The (former) K7RJ 10 GHz
propagation beacon

Figure 1:
The beacon mounted outdoors.  The solar panel provides power for a circulating fan when there is daylight while it and the sun shade help keep the beacon cool.  The omnidirectional slot antenna is on the upper-right portion of the image.
Click on the image for a larger version.
                  beacon, outside, on the roof with antenna and solar

As it turns out, Ron, K7RJ, had a location in extreme northwestern Utah at which he had the opportunity to locate a 10 GHz propagation beacon.  If you look at a map, you'll see that there's (almost) nothing there, so why a beacon in such a remote, sparsely-populated area of a sparsely-populated state - especially when we already have a beacon (see the WA7GIE Beacon page) located near the main population center? 

One of the great advantages of having a nice, stable, strong and reliable beacon nearby is that it provides an easy way to check that one's receiver is functioning properly and that it is somewhere near the expected frequency, but it doesn't really tell you too much if it's working as well as it should since it is so strong.  The K7RJ beacon was installed because it was known that signals from this location would be weak in the Salt Lake City metro area and be a bit of a challenge to receive.

Another reason:  Because we could, and it sounded like a fun project!

In mid-2013, Ron, K7RJ, moved from Utah, so this beacon is no longer on the air.  There are (tentative) plans to put this same equipment back on the air from another location in Utah.

The plan:

We (Ron and I) decided to build the beacon from the ground up and make it as cheap as possible, but we had the following goals:

With these in mind, Ron and I set to work and acquired the following parts:

Over a period of a period of about a year between mid 2010 and 2011, the two of us we managed to acquire most of the parts needed.


In digging around, I found a Vectron OCXO from some scrapped equipment that already had already been configured for 108 MHz, so I set to disassemble it to determine what it would take to "re-crystal" it to the desired frequency, in this case, one 108th of the target 3cm frequency of 10368.250 MHz, or an OCXO frequency of 96.00231481 MHz... approximately...  This unit was in Vectron's "229" series and operated from a single +24 VDC source, requiring about 400 milliamps when "cold" and taking 10-15 minutes to warm up to reasonable stability.
Figure 2:
The OCXO used for frequency control in the K7RJ beacon.
Click on an image for a larger version.
The ovenized oscillator used in the K7RJ beacon

These units are soldered shut, so the "frequency adjust" screw was removed to allow it to vent air to prevent it from "popping" hot solder when it was heated.  The unit was wrapped in a rag and firmly clamped upside-down in a vise while the outside edges of the end of the case with the soldered connections were quickly and carefully heated with a propane torch while firmly (but gently) tugging on 6-32 nuts spun onto the four mounting studs.  Just as the solder flowed on one side, the flame was moved about while each corner was eased up - the flame always being moved quickly about.  With a bit of care, the "bottom" portion will come off - but be careful not to yank out any wires.

If you were careful, the circuity and wiring in the "bottom" of the case (namely the 6.2 volt Zener reference and some bypass capacitors) will remain intact, but I've frequently had to re-work the circuits, usually due to something becoming un-soldered by the heat (usually a ground) or a blob of molten solder bridging across something that it should not.  Once it has cooled, it should be possible to extract the oscillator unit in its foam cocoon from the case and you should note the arrangement of the wires.  It's not unusual to break the bottom foam piece into 2 or 3 pieces, but just as long as you keep it from breaking apart even more and save all of the pieces, you'll be fine.  Once it is apart, use a hot soldering iron or soldering gun to remove "blobs" of solder that might impeded reassembly.

Typically, the OCXO itself is in a small, copper case wrapped with the oven's heating element.  On one end (usually that with the access hole for the frequency adjustment capacitor) there are blobs of silicone adhesive holding two protruding bumps of the circuit board in place and scraping these clean and then pushing on them will allow the copper case to come apart.

Inside, one will be able to see a TO-5 crystal case (e.g. one that looks like an old metal-cased transistor) and attached to it - or floating freely within the box - will be a small, round, paper tag on which is written a number - the operating temperature of the original crystal in Centigrade:  One should keep that number!

