensive reworking. The serial data interface portion of
    the cards must produce a clock rate within 0.001 percent of 10 Mbps to
    conform to the IEEE 802.3 specification. By the time we discovered this
    we had working "RF bit pumps:' but found ourselves without a suitable digital
    interface.

    Several years ago IBM produced a digital communications
    adapter known as the PCLANA or SYTEK 6120. This adapter was designed to
    communicate using FSK signals transmitted on a coaxial cable at a 1-Mbps
    rate. The card implemented Ethernet framing with a NetBios interface directly
    on the card. The card consists of a local ľP (80186), an Ethernet chip
    (82586), custom 802.3 serial data interface, RAM, ROM, PC bus interface,
    gnd an RF modem. While the RF modem is interesting, it was unnecessary
    for this project and was disconnected.

    We discovered that this card had all the right
    pieces: 1-Mbps Ethernet frames, defined PC interface, and because 10-Mbps
    Ethernet was displacing these cards, good availability on the surplus market
    -- sometimes just for the asking.

    To use the PCLANA card, we needed to gain access
    to the TTL signals directly (before modulating the RF modem), build an
    adapter card onto the PCLANA for converting TTL to differential ECL, generate
    DCD, and route Transmit Data (Txd) and Receive Data (RxD). You'll need
    a software driver if you want something other than a NetBios interface.
    A schematic of the card is shown in the IBM PC Technical Reference Manual,
    "Options and Adapters" section.


    How the adapter card works


    A "daughter" adapter (Figure
    9
    ) is fitted to the PCLANA to take the TTL TxD, RxD, DCD, and RTS.
    It produces a valid interface to the microwave RF modem, including power
    and differential ECL interface.

    The microwave modem interface closely matches
    the IEEE 802.3 MAU which normally connects the digital interface to a coaxial
    cable. A MAU is also known as an Ethernet transceiver (XCVR). We chose
    this interface because it allows long (120 foot) cable runs, good common
    mode noise immunity (0.6 volts), and a standard interface.

    TxD is converted to differential ECL levels with
    an open collector (OC) NAND gate and a resistor totem pole. This produces
    the right voltage level for one end of an MC10116 differential line driver
    input. The other differential input is tied to the MC10116 Vbb. The output
    of the MC10116 is differential ECL. Because the ECL drivers are open ended,
    each line is pulled to ground with a 470-ohm resistor. The polarity of
    ECL lines to the pins of the DB15 connector is important. Switching these
    lines will result in a bit sense inversion.

    RxD is converted from differential ECL to TTL
    using the differential drive from an MC10116 biasing the base emitter junction
    of a 2N3906 PNP transistor. This drives a standard TTL NAND input.

    TxD is qualified by Ready to Send (RTS) from the
    PCLANA (U1b). This is necessary because the SYTEK SIC doesn't clamp the
    TxD line to logical zero when RTS goes false.

    The SYTEK SIC interface chip is designed to work
    on a cable with multiple stations. It monitors the DCD line, and if it
    finds that it has been true too long, the SIC declares the cable jammed.
    Because the microwave RF modem provides continuous DCD with or without
    data, we used a retriggerable one shot (U2a) connected to the incoming
    RxD signal to generate an appropriate non continuous DCD. On the first
    low-to-high transition, DCD will be asserted and the one shot will start
    timing. Each low-to-high transition in the incoming data will retrigger
    the one shot and keep it from expiring. When data stops and the RxD line
    clamps to a low, the DCD will fall to a zero when the one shot expires.
    Because each end of the link expects to hear itself, the DCD is ORed with
    the local RTS (U1a).


    Building the interface adapter and 

    PCLAN adapter modification


    Adapter card construction is straightforward and
    can be completed quickly with IC sockets, perfboard, and point-to-point
    wiring. The entire circuit runs at baseband speed, so layout isn't important.
    (Contact the authors if you want a pc board layout.)

    The PCLANA modification can be simple or time
    consuming, depending upon how elegant you want the finished project to
    be. The adapter daughter board must be mounted on the PCLANA. The best
    place to do this is over the long metal cover housing the actual RF modem.
    Decide on a place and prepare your mount.

