ARRL 2008 "10 GHz and Up" contest
August 20, 2008 (UTC)
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The excuse:
As with last year we decided to go out into the field and throw signals at each other, using the annual ARRL "10 GHz and up" contest as an excuse to do so. Again, we were going to do some optical ("lightbeam") communications as well as other bands.
Unlike last year, the weather wasn't particularly threatening as there wasn't any need to repeatedly take refuge from fierce, fast-moving thunderstorms! Somewhat like last year, however, it was pretty hazy - again, due to wildfires elsewhere in the western U.S. but the density of the haze was nothing like it had been last year! From the Nebo site, it was possible to pick out the 107-mile distant outline of Willard Peak, immediately south of Inspiration Point with the naked eye - if one knew where to look!
Contacts on "low" frequencies:
The locations chosen were the same as those used last year - and why not? They were separated by a respectable distance (107.04 miles) and we were familiar with the locations, and we weren't trying to be original, but make a few contacts. This time, too, since the weather was more cooperative, we decided to make time to make attempts to work on other "low frequency" bands - such as 10 and 24 GHz!
Being that this was the first weekend of a two-weekend ARRL event, there were several others out-and-about, armed with microwave gear. While there wasn't exactly a pileup, there was about as much activity on 10 GHz this time as there ever has been around here.
Not too long after I arrived at the Nebo site, Ron, K7RJ along with his wife Elaine, N7BDZ and Robb, N0KGM arrived at Inspiration point and began to set up, with Robb documenting the event on video. A few minutes after an unsuccessful attempt by Dale, WJ7L and I to make contact, I heard a burst of off-frequency SSB. Quickly retuning, I heard Ron's signal blasting through. Quickly re-peaking my antenna - an 18" DSS satellite dish - I called him and he came back, reporting that he was using just a 17dBi gain horn: This wasn't too surprising, as we already knew that out path was line-of-sight!
As Ron put it:
"Utah has a small, but enthusiastic microwave presence. I operated the [evening] of August 17, 2008 [UTC] from a mountain ridge in the Utah Wasatch mountains at about 9000 ft. elevation, which is about 5000 feet above the average terrain. One contact was to an 80 km distant station (WJ7L) that used a homemade omnidirectional slot antenna from his home. He was warm in his shack while we were getting cold on the mountain about a mile higher in the air! The other 10 GHz SSB and wideband FM contacts were from distant mountain sites, ( WA7GIE and KA7OEI) We tried 24 GHz, but we still have some equipment issues."
Shortly after arriving on-site, Dale appeared on 2 meters and
we had tried to make a QSO on a frequency 10 kHz below that of
the WA7GIE beacon. Unfortunately, with a lot of dirt between
us (mountains entirely blocked my view of the Salt Lake
Valley) I heard nothing from Dale. In the meantime, Dale
and Bryan, W7CBM, managed to work each other across the valley
with no great difficulty.
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Swinging my antenna toward him and peaking, we more-or-less pegged each other's FT-817's S-meters and we quickly turned our attention to having Ron and Dave try to work each other. At about 128 miles - with mountains in the way - the first few attempts were unsuccessful until a strategy was decided: Being very near the 11,900+ ft. Mount Nebo, I would transmit a signal on which both Dave and Ron would peak their dishes, hoping for a reflection or refraction of some sort from the Nebo mass. This strategy worked and, despite a bit of random, deep QSB, they managed to work each other over the non line-of-sight path during the occasional, strong signal peaks.
Robb put a short video documenting Ron's 10GHz activities at Inspiration Point on YouTube:
- YouTube link: Ron
working 10 GHz SSB in Inspiration Point
As it was starting to get dark, we switched over to wideband 10 GHz FM. Using the DSS dish at my end and with Ron using his 17dBi horn, we were able to work each other, verifying that our WFM gear was actually working, so we switched to 24 GHz WFM, but with no results: Not having proven our 24 gig gear as much as we had our 10 gig gear, we weren't entirely surprised - but this left us with further work to do!
QSYing to the "Red" band:
After having made as many 10 gig contacts as we could - and now that it was getting dark - we turned our attention toward operation on the "Red" band. In the darkness it took a few minutes to disassemble the microwave gear and configure the optical gear, but before too long, we managed to "rough-in" each other's beams and a few more minutes to "fine-tune" each end's aiming.