The original plan was to use the oven built into a "brick" oscillator (see below) but when it was noted that, when tested the brick with the crystal that was originally supplied (which was not at the beacon frequency) it didn't hold the frequency as closely as we wanted, we looked around.  Having already taken this OCXO apart I noted that it appeared to use the same type of TO-5 crystal.  When it came time to order the crystal,  I contacted Ron with the particulars - including the temperature of the crystal in the Vectron OCXO - and as per my specifications, he ordered a 5th overtone, series-resonant TO-5 packaged unit from International crystal with the "turnover" frequency equal to that which had been on the label attached the original crystal.  As it turned out, this crystal was about $50 and took about 2 weeks to be cut and tested and when it arrived, I eagerly installed it, having already noted the orientation as well as the lead length, spacers and and lead dressing of the original, soldering the crystal's leads only after they had been soldered to avoid damaging it from the mechanical shock.

It worked, but I observed that the frequency it output was around 115.2 MHz.

My initial thought was "Drat, it's cut wrong!" but then it something about the frequency occurred to me so I did some quick math:

A bit of probing with an oscilloscope confirmed that not only was the crystal operating at approximately half the intended frequency, but that the design of the OCXO is such that it doubles the oscillator's frequency.  What this also meant was that I'd had Ron order the right crystal for the brick oscillator, but the wrong crystal for the OCXO!

Rather than spend another $50 or so, I decided to reverse-engineer the OCXO's oscillator a bit and determined that it was of a fairly standard design with some parallel-resonant L/C circuits operating at the oscillator's output frequency intended to promote oscillation at the desired overtone while suppressing the fundamental frequency and undesired overtones, so I rummaged around, changed some parts and managed to coax the oscillator into operating at the 5th overtone frequency of about 96 MHz rather than the 3rd overtone frequency of the original design.  What had been a frequency doubler circuit was perfectly happy to run as a straight-through, tuned amplifier and a bit of smashing/stretching of the coils peaked it up at 96 MHz with an output of about +10dBm

Figure 3:
The "brick" oscillator used on the K7RJ beacon.  A thermal switch and temperature sensor are mounted to this unit as described below.
Click on the image for a larger version.
The brick oscillator used in the K7RJ beacon
I then put the two halves of the copper case back together, put new "blobs" of silicone on the nubs from which they were originally removed, allowed the silicone to cure overnight, and then put it back in the foam, taking care to orient it so that the tuning capacitor could be accessed through the hole in the case.  After this, I carefully put the bottom piece of foam (or pieces, since it broke it when taking it apart!) and firmly pressed the bottom of the OCXO back into the case.  I powered up the OCXO again and waited 20 minutes before determining if the mechanical adjustment would tune through the desired operating frequency as well as verified that the electronic tuning was working with a 6.2-ish volt output from the reference line.  Once satisfied that it was operating properly, with solder at the ready I used a torch to quickly re-seal it, re-testing the oscillator yet again once it had cooled.

At this point I went about putting the OCXO into a die-cast enclosure, along with a simple line-driver amplifier as well as a simple PIC-based keyer, the idea being that this portion of the beacon would be located indoors where the temperate was moderated somewhat to aid in stability.  The buffered and amplified output of the OCXO was to be fed to the outdoor portion of the beacon using inexpensive RG-6 TV-type coaxial cable.


Figure 4:
The waveguide slot antenna as used on the K7RJ beacon.  This antenna provides omnidirectional horizontally polarized signals with a reasonable amount of gain.
Click on the image for a slightly larger version.
Slot antenna used at the K7RJ beacon
The "Brick" oscillator:

On EvilBay, Ron found a "Brick" oscillator that was advertised as being suitable for the 10 GHz amateur band that had been made by "Magnum Microwave Corporation" and as previously noted above, this takes a low-power input in the 100 MHz range and then, using a combination of a locked oscillator and a diode multiplier, yields the final output at a level of about +5 to +13 dBm - suitable for driving a power amplifier.  Fortunately, this oscillator had been originally designed for operation near the 10 GHz amateur band (10.45-10.75 GHz) so little difficulty was expected in retuning it.

As noted above, the original idea was to get a "Brick" oscillator with a built-in crystal oven and use it to control the transmitter frequency, but Ron was able to determine, using the original crystal that shipped with the brick, that this oven wasn't going to be suitable for our application since it kept the oscillator only within a few 10's of kHz of the desired output frequency over a wide temperature range - probably well within its original specifications, but not good enough for us!

