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.
Background: 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!
Note:
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:
Be as cheap as possible - but I already mentioned that!
Use "readily available" parts, that is, things that we could
likely find on the surplus market such as Ebay
Frequency-stable. The goal was that it should be +/- 10
kHz at the intended frequency over a wide range of temperature
conditions - preferably much better than this!
Enough transmit power so that it was at least possible to be
heard in the Salt Lake metro area. From our previous
experiments with transverters and our successfully having made
contact with Salt Lake area stations from the proposed beacon
location we figured that somewhere around 1 watt of RF with
10-15dBi of antenna gain would do.
Practical. We didn't want to go too overboard with
complexity such that it would never be completed!
With these in mind, Ron and I set to work and acquired the
following parts:
OCXO (Oven-Controlled Crystal Oscillator.) This
would be the main frequency control component, operating at a
lower frequency - in the area of 100 MHz.
A "Brick" oscillator. These are fairly old
technology units that have an :L-band (typically around 1.6 GHz)
oscillator that is locked to the 100-ish MHz reference and then
multiplied to the final frequency.
A weatherproof box. We wanted to put the RF
portions outside so that we would have minimal loss at 10 GHz -
just a few feet of coax and/or waveguide at the most!
A 10 GHz power amplifier. We wanted to have
"about 1 watt" of RF to send to the antenna.
An suitable antenna.
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.
The OCXO:
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.
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:
5th overtone at 96 MHz would put the crystal's fundamental at
about 19.2 MHz.
The 3rd overtone of 19.2 MHz is 57.6 MHz.
Twice 57.6 MHz is 115.2 MHz
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.
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.
Comments:
Having learned this lesson, when I later worked on the WA7GIE
beacon I determined that International Crystal already
had a formula for the Vectron 229 series of oscillators.
In order to save cost, we actually had them make a crystal with
slightly looser frequency tolerance than the original and for
this, they created a "new" formula which should still be on
their computer somewhere...
If you don't want to re-crystal the OCXO yourself, you may be
able to find someone on a microwave-related internet group that
may be willing to do it for you for a reasonable cost.
It's possible that International Crystal (ICM) may be willing to
re-work the oscillator (I don't know this for a fact,
however) - also for an additional cost!
There are other Vectron oscillators that output in the 100 MHz
range, namely the "225" series that may also be re-worked.
These oscillators are of a different design in that they start
out with a much lower frequency in - in the 5-10 MHz area - and
multiply it to the final 100 MHz-area frequency: ICM may
have the formula for these ovens as well, but I don't know this
for certain.
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.
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.
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.
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.
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.
Diagrams
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:
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.
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:
A smaller amount of frequency shift (e.g. 1.5-2.0 kHz) reduces
the amount of audible "chirp" that may result.
An upwards shift in frequency during "key down" works nicely
with the fact that on the microwave bands the convention is to
use upper sideband. If the beacon is tuned in so that the
code is copied properly, the "key-up" condition is "below zero"
in the audio passband and is not audible.
With just a 2 kHz shift, it is possible to hear both key-up
and key-down if desired as both frequencies will fit into a
standard SSB bandwidth which can be useful when monitoring the
signal strength as the S-meter can be more constant.
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.
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.