This year, we each used exactly the same gear as last year to complete the two-way optical QSO - see below: Both sides used high-power (3-watt) red LEDs and large, plastic Fresnel lenses. Even though it was quite hazy, it was much-less so than last year so we had fairly good-signals end-to-end with relatively little fading.
Audio clip:
- LED reception at Inspiration point, audio file - 1:31, Stereo MP3, 1.38MB Note that the use of short duration (<30 second or 10%) music clips is considered to be acceptable fair use under current interpretations of U.S. Copyright law. (Music: "Children" [Dream Version] from the album "Dreamland" by Robert Miles)
- Initial exchange after setting up the optical link.
- Left channel: Transmitted audio from Nebo
- Right channel: Audio received from Inspiration at Nebo.
- Nebo audio heard in the right channel is mostly from Rayleigh/dust scattering of the transmitted beam - plus a little bit of receive audio retransmitted from the far end.
- Because of the haze in the air, signals were significantly reduced from what they would have been with crystal-clear air.
- We have observed that with haze, scintillation is usually greatly reduced, as is apparent from this recording if it is compared with the "clear air" recording made last year. See this page for some of those audio files.
Robb put together another short video documenting Ron's 24 GHz attempt and initial "Red-Band" activities at Inspiration Point on YouTube:
- YouTube link: Ron on 24 GHz and the "Red" band
- Please note: When it gets dark - as it was when
this video was shot - it becomes rather difficult to shoot
video!
This year, we decided to try something that we didn't get a chance to do last year due to weather and/or time constraints: Make the 107 mile path using cheap, standard laser pointers. For a number of reasons lasers, using small apertures, aren't particularly well-suited for high-quality optical communications over long paths - see this page for an explanation of this phenomenon. Somewhat fortunately, with the significant haze present, scintillation was significantly reduced, but as you can hear from the following audio clip is still very apparent:
Audio clip:
- Laser Pointer reception at Nebo, audio file - 1:04, MP3, 980kB Note that the use of short duration (<30 second or 10%) music clips is considered to be acceptable fair use under current interpretations of U.S. Copyright law. (Music: Theme song of the movie Dark Star by John Carpenter)
- For both ends, the already-aligned optical receivers for the LED QSO were used.
- This is a "2-channel Mono" recording from the receiver at Inspiration point only.
- The occasional "squeak" that is heard is from a long-range
FAA RADAR, its RF getting into the optical receiver's front
end.
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Quite apparent in this audio clip is a sort of "rumbling hiss" caused by the scintillation of the laser's light: Measurements indicate that there is at least 40dB of scintillation present on the audio, but the redundant nature of human speech and the brevity of the most severe of these "dips" in amplitude still allow good intelligibility, albeit with rather poor audio quality.
Aiming the laser pointers - the challenge:
While the aiming of the LEDs is fairly easy, we knew from past experience that aiming even rather poorly-collimated laser pointers was a significant challenge. After prior arduous and hair-pulling attempts in aiming lasers (and the LEDs!) an "audible S-Meter" system was devised where an audible tone was used to convey the strength of the received optical signal - modulated with a 1 kHz "alignment tone" - via pitch of the audible S-meter: The operation is simple: The higher the pitch, the better the signal! This device allows much simpler aiming as it is a pitch - subtle differences of which are much more-easily discerned by the human auditory system than absolute amplitude and as such can be readily transmitted across a radio or optical link. Additionally, the response of this tone is instantaneous: If one is listening to the receiving end's audible S-meter - say, via a radio link - and one even briefly "flashes" the laser across the detector at the receive end, that burst of modulated light at the receiving end will be instantly heard as a "pip". Needless to say, with such instant feedback the aiming is greatly simplified as it becomes very practical to perform a manual "scan" to determine the rough aiming!
Having a good system for alignment is one thing, but actually aligning a laser is another problem! For this task, I simply mounted my laser pointer to the camera mount atop my 8" reflector telescope, using its polar mount and verniers to provide both a stable mount and fine-tuning.
What if you don't happen to have a stable telescope mount handy?