Removing and un-powering the oven to reduce current consumption, the original "Xtal Out" connector that had provided a sample of the crystal oscillator's frequency was re-purposed and using a blocking capacitor, was reconnected to the side of the crystal socket that, when driven with an external signal generator, caused the "brick" to lock to it, essentially converting the brick to an "external" reference unit.  Using that same signal generator, operating near the frequency of the original crystal, we then checked the lock range of the brick itself and found that it very closely matched the frequency range on the label in that it was barely working at the intended transmit frequency around 10.368 Hz. 

Fortunately, the oscillator portions of these bricks are fairly easy to retune via a screw on the side so we first set the generator so that it operated at the original crystal  frequency and measured the voltage on the "Phase Lock" pin to determine where the likely optimal center frequency and tuning conditions would be.  Then, putting in the desired frequency of 96.00023 MHz from the signal generator, the main oscillator was re-tuned so that it produced an output at approximately 10368.250 MHz with the same voltage on the "Phase Lock" pin that had been present at the original frequency.

In the next step, we then moved the 96 MHz frequency from the signal generator up and down to see what the locking range of the brick was at its new frequency and found that 10368 MHz was approximately in the middle of it.  We also observed that the filter within the brick oscillator, while it started to drop off just a little bit at 10368 MHz, was "close enough" and put out the proper amount of power (+7 to +10dBm) that we did not have to re-tune the filters!  While I have retuned the filter sections in these brick oscillators in the past, it's a task that I was happy to avoid!

These "brick" oscillators, being fairly old technology, have the downside is that they typically require a negative supply in the -20 to -24 volt range and consume 250-400 mA with the crystal oven disabled, depending on the model and make.  For this, we knew that we would need to construct an appropriately regulated power supply.

The 10 GHz power amplifier:

Originally, we anticipated that we' have to "bite the bullet" and spend several hundred dollars to get a 1-watt amplifier unless we were lucky enough to find a suitable device on EvilBay.  While we were keeping our eye out for one I happened across some defective transceiver modules intended for use at Ku band on satellite terminals and was able to extract the amplifier module, power it up and then test it.  As luck would have it these modules proved to operate satisfactorily at the 10368 MHz operating frequency and providing between 0.5 and 2 watts of output with less than +2dBm input, the power level depending on the individual unit and model number.

The use of this amplifier unit is described on the web page "A 10 GHz power amplifier using a VSAT power module" located on this site.

The weatherproof box:

This turned out to be one of the first things that we ran across.  A few years before, a friend had given me a nice, waterproof fiberglass box that had some minor damage, probably from having been dropped:  One of the lower-corner mounting tabs and been broken off - but there was no damage to anything that would compromise its structural integrity or its "waterproof-ness."  Fortunately, it appeared as though it would be about the right size for this project so I handed over to Ron who worked around the broken tab and mounted to the rear of it a large, aluminum plate that accommodated clamps to allow the entire box to be attached to a mast.

Figure 5:
The power supply modules, mounted in the door of the box.
Top right:  The amplifier control board
Top Left:  +8 volt switching regulator for the power amplifier
Center-left:  The -20 volt switching regulator for the "brick"
Bottom-left:  AC-to-DC rectifiers and unregulated supplies.
Click on the image for a larger version.
                supply and interface modules in the door of the beacon
The antenna:
A few years before, one of our local microwave group, Dale, WJ7L, made several omnidirectional waveguide slot antennas.  This antenna, about 2 feet long, offered about 13-15 dBi gain in a "flat" pattern and over the years these antennas had been successfully used both during contests and permanently installed at home stations and Dale graciously provided an antenna for the project.

With the open "slots" these antennas are not inherently waterproof.  For a while, Ron had one of these antennas at his QTH and had placed it inside a "radome" (a protective cover) made from white PVC pipe (which turned out to offer minimal loss) so this scheme was used by several others with success.  One problem that Ron had was that in the winter, the inside of the antenna accumulated moisture due to condensation and the water droplets gathered at the bottom in the coax-to-waveguide transition, rendering the antenna useless until it could either be cleaned out or until it evaporated!

When the weather permitted, Ron removed the antenna and, using some polyimide (a.k.a. "Kapton" tm) tape, we sealed the slots and the waveguide opening at the bottom of the antenna.  Over the next week or so that the antenna was laying around it was observed that the tape covering the waveguide opening would either bulge outwards or inwards being affected by the change in barometric pressure and also indicating that the antenna was now hermetically sealed!  Since it was winter when it was sealed up - a time during which the relative humidity here in Utah is in the 15%-20% range, we also knew that the air sealed within was going to be fairly dry as well.

