[Laser] 173 mile optical 2-way QSO completed

[C. Turner]

Hello,

On the night October 3, 2007, two groups here in Utah were able to 
complete an electronic 2-way optical QSO over a path that, according to 
the RadioMobile program, was 278.56km (173.1 miles) in length.

The north-to-south bearing was 177.1 degrees true, with the southern end 
of the path being a site on a ridge just below Swasey Peak in the House 
Range, west of Delta, Utah, to the northern site in the Raft River range 
in northwestern Utah, south of Standrod, Utah, just south of the Idaho 
border.  The elevation of both locations was just a hair under 2865 
meters ASL (9400 feet.)  While it was possible to drive to the north 
site using 4-wheel drive and tire chains to combat mud and snow, it was 
required that the party attaining the southern site hike in from the 
vehicle:  Their path required route-finding on rugged terrain, several 
kilometers of hiking, and quite a bit of elevation gain - not to mention 
a return path in darkness while fighting a fierce wind.  This optical 
path crosses some of the flattest land on Earth - the Utah Salt flats.  
There was also zero light pollution, owing to the fact that the majority 
of the path crossed over an uninhabited military reservation.

On the south end near Swasey Peak, the party was lead by Ron K7RJ with 
Gordon K7HFV and Brett, N7KG.  On the north end near George Peak was 
Clint, KA7OEI, solo.

Air conditions on this occasion were not particularly good:  Owing to 
winds preceding a cold front, a fair amount of dust had been kicked into 
the air, causing significant decrease in visibility, but the winds may 
have helped in maximizing thermal homogeneity of the air between our two 
sites, reducing the amount of scintillation:  Preliminary analysis of 
scintillation shows it to be much less than that experienced during 
previous 15 and 107 mile experiments.

A two-way MCW exchange was completed fairly easily, followed by the 
successful reception of a voice report at the south end - but it was 
noted that the link margin in the north-to-south direction was a few dB 
better than the south-to-north direction, making the reception 
south-to-north voice report somewhat "iffy" - the voice was detectable, 
but copy was extremely marginal.  Also transmitted in both directions 
were test tone - to provide later analysis of scintillation, and several 
SSTV images were transmitted from north-to-south.  All exchanges were 
recorded live in a lossless .WAV format to identical digital audio 
recorders.

For both ends, the transmitting gear was similar, the optics based on 
work done by the Australian optical group:  The transceivers used 
separate large Fresnel lenses for transmit and receive.  On the north 
end, a pair of 430x404mm lenses were used while the south end used a 
pair of 318x250mm lenses.  On both ends, current-modulated 3-watt red 
Luxeon LEDs were used for transmit, while both receivers used a 
reverse-biased BPW34 PIN photodiode with a cascode amplifier using a 
saturated JFET as the input stage.

The reason for the link margin disparity between the  north and south 
ends is currently unknown.  At least a portion of this may be attributed 
to the fact that it is known that the transmitter used at the north end 
has a couple dB higher optical flux output - a fact attributed to the 
fact that the secondary lens was a custom-ground aspherical lens that 
was designed to provide better illumination of the Fresnel lens.  It is 
also suspected that the receiver used on the north end may be deficient 
by just a couple of dB:  Even though the designs are identical, it is 
possible that the difference is related to normal component variation.

During this event, at no time were the lights from the opposite ends 
ever visible with the naked eye:  We spent several hours trying to 
acquire the signal, but just before we were about to give up, I was 
finally able to spot an the extremely dim red light from the south end 
using an 8" telescope:  Using electronic pointing aids (e.g. an "audible 
S-meter" system) we were able to acquire each others signals.  
Eventually, Brett (at the south end) was able to spot the light from the 
north end using a very large set of binoculars.

After our MCW, voice, and SSTV transmissions, we attempted to acquire 
signals from the Lasers:  Using the electronic pointing aid, this was a 
trivial task during a test over a  172km (107 mile) path a month earlier 
when using both the 8" reflector telescope (which was already aimed 
directly at the far end, requiring only a minor tweaking of the 
telescope's vernier to maximize the signal once the eyepiece was 
replaced with the emitter) and again with a Laser pointer attached to 
the telescope's sighting-scope mount.  On this (the 10/3) occasion, 
however, the Laser signal could not be acquired at the south end:  A 
attempt was also made to acquire a Laser signal at the north end that 
was transmitted from the south end, but this, too, was unsuccessful.

It is expected that were the air conditions better (e.g. less airborne 
dust) that 2-way voice communications would have been easy and that 
acquisition of the Laser signals in both directions would also have been 
possible.

In comparison, a 2-way optical voice and SSTV contact had been completed 
over a 107 mile path exactly a month earlier, on September 3.  On this 
occasion, S/N of over 20 dB was noted using the exact same equipment.  
Also on this occasion, experiments were done using coherent light (diode 
Lasers) - both using a beam collimated using an 8" reflector telescope 
and an "uncollimated" beam using just a standard $3 Laser pointer.  On 
this occasion, while voice communications was easily accomplished using 
all three transmission methods, it was noted that the coherent light 
transmissions suffered severely from scintillation, with troughs 
dropping well below the measurable level - with the scintillation 
experienced on the beam emitted by the 8" telescope having a notably 
lower rate of scintillation with fewer instances of extreme fading.  It 
was also noted that the amplitude peaks of the coherent light (during 
brief instances of constructive interference) were higher than those of 
the noncoherent light, but that the average level was significantly lower.

At present, pictures and written accounts of the event are still being 
gathered and analysis of the recordings is only in a preliminary stage.

73,

Clint
KA7OEI