Past experience had shown that even with a reasonably good-quality photographic tripod, it was not practical to point with the finesse and precision required to aim even a cheap laser pointer! The main problem with a tripod is that when one attempts move it a very small amount (fractions of degrees) it's difficult to gauge exactly how far it actually moved - which makes repeatable or proportional motions practically impossible! To make matters worse, most tripods have a viscous grease (e.g. "fluid head") that provides smooth movement for photographic purposes, but provides unpredictable amounts of backlash when very tiny changes are attempted - especially at cold, mountaintop temperatures!
Something else had to be devised, so Ron took up the challenge. The result can be seen in Figure 3. This device mounts to a standard tripod, but using bolts, precise Azimuth and Elevation adjustments can be made after initial "rough aiming" with the tripod and locking it down.
This device, made in a single evening from scraps of plastic and hardware that Ron had laying around, looks crude - but it works very well:
- Pieces of plastic are used as "hinges" on the moving blocks to maintain Az/El orthogonality.
- Tee nuts (not visible) are pressed into the plastic to provide threads for the adjustment bolts.
- Rubber bands are used as "springs" to provide a "return" tension for the adjustments - as well as to mount the laser pointer module.
Comment:
There is actually a web page
that describes using laser pointers for long-distance optical
communications in some detail, and it's called "Using Laser Pointers for Free-Space
Optical Communications".
After several hours of shooting microwaves and photons at each other, we all decided that it was getting cold and late, so we threw our gear in our vehicles and headed back...
Additional details:
I'd like to thank those that helped, including:
- Ron, K7RJ.
- Elaine, N7BDZ, Ron's much better half, who took the photos at Inspiration point.
- Robb, N0KGM, documenting things at Inspiration point on Video.
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At the south end of the QSO:
Present: Clint, KA7OEI.
Location: Along the Mt. Nebo Scenic Loop Road that goes between Payson and Birdseye, Utah. This location is about 525 feet southwest of the one used during the August 18th, 2007 expedition.
WGS84 coordinates: 39°, 51' 16.9" North, 111°, 42' 14.7" West, Altitude was 9406' (2867 meters) according to GPS.
Grid square: DM49du
At the north end of the QSO:
Present: Ron, K7RJ with his wife Elaine, N7BDZ, and Robb, N0KGM
Location: A place called "Inspiration Point" that is slightly north and west of Willard Peak, which is north of the city of North Ogden, Utah - the same place as last time
WGS84 coordinates: 41°, 23' 26.6" North, 111°, 59' 9.6" West. I don't have Ron's GPS reading for the altitude, but according to the USGS topographical maps, the altitude is almost exactly 9400 feet (2866 meters).
Grid square: DN41aj
Distance:
The calculated distance (as a crow flies) using the Haversine method is 107.09 mi. (172.34km) using the RadioMobile program version 8.0.5. This is about 230 feet (70 meters) farther than the August 18th expedition.
Other path statistics:
- South-to-North azimuth: 352.1° (true)
- Elevation angle at each end: Approximately -0.77°. Because our altitudes were pretty much the same, this is downward angle is due to Earth curvature.
- North-to-South azimuth: 172.0° (true) This is slightly different
than 180 degrees from the reciprocal bearing due to
rounding off.
- Maximum difference in elevation along path: Approximately 5150 ft. (1570 meters)
Inspiration Point:
- For 10 GHz narrowband (SSB) use, Ron used a DEMI (DownEast Microwave) transverter using the internal local oscillator reference. This unit converts the 10 GHz signals to/from the 2-meter band.
- Ron has since "upgraded" the local oscillator on his
transverter to use an N5AC synthesized local oscillator
board, allowing the use of more-stable 10 MHz frequency
references - which "fixes" the drift problem that he had
been experiencing during this event.
- The "IF" radio used was a Yaesu FT-817.
- Ron used both the "bare" horn antenna (about 17dBi gain) as well as a horn-dish antenna combination, providing about 30 dBi gain on the transverter.
- For 10 GHz "wideband" (FM) he used a varactor-tuned Gunn transceiver along with a homebrew controller and a 30 MHz IF receiver. Similar gear was used for the 24 GHz attempts.