When it came time to install this beacon, the already-proven antenna was put into service!

Power supplies:

For operating the beacon, we decided in a somewhat low-tech approach:  Run everything from 24 volts AC.  By feeding this voltage up to the beacon, the effects of the ohmic losses of the control/power cable could be minimized and both positive and negative voltages could be extracted from the AC source.  All we needed was to provide the +8 volts for the power amplifier and -20 volts for the brick oscillator.

The original power supply for the brick oscillator was fairly simple:  An LM337 adjustable negative regulator set to -20 volts, fed with the rectified negative voltage from the 24 VAC supply.  For the +8 volts, things were a bit more complicated, particularly since we knew that we needed to minimize the heat load inside the box so a simple switching supply was constructed using an LM2575-5 "Simple Switcher tm" 5 volt regulator that was re-biased for +8 volts.  This supply provided fairly high efficiency (around 85%-90%) and with its extensive input and output filtering, clean power.  Connected to the +8 supply was the servo controller and protection circuitry for the power amplifier described in the link mentioned above.

Putting it all on the box:

There was about 1 year between the time that the oscillator unit had been assembled and when we started putting the beacon together and in the interim, Ron had kept the unit with the oven-controlled oscillator powered up most of this time, both to do long-term testing and to (hopefully) help the crystal and the oscillator components age more thoroughly.  From the time it was brand new and "green", the crystal drifted down in frequency 20-30 kHz (at the 10 GHz transmit frequency) and as these things do, finally settled in, with the drift due to aging being undetectable amongst the minor variations due to temperature.  Using both the "coarse" tuning of the OCXO's tuning capacitor and the electronic tuning, the oscillator was then brought back on frequency.
Figure 6:
The "other" side of the box, showing the brick oscillator (above) and the 10 GHz power amplifier (below).  At the very bottom can be seen the solar-powered fan used to circulate air within the box.
Click on the image for a larger version.
Brick and
                power amplifier

In the fall of 2011, both Ron's and my schedule meshed and we got together to do a marathon session of putting the beacon together, the first task being to mount the various modules in the box, starting with the various power supplies.  Finding a chunk of aluminum plate, this was cut to size and on it were mounted the various modules:  This box did not have any attachment points in the door, so it was initially required that we just set the plate into position and Ron would later install some attachment points - but it would be good enough to test.

The main portion (the rear) of the box was intended to allow the attachment of a backplane, but since this was missing from the box when we acquired it we cut another piece of aluminum and to it, we mounted the brick oscillator and the 1 watt power amplifier, using the plate as a heat sink. At this point we also made a short jumper cable from 0.141" coaxial cable that connected the SMA output of the amplifier to a bulkhead-mount N-type connector to which the waveguide and antenna would be attached.

"Thermal management" problems:

At the end of the day, we got everything in the box and operating, but we started to realize that we had a problem:  Getting rid of the heat!  Carefully, we closed the lid on the cables and allowed the beacon to sit overnight and monitor the internal temperature.  In the 70F (21C) room, it was observed that inside the beacon enclosure the temperature had risen to about 155F (68C) - and that was with the door open slightly to allow air to escape!  Clearly, with this beacon installed outside, in the sun, in the Utah desert where the temperature could easily reach 105F (41C) the internal temperature would be much higher and these kinds of temperatures would surely damage the equipment within!

Immediately we decided that we did not want to have any sort of thermal control system that would rely entirely on fans as these would be a liability as they would certainly last only a couple of years.  Since the goal was now to make the heat management as passive as possible, we did a bit of brainstorming and came up with a solution.

Ron was able to find a thick plate of aluminum and cut out the backside of the box to its size.  Since there was already an aluminum plate for mounting to the mast, it and this new plate were bolted together and the back of the box sealed up and made waterproof with RTV adhesive and to the interior portion of this plate the brick oscillator was mounted.  The location of the 10 GHz power amplifier was below the edge of the plate, so another chunk of thick aluminum was cut, the amplifier mounted to it and this new plate was then bolted to the larger plate with heat-transfer compound as well.

The result of this was that the majority of the heat generation (the power amplifier and the brick) were now connected to a large, thick aluminum plate that passively transferred the heat from the inside of the box to the outside, on the back, facing north and away from the sun.  In addition to this passive heat transfer, the -20 volt supply was rebuilt from the original LM337 linear regulator to a switching converter so that the several watts dissipated in the former linear regulation would be greatly reduced and further-minimize the heat load.