    You might consider removing enough of the RF modem
    components to make room for the daughter adapter board. If you do, be aware
    that the RF modem reports to the onboard ľP (80188) as to the validity
    of the -12 volt DC power line (for historical reasons). If you remove the
    RF modem without connecting this signal, the ľP will report an error whenever
    you attempt any operation. Locate Q20 and R121 on the left side of the
    pc board next to the lower left corner of the RF modem cover (if it's still
    installed). The left end of R121 is tied to the base of Q20. Cut the trace
    to the right end (or unsolder R121) and reconnect it directly to -12 volts
    DC on the board (bus pin B7).

    You need four signal lines to connect the daughter
    card to the PCLANA. They are: TxD, RxD, DCD, and RTS. You also need three
    power lines: +5 volts, +12 volts, and GND.

    You'll find the connection to ground underneath
    the screw holding the mounting bracket to the rear of the card. Positive
    5 volts DC is taken from a trace leading to edge connector Al (component
    side of the card, on the right). Positive 12 volts DC is also taken from
    a trace leading from B9 (solder side of the card, on the left). This trace
    also "comes through" to the component side of the card and is easier to
    solder to. The current demand is low, so you can make contact by cleaning
    a portion of a wide trace, tinning it, and soldering to it.

    Make signal connections to the SYTEK SIC chip
    by bending pins up out of the socket and soldering to them. Find IC U16
    in the upper left of the board. It's designated SIC and is a wide 28-pin
    device. Pin 1 should have a red dot and be on the lower left. Remove the
    device carefully; you probably won't be able to find a replacement easily.
    Bend pins 12, 13, 17, and 18 up so they won't make contact with the socket
    when the IC is reinserted, and so you can solder to them. Reinsert the
    IC. Connect the daughter board signal leads directly to the exposed leads.
    Pin 12 is RxD, 13 is DCD, 17 is RTS, and 18 is TxD. It's a good idea to
    use shrink tubing on each lead. Be careful not to heat the device unnecessarily
    while soldering.

    With the daughter card mounted on the PCLANA and
    all seven interface lines hooked up, measure the resistance from the +5
    and +12 lines to GND. Find the cause of any reading less than 700 ohms
    before installing the card in the PC.

    If everything checks out, install the board in
    a PC bus slot and power up the computer. If the microwave hardware isn't
    attached to the 15-pin daughter board connector, the PC will delay for
    about 15 to 45 seconds during the boot cycle. It may display an error 3015.

    To complete testing, install a similarly modified
    card in another PC, connect the microwave hardware, power up both computers,
    and check to see that both FSK receivers show DCD. It may be necessary
    to reboot the PC by selecting ctl-alt-del after DCD has been established.
    When the PC is booted, the PCLANA runs through a series of diagnostics
    which include frame loopback. The loopback will fail if the microwave hardware
    doesn't have DCD. Depending on the PC, the PCLANA card will declare itself
    inoperative until you run diagnostics again.

    If you have a copy of the IBM PC network program,
    you can now start it on both machines and share disks. Playing with the
    network can provide lots of creative fun. With the link running to a fellow
    ham, you can access each other's hard disks. Suppose you just finished
    some nice graphics and you want to show them off. Instead of reaching for
    your An/ camera, bit dump the screen image to disk and do a DOS copy file
    from your disk to your friend's. Your friend will be able to see your work
    in seconds. A driver is also available to provide packet interface support
    for Phil Karn's (KA9Q) TCP/IP package for the PC. (Contact the authors
    for further information.)


    Tune-up and testing


    Unless you have a 10-GHz microwave signal generator
    available, you should build these units as a pair -- although two dishes
    aren't necessary for short range use or testing. You should build the microwave
    assembly and set the bias voltage from the three-terminal regulator' to
    approximately 6.3 volts before connecting to the transceiver. Verify that
    the deviation control can be set to give 0.25 to 0.5-volt variation when
    the ECL line receiver is toggled between states. Once the biases are correct,
    hook up the transceiver. Most transceivers as shipped will be close to
    10,525 MHz. With two transceiver/horn assemblies pointed at each other
    and separated a few feet, hook a frequency counter or general coverage
    receiver to the preamplifier output. Leaving one unit's tuning unchanged,
    tune the second unit while monitoring the IF frequency with a counter or
    listening for it in a general coverage receiver. Turning the mechanical
    tuning screw further into the cavity will reduce frequency. As the unit
    is tuned 70 MHz (or more) lower, you should be able to read the difference
    in frequency with a counter. You should be able to "walk" the two units
    lower in frequency into the hamband using this technique and space them
    105 MHz apart. If you get "lost" and don't know the absolute frequency,
    try using a local supermarket door opener as an approximate 10525-MHz reference.
    If a microwave frequency indicator is available, adjustment is trivial.