- Clint's 10 GHz transverter is assembled mostly from
modified parts, using a commercial diode-ring mixer, a
re-tuned interdigital bandpass filter, a modified "brick"
oscillator and a 5 MHz oven-controlled oscillator, a pair of
SMA relays, and a DEMI 2-watt power amplifier from a kit,
and a military surplus GaAsFET microwave preamp.
Homebrew components included a 70cm GaAsFET post-mixer
amplifier, control logic, and a VCXO-based synthesizer to
generate a signal to lock the "brick" oscillator to
frequency. For a link showing the 10 GHz gear, go
here.
- An FT-817 was used for the IF at this end as well.
- The 10 and 24 GHz wideband (FM) gear was also homebrew, using Gunn transceivers, a computerized frequency controller and a 30 MHz IF receiver.
- As with Ron's station, a 18" DSS antenna and standard
"bare" horns were used for contacts.
Equipment common to both sides of the QSO:
- The LED was amplitude modulated with a current-linear
modulator with a resting current of 1.1 amps. Details
of the modulator are here: LED_linear_modulator.html
- The transmit LED in both cases was a Red Luxeon III emitter module (Lumileds M/N: LXHL-PD09) epoxied to a heat sink.
- The optical receivers were my "version 3" design, described here: optical_rx1.html#ka7oei_rx_ver3 with both receivers using BPW34 photodiodes.
- Audio interface units, incorporating audio amplifiers,
audio recorder interface, audible S-meter, and a few other
features were used - details are here: optical_comm_audio_interface_device.html
- Both transceivers have separate and identical TX and RX lenses mounted side-by-side.
- Digital audio recordings were done on both ends of the
path using Insignia NS-DV4G portable audio players,
recording using an uncompressed .WAV format.
- This enclosure for the LED transceiver is described in detail here: Optical_enclosure_first_version.html
- Lens size: Unmounted, the Fresnel Lenses are 250mm x 318mm and have a focal length of 318mm. The mounting frames vignette the lenses by about 10mm in each dimension, so the available lens area is about 240mm x 308mm. Each lens is protected by a sheet of Plexiglas and the front surface has been coated with a protective polymer to prevent scratching and moisture accumulation.
- For optimal far-field optical flux density, a glass PCX (Plano-ConveX) lens is used in front of the LED to appropriately illuminate the Fresnel, the LED-Lens distance being set empirically for best output.
- Ron used an inexpensive laser pointer, pulse-width
modulated by a circuit of his own design, mounted to a
photographic tripod along with the device shown in Figure
3. For reception, the already-aimed
Fresnel-based optical receiver was used.
Optical gear used on the South-to-North link:
- The enclosure for the LED transceiver is described
here: Optical_enclosure_foldable_version.html
- Lens size: Unmounted, the Fresnel lenses are 404mm x 430mm and have a focal length of 229 mm. The mounting frames vignette the lens by about 10mm in each dimension, so the available lens area is about 394mm x 420mm. Each lens is protected by a sheet of Plexiglas and the front surface has been coated with a protective polymer to prevent scratching and moisture accumulation.
- For optimal far-field optical flux density, an optical acrylic DCX (Double-ConveX) lens was reground to an aspherical shape to provide optimal illumination of the Fresnel. This turned out to be necessary owing to the very short focal length of the lens that made it difficult to efficiently illuminate the lens. After adjustment, this LED/Lens combination produces about 25% higher far-field flux than the other assembly, with an almost identical half-power beamwidth.
- An inexpensive laser pointer was used, mounted to the
telescope to allow precise Az/El adjustment. For
modulation of the Laser, the Pulse-Width
modulator was used, while the
already-deployed Fresnel Lens receiver was used..
Notes about the audio clips on this page:
- The audio clips on this page have been edited to remove "dead" time and irrelevant bits of dialog. This editing has been done solely to make them more "listenable" and to keep the file sizes manageable.
- In the audio clips, amplitude and gain adjustments have been made to improve listenability. At the time of the actual event, the volume control was used to similar effect for the benefit of the local listeners.
- Except as noted, no noise reduction or audio filtering has been done, other than some lowpass filtering that was done during the MP3 encoding process.
- For the audio clips transmitted via Laser, the audio level was reduced to prevent clipping during the occasional bright peaks.
- With the LED, the average audio level could be higher,
owing to the lower amount of scintillation - a fact that
brought up the background noise to a higher level.
Final comment: "Is it 'coherent' enough?":
Over the years, there has been some discussion as to whether or not "Lightwave" communications - not being covered directly by the same FCC rules that govern amateur radio communications - were really amateur radio communications. This was taken up by the ARRL contest committee and is spelled out in this document: http://www2.arrl.org/contests/announcements/rules-vhf.html.
In particular, there was section 1.12 in the General Rules for ARRL Contests above 50 MHz which stated:
1.12. Above 300 GHz, contacts are permitted for contest credit only between licensed amateurs using coherent radiation on transmission (for example, laser) and employing at least one stage of electronic detection on receive.Unfortunately, this statement is rather vague and could be interpreted in several ways. At the time that this ruling was made (in mid-1980, perhaps) one of the few methods that had been historically been used for "all-electronic" lightwave communications had been via laser - but it most certainly does appear to limit the scope to only lasers. Although I am not privy to the internal discussions behind this, it would seem that the intent was to prevent amateurs from using blinking lights to send Morse code to each other, hence the necessity for "...at least one stage of electronic detection on receive" but I would hope that there was never any intent to straightjacket experimentation in so-wording the rule.
The early portion of that statement, namely "...using coherent radiation on transmission (for example, laser)..." was a bit more mysterious. It would seem that it was be worded to preclude the use of a tungsten light source (e.g. light bulb) from being modulated, but why, exactly did the authors feel it necessary to narrow the possibilities? Perhaps a "light bulb" was considered to be too passé, or maybe it was considered to be too far distant from being any sort of "transmitter" in the conventional sense in that it was more of a broadband noise emitter than a device that generated a "signal" on a specific frequency.
What is interesting, though, is that there is that statement "(for example, laser)" that suggests that the means used need not be a laser, specifically. At the "low" end of this scale, say - in the millimeter-wave range - it is unlikely that Lasers would be applicable at all. It would, therefore, imply that the use of a laser, specifically, wasn't required!
What about the "coherent radiation" portion of the statement? The degree of coherence isn't stated and its purpose would seem to be to remove the use of "noise" sources (e.g. incandescent light bulbs) from consideration. At the time of writing, the mostly likely laser source available to the radio amateur was a gas laser tube, which arguably puts out a fairly coherent (single-frequency) light source and it is entirely possible that other suitable types of light sources fathomable at the time. Since that statement was written, however, a number of other light sources have become available - including semiconductor lasers. This, again, brings the question of "coherence" to the forefront: Compared to a gas laser - such as a typical Helium-Neon laser tube - the spectra of a laser diode is not particularly coherent in that its energy is spread over a fairly wide range of frequencies - but it seems to be "coherent enough" for the definition.
Since the above rule was written, other distinctly "non-laser" but monochromatic devices - such as LEDs - have become available with capabilities that make it practical to use them instead of lasers for long-distance communications. Had such high-power LEDs been available at the time of writing been available, would they have included or excluded them - and in either case, what would have been the justification for doing so? We'll probably never know, and there's probably little reason to debate that point!
Fortunately, the ARRL does respond to changes in technology - especially if they are lobbied to do so - and on July 16, 2010 the "Rule 1.12" was changed to:
1.12. Above 300 GHz, contacts are permitted for contest credit only between licensed amateurs using monochromatic signal sources (for example, LASER and LED) and employing at least one stage of electronic detection on receive. LASER usage is restricted to ANSI Z136 Class I, II, IIa, and IIIa (i.e., output power is less than 5mW).This revision would seem to increase the options of light sources used for communications but clearly precluding unfiltered "thermal" sources (such as tungsten lamps). What is interesting is the inclusion of a 5mW limit for laser use - but this probably has to do with safety and legal issues that become increasingly important as power levels go up.
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Keywords: Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, LED communications, laser communications, LED, laser, light-emitting diode, lens, fresnel, fresnel lens, photodiode, photomultiplier, PMT, phototransistor, laser tube, laser diode, high power LED, luxeon, cree, phlatlight, lumileds, modulator, detector
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