The result?  On a hot (100+ degree F) sunny summer day, the passive heat transfer system yielded an interior temperature of about 160F (71C).  To improve this even more, Ron constructed a sun shield using a solar panel that also drove a small DC fan to move air around inside the unit.  While adding the heat shield alone resulted in a 15-20F (8-15C) temperature drop, the fan lowered this even more, with the ultimate result being that on a 100F day, the interior temperature was around 130F (54C) or so.  It's worth mentioning that throughout, we used high-temperature (105C) low ESR capacitors so even temperatures in the 150F (65C) range (e.g. that which might occur if the fan were to fail) could be tolerated for extended periods.

As an additional protection against high temperature Ron mounted a "Klixon" tm thermal switch to the brick oscillator so that if its temperature exceeded 165F (74C) or so power would be cut off from the beacon until the temperature dropped below the "cut in" temperature, which is probably in the 150F-160F range.

Since it was permanently installed in early 2012, it has been on the air continuously, having survived the long, hot summer.  Telemetry has indicated that the temperature of the brick itself has maintained a safe level even on hot, sunny days!

Figure 7:
Diagrams of the various modules comprising the K7RJ 10 GHz beacon
Top Left:  OCXO and keying interface, line driver and power supply.
Top Right:  The +8 volt switching regulator for the power amplifier.
Center Left:  The beacon controller with 3 temperature sensors.
Center Right:  The -20 volt switching converter.
Bottom Left:  The unregulated DC supply.
Bottom Right:   The physical layout of the -20 volt switching regulator.
Click on an image for a larger version.
Oscillator and controller for the K7RJ beacon
8 volt switching converter for the K7RJ beacon
Beacon controller with 3 temperature sensors - K7RJ
-20 volt switching regulator for the brick
                oscillator - K7RJ beacon
Unregulated DC supply for the K7RJ beacon
Layout of the -20 volt switching regulator - K7RJ


OCXO, power supply, line driver and keying interface (e.g. the "Indoor Unit"):

The upper left drawing in Figure 7 shows the connection of the OCXO, its +24 volt regulated supply and the line amplifier.

The 24 volts AC enters via a feedthrough capacitor to choke RF, followed by an RF choke to do the same.  Following this are a pair of 4.7nF bypass capacitors to eliminate the possibility of the rectifier, D1, from generating noise.  After this is R1 which is used to drop the voltage slightly to assure that the rectified and filtered voltage is within the safe zone for C2, C3 and U1, the 24 volt regulator, a 7824.  C4 and C5 are used to guarantee stability of the regulator.

The OCXO itself is a Vectron "229" series unit that has been rebuilt as noted above so that it outputs a frequency at 1/108th of the ultimate transmit frequency of 10368.250 MHz which requires an oscillator frequency of 96.00231481 MHz.  Operating from +24 volts DC, this unit pulls about 400 mA when cold and takes 10-15 minutes to achieve reasonable stability and accuracy.  This oscillator has a electronic frequency control which is provided by an internal 6.2 volt Zener reference and an electronic frequency tuning line which is brought out and applied to a multi-turn adjustment potentiometer to provide a frequency adjustment range on the order of 25kHz-40 kHz.  Applied to this tuning line is the FSCW (Frequency-Shifted CW) keying line via R4 which comes from the keyer (see below).  It was determined that both C14 and C6 were required to "clean up" the frequency stability of the oscillator due in part to low-frequency noise from the onboard Zener regulator as well as possible pick-up of low-level noise from the keyer itself.  Without these components the note would be distinctly more "wobbly" and slightly "hissy."

In testing it was determined that the OCXO itself was slightly susceptible to "pulling", moving several kHz (at the 10 GHz frequency) as the load varied on its output and to this end, a 3dB attenuator pad (R5-R7) is employed followed by an emitter-follower amplifier consisting of Q1.  Because the 96 MHz energy is conveyed via coaxial cable to the beacon, it was decided that a fairly high level (+15 to +20dBm) was to be used so that it could be attenuated at the beacon itself to the appropriate level.  Consisting of Q2, this tuned amplifier provides this amount of output and in practice, a 3dB 75 ohm attenuator (TV-type) is used at the output of the oscillator to assure a proper 75 ohm source impedance.