    Once the two ends are operating 105 MHz apart,
    you can align the receivers. Select MFC and midrange control setting, and
    use a counter or 2-meter receiver to monitor the VCO frequency. Set it
    to 148 to 150 MHz at midrange. You should be able to tune several MHz on
    either side of this center with the manual tuning control. You can use
    AFC to tune it even further once the other circuits are operating. Tune
    the VCO to 45 MHz above the previously measured frequency of the microwave
    IF and adjust the 45-MHz bandpass coil for maximum C/N reading. It may
    be necessary to separate the units or use conductive foam material to keep
    the signal strength reading on scale. Once the receiver is peaked on a
    45-MHz IF, tune the discriminator coil to center the detector output voltage
    on pin 10 or 11 of the MC13055. With the squelch control set to maximum
    resistance of 5 k, a 0.35-volt change on pin 12 corresponds to 10-dB change
    in signal strength. Keeping the receiver tuned to center with MFC, adjust
    the position and absorbers to produce about 10 dB of C/N. Then adjust the
    squelch control so the DCD light just extinguishes. Measure the squelch
    control resistance again and calibrate your C/N reading by calculating
    sensitivity:

    V(p12) = 0.070 * Rsquelch (1)
    Rsquelch = squelch resistance in kilohms. 
    V(p12) = voltage change at MC13055 pin 12 in volts for 
            10-dB change in signal.

    Adjust the discriminator output sensitivity to give
    full scale on your meter as you use the MFC to tune across the incoming
    signal. Finally, verify that the search oscillator runs when you select
    AFC and that there's no incoming 105-MHz IF signal. This should appear
    as a sawtooth oscillation on the VCO tuning line.

    The audio channel should work without further
    adjustment. Some background noise may be audible even when signal strength
    is high due to the phase noise of the microwave oscillators, but the level
    shouldn't be objectionable.

    Use an oscilloscope to verify data throughput
    and correct transmitter deviation setting. Monitor the discriminator output
    with the scope and set the scope sensitivity to give full screen display
    as you use the MFC to tune through a signal. When you reach this stage,
    the transmitter may be modulated with data and the deviation adjustment
    may be used to set the discriminator output to slightly less than full
    screen. It may be necessary to iterate with the bias setting to keep the
    transceiver bias centered on 6.25 volts.

    At this point, both the transmitter and receiver
    should be functioning properly and may be used for audio or digital communications.
    When units are separated by a great distance, or signals are otherwise
    weak, it may be beneficial to monitor the audio channel as an aid to link
    adjustment. Audio communication will be possible even when noise is causing
    excessive errors on a data channel.


    Performance, results, and 

    remaining problems


    Getting data to flow on the bench led to a couple
    of surprises. While debugging the software drivers, we learned new elements
    of timing relationships. Since this is point to point, and the only start-up
    delay is software latency (no hardware TXDELAY), data frame ACKs arrived
    from the other end before we'd finished processing the send request. Pieces
    of software that were just fine at lower speeds had to be rethought and
    streamlined to achieve throughput.

    Aligning the microwave dishes requires some skill.
    If you haven't done this before, allow plenty of time for alignment, have
    solid mounts, and don't expect it to be like 2-meter work. Use the audio
    channel to listen for receiver quieting and get a feel for the narrow beamwidth.
    Don't try to hand hold the antennas at both ends; both must be pointed
    correctly before either end hears anything. It may be useful to use manual
    frequency control at first. If you can put one end of the link at a high
    elevation temporarily, you can power the other end from 12 volts DC in
    your car and drive around to see what microwave communication feels like.
    The experience you gain doing this will help to make you a good judge of
    final locations for the digital link. Because these are low budget systems,
    line-of-sight transmission is probably necessary for anything other than
    fairly short links. An exception would be if a good planar reflector were
    located close to one end and used as an efficient mirror. You can try this
    technique to keep the hardware at ground level using a mast or tower-mounted
    "billboard" reflector. This has environmental advantages for the hardware,
    too.

