Clear Air Scatter Tests On 458THz

After patiently waiting for the bright moon to clear the early evening skies, I was finally able to venture out for my first clear-air scatter test this past Sunday night. I had plotted the path on my Mayne Island map and determined bearings as best I could, but the path was going to be very tight. If the path plan was right, my signal should just clear the high beachfront bluffs at the chosen sea-level receiving site.

After carefully aiming the light, I set off for the receive site at around 7:45PM and was all set up with the new lightwave receiver about 30 minutes later. The site appeared fairly quiet and the Argo screen confirmed that there was little QRN coming from the local houses up on the bluff. I listened for over an hour, trying various slow changes in pointing ... varying the azimuth a few degrees at a time, and then the elevation. Unfortunately not the slightest indication of my ~549Hz tone was seen. I was confident that the system was working as several strobes were heard from distant aircraft (near Vancouver), as their flashing lamps skimmed the edge of the far treeline.

It seems likely that either my aiming or bearing calculations (or both) were off and that the signal was probably slightly to the west of me, with the bluff blocking any hope of reception ... I knew it was going to be close but was hoping for a little luck.

I left the transmitter outside overnight (it was set up two properties to the SE) and decided to try a second shot on Monday night. This path, although shorter by a mile, would require the signal to pass over two high hills ... the first topping out at 667' and the second at 567'. The overall direct-path distance was 1.7 miles (2.7 km). A cross-section of the signal path is shown below as it hugged the edges a little lower than the peaks:

courtesy: http://www.heywhatsthat.com/profiler.html

I have been using a 'compass' app on my I-Pad to determine directions when aligning the transmitter and receiver setups. I'm not 100% convinced of its accuracy at all times, as readings can sometimes be a bit flaky. Before doing any more testing, I'll need to solve this, either with a better app or with a real compass.

The transmitter was set up just before darkness, pointing right at the edge of the treeline along the 667' ridge and elevated at a 28 degree takeoff angle. The deep-red, 640mw LED, was switched-on just before departure at around 8:30PM.

It didn't take long to get set up in the back of the CRV, with the receiver temporarily set in no particular direction and plugged into the computer.

When Argo came to life, I went to the front of the car to grab the I-Pad so that the receiver could be aligned but was surprised to see a bright line at 549Hz when I came back! It seems that my 'rough' placement of the receiver was spot-on, and not exactly where I had originally intended. In fact, there appeared to be about a 10 degree error in where I had planned to point. I later traced the error to my path drawn on the paper map as it was difficult to determine my exact receiving location on the older map, which didn't show the new road where I had set up on.

Monday night's path

I was eventually able to fine-tune the aiming and hone-in on the best spot but was surprised that there was a broader degree of acceptance in both azimuth and altitude. At around 8 degrees of elevation, and although I was pointing well into the nearby trees, the signal was still observable with just tiny patchs of sky poking through behind the trees. At 10 degrees I was above the trees, with a solid signal. Surprisingly, the signal was not lost until pointing past ~ 15 degrees ... I had expected sharper pointing.

With the strength of signals recovered on this path, two-way communication could have easily been established on any of the CW QRSS modes ... if quieter, probably on normal audible CW. Signal strength indicated that there was still plenty 'left in the bucket' for greater distance paths, probably much further than I am able to test here on the island.

This was the first thing I saw, at the QRSS60 mode in Argo ... a fairly narrow passband and a ~25+ db dig into the noise.

Backing off to a wider bandpass (less sensitive) but faster QRSS10 mode showed the signal still very apparent:

The almost 'real time' QRSS3 mode, although showing a much weaker signal, indicated that the signal would have been almost audible had it not been for the high level of background noise at this site. Don't confuse the lightwave signal with the much stronger 9th harmonic of 60Hz on 540Hz!

The ferry terminal was just down the hill about 1/2 mile and with several kilowatts of spectrum-polluting 60Hz sodium vapor lighting, the cloudy skies were a sea of bright-pink. There was a high level of audible hum in the phones, right from the start, that unfortunately, masked any hope of an audible detection. The waterfall screen capture shown below, illustrates the massive QRM at this otherwise nice site!

The night was not going to be complete without a strobe signature, captured on Argo from a high passing jet aircraft:

strobes

All-in-all, it was a very successful outing, considering the obstructed path and the $5 fresnel lens used in the portable receiver! I've examined the island map for any other possibilities and there are not many suitable candidates. I had hoped for one other possibility towards the west, which would stretch the path by almost another mile, but I'm not really sure that I can get a clear shot without hitting the very close treeline at this end.

I think the next round of testing will be in the other direction ... across Georgia Strait, with John, VE7BDQ, who has expressed interest in doing some deep overnight Argo searches for my signal in the clouds.

I'm not sure which mode would offer the best chance ... 'clear air scatter' or 'cloudbounce'. John has a very good receiver, with a slightly larger and better-quality fresnel than the one used in these tests. Working from his suburban backyard, directly across the strait at 13 miles (21 km) distant, his direct path to me is somewhat obstructed and will require an elevation angle of around 30 degrees at his end. I think, ideally, we would both like to be skimming just above the ocean, with only a slight elevation. A lower and less obstructed shot from his yard would mean an oblique path so this also remains a possibility. We will play with what we have and hope for the best ... even just a trace of signal would be a measured success.

I think that a non-line-of-sight (NLOS) contact would make an exciting challenge and a great project for two amateurs living in the same city or town, and ... you really don't even need a ticket!

For more technical details on the equipment used in this test, see "A West Coast Lightwave Project" describing the activities between here and Markus, VE7CA. We have just learned that this article will be published in the 2016 Radio Amateur's Handbook ... hopefully inspiring  more new lightwave activity!