Because RG-6 TV-type coaxial cable was used to convey the signal to the beacon, a chassis-mount "F" connector was fitted and at the other end, inside the box, a right-angle BNC adapter was used followed by a BNC-male to F-female adapter - along with a 10dB pad - to the input of the brick oscillator to set it to approximately +5dBm and to terminate the other end of the cable.

+8 volt switching converter

The upper right drawing in Figure 7 shows the +8 volt switching converter.

Because a 24 volt AC supply is used for the beacon, a 24-35 volt DC source is available and to use a linear regulator to provide the 800 mA or so at 8 volts for the 10 GHz power amplifier would result in between 13 and 22 watts of heat to be generated inside the box!  Because thermal management was already a concern, a switching down-converting voltage regulator was built using an LM2575-5 "Simple Switcher tm" regulator resulting in a conversion efficiency of 85% or better, or dropping the heat generation to the 3 watt level!  Because the 5 volt version of the LM2575 was what was on hand, R301 and R302 rescale it for about 8.2 volts with the expectation that the extra 0.2 volts will be lost in the wiring and power controller by the time it gets to the amplifier.

Because we are mixing switching regulators with RF, we need to be certain that we don't allow switching energy to get where it shouldn't, so C301 and L301 keep such energy from being conducted on the voltage input line while L303 and C304 keep it from finding its way on the output line and into the 10 GHz power amplifier.

10 GHz power amplifier and controller

As noted above, the power amplifier and the circuit that controls and protects it are described on another web page at this site see:
 A 10 GHz power amplifier using a VSAT power module

Figure 8:
A look inside the indoor unit.  To the upper left is the OCXO while below it is the line amplifier.  On the upper-left is the PIC-based beacon keyer.
Click on the image for a larger version.
Inside the
                indoor unit
Beacon controller

The center left drawing in Figure 7 shows the beacon controller.

For generating the keying signal a simple, keyer based on an 8-pin PIC is used.  When I wrote this code I wanted it to be as flexible as possible and have produced several different versions on different beacons that I've helped construct, but they all have one common trait:  In FSCW mode they all output differential keying.  This is a fancy way of saying that when one of the keying outputs goes high (say, pin 2) the other output (pin 3) goes low in voltage and vice-versa.

Connected across the keying line is a 10k potentiometer and with this differential keying, the net result - if the potentiometer is set exactly in the middle, the voltage on the wiper will always be one-half of the supply voltage (2.5 volts in this case) no matter what the keying state may be.  The advantage to this is that simply by adjusting this potentiometer, one may select both the magnitude (amount of shift for the FSCW) and the sign (whether a "key-down" is a higher or lower frequency) with one simple adjustment.

Our preference has been to set the magnitude (amount of shift) of the keying to something on the order of 1.5-2.0 kHz while the sign of the keying is such that a "key-down" condition causes a shift upwards in frequency.  The reason for doing it this way are three-fold:

In addition to keying, the differential lines also connect to a 2-leaded dual-color LED and key-up/key-down is indicated by a red or green LED as desired.  Of course, one could use just a single LED or two separate LEDs for this indication!  The advantage of the dual-color LED it is always on no matter what the keying state so that one could easily tell if it was powered up - plus it required only one hole to be drilled.  For convenience, Ron also connected a piezo beeper with an on/off switch to one of the keying outputs so that an audible monitor could be turned on, making it easier to copy code than watching the LED!

The PIC used also has an A/D converter with multiple inputs and these are used to read the voltage from three LM335 temperature sensors.  These sensors output a voltage that is proportional to their temperature (10 mV per Kelvin degree) and the PIC reads this and converts them to Fahrenheit and includes these readings in the beacon's message to provide a temperature reading of inside the building, outside, and the temperature of the brick oscillator in the beacon enclosure itself.
Figure 9:
The indoor unit, with the 24 volt "wall wart" and terminal block connecting the control cable.
Click on the image for a larger version.
The indoor
                unit with power supply

Note:  If you are interested in obtaining a customized PIC for building a similar keyer, please contact me using the email address at the bottom of this page.

-20 volt switching converter:

The center-right diagram in Figure 7 has the diagram of the -20 volt converter while the bottom right diagram shows the layout.

As with the case of the +8 volt converter, the use of a linear regulator such as the LM337 caused excess heat to be generated within the enclosure - in this case, about 3-6 watts:  Using a switching-type regulator, this was reduced to well under 1 watt.