    Measurements indicate that the unit passes data
    with a low bit error rate (BER) down to signal strengths below 15-dB C/N.
    It's important to use direct paths; severe distortion can occur when multiple
    paths exist between the ends of the link. Such multipath conditions can
    cause link failure, even with very large C/N. This sensitivity should provide
    low error data transmission on a line-of-sight path of more than 40 miles
    with well-stirred air. In many locales, marine air layers and other causes
    of fading and ducting may require shortening the path to guarantee high
    linkup time.

    With the link installed at two locations 13.5
    miles apart in northern California, C/N measurements show that there is
    at least 10 to 15 dB more signal strength than the minimum necessary. This
    indicates that more than 40 miles should be possible with this hardware
    as shown. However, because longer paths are more likely to experience propagation
    anomalies and heavy rain could decrease signal strength temporarily, it's
    desirable to use slightly larger dishes for longer paths. The audio link
    has proved useful in system troubleshooting too.

    As with any ham project, there's certainly room
    for improvement. Because the original RF design was for a 2-Mbaud link,
    you should be able to improve DX by optimizing the receiver detector bandwidth.
    If you operate with a mast, the equipment needs to be waterproofed for
    all- weather use. As an alternative, you could mount a coax waveguide adapter
    to the feedhorn and locate the microwave circuitry remotely in a more protected
    environment. Use low loss semi- rigid coax cable for connecting to the
    antenna if you do so. A suitable coax waveguide adapter was described in
    an earlier article.2

    If you want to use them, almost any of the surplus
    radar detector, motion detector, or burglar alarm Gunn transceivers should
    work well. M/A-COM GunnplexersTM, although somewhat more expensive, will
    work too. They have built-in electronic tuning that permits modulating
    at higher rates for full 10-Mbaud data links or An/ uses. It should be
    possible to frequency modulate them by driving the electronic tuning input
    from the ECL output, properly scaled and offset with a resistor network.

    The first prototype of this link was built using
    24-GHz radar transceivers and metal lamp reflectors for antennas. This
    arrangement works very well. Because of the modular system design, you
    may substitute these microwave assemblies for the 10-GHz ones described
    here without making any other adjustments. The higher gain available for
    a given dish antenna size at 24 GHz can actually provide better performance
    over some paths.

    You can use a larger antenna for greater DX or
    more difficult paths. If you use something other than a 0.5 F/D reflector,
    you'll need to design a new feedhorn to produce maximum performance. Other
    diameters are available from the indicated supplier.


    Where to go from here


    Ham radio has often made use of surplus and obsolete
    gear. Many have designed and built their own equipment. This project is
    no exception. By the time you read this, there will be a PC card (designed
    primarily by K3MC) capable of two-channel, full-duplex 2.5-Mbps operation.
    You'll be able to split the channels into four half duplex if you wish.
    The card will have a V40 ľP, RAM, and Zilog 85C30 SCC devices, plus the
    usual glue logic. Bit rate will be software selectable. The card will have
    enough capacity to run as an IP router, allowing the PC to perform other
    functions while continuing to provide network access.

    We are also working to build inexpensive 250 to
    500-Kbaud 900 and 1200-MHz radios to give the individual user access to
    other hams and to a "backbone:' using this higher speed microwave hardware.
    We hope to have a fledgling moderate speed network in place in northern
    California and Colorado by the time this article goes to press. As the
    hardware is put into place, the platform for some really exciting applications
    and a whole new era of Amateur Radio becomes a reality.

    The authors would like to thank WN6I, N3EUA, K3MC,
    and N6TTO for their encouragement and perseverance during testing. We'd
    also like to thank our XYLs, Sharon and Lynn.



     


     

    REFERENCES


    1. Glenn Elmore, N6GN, "Designing A Station For
    The Microwave Bands. Parts 1-3," Ham Radio Magazine, February, June, and
    October 1988.

    2. Glenn Elmore, N6GN, "Designing A Station For
    The Microwave Bands. Part 2," Ham Radio Magazine, June 1988, page 35-37.



     


     


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