For this circuit, an LM2577-12 regulator (the 12 volt version) was used since the "-Adj" version was not available at the time of construction.   Since the "-12" version was all that was available, the "dropout" voltage (e.g. the minimum difference between the input and output voltage) would normally have been about 15 volts had the diagram on the LM2577 data sheet been exactly followed so a simple comparator was constructed using Q1, Q2 and the associated components.  If the output is not "negative enough", Q2 stops conducting and the '2577's output voltage increases whereas if the voltage is "too negative", the regulator is cut off:  The ratio of R4 and R5 (1.8) along with the fixed 12 volt setting of the LM2577-12 effectively multiply the result, yielding about -20 volts.

As with the +8 volt regulator, it's important to make sure that the switching energy is removed and C1/L1 keep it from being conducted on the input voltage line while L3 and C6 keep it from finding its way into the brick oscillator's -20 volt line.

The -20 volt switching supply - as well as the +8 volt supply - were built "dead-bug" on a piece of double-sided copper-clad circuit board and "islands" of copper were cut out to isolate the various sections and along the edges, the various ground busses were tied to the backside of the circuit board with soldered copper foil to reduce ohmic losses.  The bottom right image in figure 7 shows the layout of this board along with the location of the parts and the sections of copper that were isolated.  Constructing the power supply in this way is quick, cheap and easy and it allows for very easy modification while the solid copper ground busses allow the switching supply to operate more "cleanly" since the likelihood of significant I*R drops across the board minimize the likelihood of switching transients from being conducted on the DC input and output lines.  Another advantage of this technique is that since the rear of the board is just ground, it can be flush-mounted to the metal backplane - which is also ground - doesn't need any standoffs and it can dissipate what little heat is produced by the regulator.

Unregulated DC supply:

The bottom-left diagram in Figure 7 has the diagram of the unregulated DC supply.

A 24 volt AC source is used because it makes it easy to produce both the positive and negative voltages needed for the beacon.  Using a transformer-type "wall wart" one side of the 24 volt supply is declared to be "ground" (AC Low) and the other side is half-wave rectified as needed in both the indoor beacon controller and within the outdoor beacon enclosure.  Another advantage of the 24 volts is that this higher voltage, along with the switching converters, reduces the current and makes the resistive losses of the control cable less important.

In the beacon itself, the 24 volt AC input is passed through a bifilar choke to remove any stray switching supply components that might find their way from the +8 and -20 volt converters and be conducted or radiated on the control cable.  This choke - along with the capacitors C1 and C2 - also provide a degree of lightning protection as they will effectively quash transients that appear on the cable.

From the bifilar inductor, the "high" side of the AC line goes to a bridge rectifier which is used as a pair of half-wave rectifiers to provide both positive and negative DC rails on the order of 30 volts.  When it was constructed, a 4-amp bridge rectifier was handy and both "sides" of it were used, but a pair of ordinary 4-10 amp, >=200 volt diodes could have been used instead.

YouTube video about the former K7RJ 10 GHz microwave beacon in grid DN31it in remote northwestern Utah.

The results:

This beacon was put on the air permanently in early 2012 and has been heard throughout the Wasatch Front from Salt Lake county northwards.  As expected, it provides a weak signal source for the 10 GHz operators in the Salt Lake area, largely via an indirect "bounce" off the tops of the Wasatch mountains to the east of Salt Lake which are in direct line-of-sight of the beacon itself, some 100 miles (160km) away.

During the initial testing phases, we had the problems of getting rid of the internal heat as well as the yet-unexplained failure of the first 10 GHz power amplifier that we'd put together which had failed on the workbench after having operated for weeks outside, in the summer heat.  After chasing down these minor bugs, it's been solid as a rock.

Because the crystal was already about 18 months old when the beacon was permanently installed, and because the oscillator had been powered up during most of this time, the beacon's frequency has drifted less than 2 kHz from where it was originally.  Indications are that it moves less than 1.5 kHz between winter and summer temperature extremes making it relatively easy to find on a well-calibrated receiver.

If there is interest in obtaining a PIC for your beacon project or if you have other questions about this beacon, please contact me using the email link below.

Go to the KA7OEI microwave page, or go to the KA7OEI main page.

If you have additional questions, you may send email using the link at this page.

This page updated on 20131126

Since 1/2013: