Figure 1: Map and
elevation
profile showing the path between the location near Mt.
Nebo in the
south to Inspiration point to the north.
Click on the image for a larger version.
|
Better weather!
Our
August
18, 2007 Expedition, although successful, left us
with mixed
feelings: Even though we were able to complete the contact
despite terrible optical conditions (e.g. thick haze and light
pollution) we still wanted to try a "lightbeam" contact again
when
weather and air
conditions were better.
Such an opportunity arose on September 3, 2007 when a
coincidence of
schedules and good air conditions occurred, so we again headed
to the
same places as last time: Ron, Elaine and Gordon headed
north to
Inspiration Point, while I went south, again to the area around
Nebo,
this time with Dale, WB7FID, as Tom was out of town.
Although there had been a few clouds in the morning, it had
cleared up
by the evening, with no imminent threat of storms anywhere along
the
path. Additionally, previous weather and wind conditions
had
resulted
in very clear air with only a hint of haze, with each site being
clearly visible from the other.
Same gear, same path - almost:
Since the last time, minor modifications had been done to the
optical
gear:
- The beamwidth of the optical receivers were narrowed by
masking
off a portion of the photodiode's active area: A small
drill bit
was used to make a clean, round hole in self-adhesive copper
foil that
was centered over the diode. After masking, the
focusing and
paraxial alignment was checked and adjusted as
necessary. The
result was narrower and sharper beamwidth with no loss in
sensitivity,
as the holes were larger than the blur circle of the lens,
plus better
rejection of off-axis light, such as that from light
pollution.
- A simple pushbutton was added to the modulator, in
parallel to
the "LED On/Off" switch to allow easier on/off keying of the
LED using
Morse: This is useful for signaling - particularly
during
alignment.
- It was determined that, if the audio interface box and
modulator
shared the same battery, a slight amount of crosstalk
occurred.
By adding shielding to the enclosure and using separate
batteries, the
only remaining crosstalk was that resulting from light
scattered in
front of the transceiver.
While, at Ron's end (Inspiration Point) they were within a few
feet of
the same location
as before, a slightly different location was used at the Nebo
end: Last time, just to be safe, we'd moved along a fence
line to
better-clear the optical path from a nearby ridge. As it
turned
out, the clearance with that ridge was sufficiently large that
upon
return, we parked at a spot much closer to the microwave radio
site. This simplified setup considerably, as we didn't
need to
hassle with a fence, and the gear was within a few feet of the
vehicle. This also meant that owing to geography, we were
about
230 feet (70 meters)
farther from Inspiration point,
thereby
beating our previous record - if only by a little.
Setting up:
As it turned out, Ron and company arrived at their site not too
long
after we arrived at ours. We had arranged our departure
times so
that we would be arriving at or just after sunset, which
occurred at
about 7:57 PM
on that day. Before unloading too much gear, however, we
decided
that it would be a good idea to verify that, at our new
location, we
had a clear, line-of-sight path to Inspiration Point. This
was
easily verified by sighting, through binoculars, the headlights
from
Ron's
vehicle, even though it was still light enough to easily read
by.
We quickly verified that the headlights were very visible - even
with
the naked eye. This was a good omen, indeed
- especially since, this time, the headlights appeared to be
white in
color rather than, during the previous attempt, a muddy brown,
filtered
through dense haze and visible only through the telescope.
Figure 2: Computer
simulated
views of the paths. Top: Looking to
the north,
toward Inspiration Point. Bottom:
Looking to the
south, toward the Nebo Loop.
Click on an image for a larger version.
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|
With the remaining daylight, Dale and I proceeded to set up the
gear -
a process more involved than the setup at Inspiration
Point: Our
setup
involved not only deploying the tables onto which the gear was
set, but
also an 8 inch reflector telescope as well as other optical gear
that
had been brought along for other experiments and tests.
Again using the 146.76 repeater on Lake Mountain for
coordination, we
continued our setup but this time, Chris, VK3AML was unavailable
to
join via IRLP owing to a schedule conflict. On North end,
setup
was somewhat
less-complicated: Owing to the design of the
optical
transceiver,
one need only open the front cover and flip it underneath the
enclosure, at which point it becomes the elevation adjustment
platform. The only other steps required for assembly are
to
verify that the optical transmitter and receiver electronics are
properly seated and aligned and that the electronics are plugged
into
the appropriate places, powered up, and functionality
checked. On
the south end, however, setup is slightly more
complicated: The
optical
transceiver, being foldable, requires a few extra minutes
of
assembly - a process that involves the installation of about two
dozen
screws (with wing-nut heads) used to hold everything together
and in
precise alignment. Not only this, we were also embroiled
in the
setup of the telescope which was not only to be used for
sighting, but
for emitting as well.
One of the final steps taken before we started to lock onto each
other
signal was to start our digital audio recorders. For
documentation and analysis purposes, both transmitted and
received
audio was recorded to digital audio recorders in a "lossless"
PCM
(.WAV)
format with a sample rate of 32,000 sample-per-second. It
is
important to note that the PCM format is chosen intentionally,
as it
accurately records each cycle's waveform, unlike a compressed,
lossy
format such as MP3 or WMA. The sample rate of 32 ksps was
chosen
because it is high enough to capture suitable detail with good
frequency response, but it takes less storage than, say, a file
recorded at 44.1 or 48 ksps.
Initial lineup:
Because of our past experience, we already knew the "lay of the
land"
and precisely where, in the distance, that we should be pointing
and
with the excellent air clarity, aiming of the gear was quite
easy. Even though none of our transceivers have any sort
of
aiming aids (such
as alignment scopes or gunsight-type hardware) it is still
fairly easy
to point it in the general direction by sighting the beam off
the
ground below and then raising the elevation. Another
helpful
sighting
aid is the beam itself: Due to Rayleigh scattering, it is
possible -
if one's eyes have adjusted sufficiently and if it is dark
enough - to
see the shaft of red light going off into the distance, and it
is
simply a matter of pointing that shaft of light in the direction
of the
other end of the path.
First, we started out by having Inspiration Point roughly
pointing
toward us while we looked toward them with a pair of
binoculars.
It only took a few seconds before we saw a brief, red flash that
was
easily visible with the naked eye. After a bit more
"talking
in" via radio, we could see a fairly steady, red dot in the
distance: Now, it was time to switch over to the
electronic
aiming system.
More precise aiming:
Once we had the light from Inspiration Point "on visual," we had
them
modulate their end with a 1 kHz tone generated by the
modulator.
Switching our end to the "
audible
S-Meter" mode, it only took a few
seconds of moving our transceiver around before we got a "hit" -
and
then were able to do further tweaking of our receiver to
more-precisely
aim it. Because we had already turned on our light, they
could
now
see our end as well.
We now switched roles: We transmitted a 1 kHz tone (while
they
shut
their tone off) while they peaked their receiver on our signal
using
their
audible S-meter. After a few more back-and-forth exchanges
to
verify that we'd properly peaked each end, we switched to voice
mode
and began talking:
Initial exchange via the optical link:
- Initial
2-way
exchange (mp3, 0:47, 560 kB)
This is the
first exchange, just after using alignment tones and
completion of
final aiming: A brief segment of the 1 kHz alignment
tone may be
heard at the beginning.
- The audio as received at Nebo (on the
south end)
is in the LEFT channel.
- The audio as received at
Inspiration Point (on the north end) over the 107 mile
path is in the RIGHT
channel.
- Into both channels - particularly into the LEFT channel
- there
is some crosstalk which causes the local transmitted audio
to be heard
in the receiver, so keep this in mind when you are
listening. You
may want to mute one channel (or lift off an earphone) to
hear just one
end at a time.
Figure 3: Waveforms
showing
the scintillation on a 4 kHz test tone from the Laser
using the 8"
telescope. The top graph shows scintillation over a
2 second
period, the middle graphs shows scintillation over a 0.2
second period,
and the bottom graph shows scintillation over a 25
millisecond
period. Note that the vertical and/or horizontal
scales may be different for these graphs.
Click on the image for a larger version.
|
Shortly after the time of this recording, we re-checked our
aiming and
improved signals even more and began to carry on normal
conversations
without any difficulty at all as demonstrated by this short
clip:
- N7BDZ
and
KA7OEI
exchange (mp3, 1:47, 1.23 Meg)
Clint,
KA7OEI and Elaine, N7BDZ (and later, Gordon, K7HFV) exchange
greetings across the 107+ mile optical link. 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)
- The audio as received at Nebo (on the
south end)
is in the LEFT channel.
- The audio as received at
Inspiration Point (on the north end) over the 107 mile
path is in the RIGHT
channel.
- Into both channels - particularly into the LEFT channel
- there
is some crosstalk which causes the local transmitted audio
to be heard
in the receiver, so keep this in mind when you are
listening. You
may
want to mute one channel (or lift off an earphone) to hear
just one end
at a time.
After getting everything set up, we began to talk back and forth
casually, discussing what it was that we were going to do next.
At Inspiration Point, the operation of the optical gear
attracted a
little bit of attention: Because it is a popular
destination for
people all-terrain vehicles, riding their mountain bikes, and
just to
see the
(ahem) inspiring view, some of the visitors were
naturally curious as to what it was that
was happening. Of course, Ron, Elaine, and Gordon were
happy to
explain that they
were "listening" to the distant red dot and that there were
other,
equally strange people at the other end, over 100 miles away.
Planned Experiments:
For our August 18 test, we'd planned to conduct some
experiments, such
as sending test tones, pictures using SSTV, and even using
Lasers - but
conditions, being what they were, precluded such. This
time, with
the beautiful weather, we now put our minds conducting them.
Red Laser
diode
module - using an 8-inch telescope:
In our past tests, we compared the signal quality when a
collimated red
Laser emitter was used with the signal quality obtained when a
similarly collimated LED was used - but
using
PWM techniques, as Laser
cannot be conveniently modulated using linear techniques.
Using
the same setup as in previous tests, operating at a wavelength
of
around 660 nM or so - somewhat longer than the 627 nM wavelength
of the
LED, but still fairly close. I
installed
the
laser
module in the
telescope and, using the audible S-meter system, was able
to use
the
telescope's micrometer adjustments and quickly peak the signal:
- Laser
with
telescope (mp3, 1:17. 605 kB)
Stereo audio
file recorded at Inspiration Point
- The LEFT channel contains local
audio
transmitted from Inspiration Point
- The RIGHT channel contains the audio received
at Inspiration point, having been transmited over the 107
mile path via
the Laser in the telescope.
- 0:00-0:16: Sighting-in of the Laser using
the
telescope. In the LEFT channel, one can hear
the audible
S-meter while the RIGHT channel contains the 1 kHz
"alignment"
tone being received, having been transmitted via Laser,
being used to
"key" the audible S-meter. In the first few seconds,
one can hear
the Laser "swoop" past the receiver and then get "dialed
in" to peak
the signal. The "wobble" of the S-meter's tone is
due to the
scintillation of the received signal.
- 0:16-0:50: Voice commentary about the
lightbeam.
- 0:50-1:17: Music clip. 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)
As can be heard, scintillation is more apparent than with the
LED and
Fresnel Lens - the main difference being a markedly faster
rate-of-change of amplitude and an increased depth of the
nulls.
It is also interesting to note that during the peaking, the 1
kHz tone
was, at times extremely "rough" sounding - probably from far-end
illumination by the edges of the Laser's collimated beam where
additional distortion due to "beam wander" was evident. In
our
prior 15 mile tests, it was noted that the edge of the beam
produced by
the telescope was extremely sharp, going from a percieved
full-brightness to being invisible over a lateral distance
of
just a few feet.
Analysis:
Figure 3 shows samples of waveforms of the 4 kHz pilot
carrier,
showing the amount of scintillation present on the received
signal. Analysis of the resulting audio and the waveforms
themselves indicate that the depth of scintillation is well over
40dB. The scintillation depth may be greater than this,
but the
finite signal-to-noise ratio of the received signal limits the
ability
of making "deeper" measurements.
Close inspection also reveals that the rate of scintillation
appears to
have several overlaid periods: The most obvious period is
one of
around 70 milliseconds or so, but harmonics of this period can
be seen
in the 1.5-2 millisecond area - but the temporal resolution of
the 4
kHz tone (plus the bandwidth limits imposed by the receiver and
subsequent processing) limit the observation of even faster
scintillatory periods that may be present.
Another interesting and aspect is the rate-of-change of the
amplitude
of the received signal: Several examples can be spotted
where the
audio changes by more than 20dB in under 50 milliseconds.
Standard Laser
pointer:
Figure 4: Waveforms
showing
the scintillation on a 4 kHz test tone from the Laser
pointer.
The top graph shows
scintillation over a 2 second period, the middle graph
shows
scintillation over a 0.2 second period, and the bottom
graph shows
scintillation over a 25 millsecond period. Note
that the vertical
and/or horizontal scales may be different for
these graphs.
Click on the image for a larger version.
|
Another test that was planned was the use of a
simple,
cheap red Laser pointer. This Laser pointer originally
used a
pair of AAA batteries and was bought for just $3 and uses
same type of
standard red Laser diode as was used in the telescope.
As in the
case of the Laser module in the telescope, Pulse-Width
Modulation was
used.
The Laser pointer was "nondestructively" modified by
inserting a dummy
battery made from a wooden dowel with connecting wires,
taping down the
"on" button, and
using thermoset glue to attach it to a small plastic
box. To
simplify aiming, the box to which the Laser pointer was
attached to the
spotting scope mount of the 8" telescope. Using
the telescope and eyepiece, the aiming of the Laser pointer
was roughly
adjusted to be pointed in the same direction as the
telescope itself by
looking for the red dot in the telescope's eyepiece.
This Laser is a standard pointer with an aperture size of
just a couple
of millimeters with no other collimation optics and because
of this,
aiming was somewhat touchy. As with the Laser in the
telescope,
the pointer was modulated with a
1 kHz alignment tone and, using feedback from the audible
S-meter from
Inspiration Point, after a minute or so of sweeping, I heard
a
"hit" as the Laser pointer flashed past the far end's
receiver.
After a bit more gentle tweaking, I was able to
dial the telescope's vernier adjustments to peak the signal
at the far
end.
- Laser
pointer (mp3, 2:20, 1.07 Meg) Stereo
audio file
recorded at Inspiration Point
- The LEFT channel contains local
audio
transmitted from Inspiration Point.
- The RIGHT channel contains the audio received
at
Inspiration
point, having been transmitted via the Laser pointer
over the 107 mile path.
- 0:00-0:29: Sighting-in of the Laser pointer
clamped to the telescope. In the LEFT
channel, one can
hear the audible S-meter while the RIGHT
channel contains the 1 kHz "alignment" tone being
received, having been
transmitted via Laser, being used to "key" the audible
S-meter.
In the
first few seconds, one can hear the Laser "swoop" past the
receiver and
then get "dialed in" to peak the signal. The
"wobble" of the
S-meter's
tone is due to the scintillation of the received signal.
- 0:29-0:58: Music clip. 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)
- 0:58-2:20: Voice commentary about the
communications. (There's a bit of acoustic
feedback at the
beginning due to my microphone gain initially being too
high.)
As can be heard, scintillation is even more severe than that
observed
with the larger aperture telescope, yet the intelligibility is
still
reasonably good - mostly owing to the redundant nature of human
speech
and the fact that the scintillatory periods were, on average,
far
shorter than syllables: This is an example of the ear and
brain
doing a good job of "filling in" the gaps.
Analysis:
Figure 4 shows samples of waveforms of the 4 kHz pilot
carrier,
showing the amount of scintillation present on the received
signal.
Analysis of the resulting audio and the waveforms themselves
indicate
that the depth of scintillation is also well over 40dB, and
again, the
scintillation
depth may be greater than this, but the finite signal-to-noise
ratio of
the received signal limits the ability of making "deeper"
measurements.
Close inspection also reveals that the rate of scintillation
appears to
have several overlaid periods: The most obvious period is
one of
around 18 milliseconds or so, with harmonics apparent in the
1.2-1.5
millisecond area. Again, the temporal resolution of the 4
kHz tone (plus the bandwidth limits imposed by the receiver and
subsequent processing) limit the observation of even faster
scintillatory periods that may be present.
It can also be seen that the maximum rate-of-change of the
amplitude
has increased: Several examples can be spotted where the
audio changes by more than 20dB in under 20 milliseconds -
somewhat
faster than that observed with the Laser collimated through the
telescope.
It should be noted that such an increase in scintillation is not
unexpected, for it is well-known that an increase in aperture
the
emitting and/or receiving aperture will cause an effect known as
"aperture averaging" - that is, if scintillation quashes the
luminous
flux at one point across the area of the aperture, it is
statistically
more likely that as the aperture is made larger, some other
portion of
it will still be intercepting some of the signal.
It is also worth mentioning that the Laser pointer - with its
inexpensive, plastic lens, does not offer nearly the
minimization of
divergence that would be found in a higher-quality, collimated
laser
source using wavelength-accurate optics. With the cheap
Laser
pointer, the beam need only travel a small fraction of the total
path
distance before it has diverged to a size larger than that of
the
receive aperture - a property that
artificially
increases the
aperture size of the Laser pointer. For the most part, the
worsening of the Laser pointer's scintillation as compared to
the
scintillation of the beam from the telescope is that which is
incurred over the first portion of the overall path.
Ironically, this also indicates that if a Laser with
higher-quality
optics used, the results would be even worse as the
self-divergence of
the beam require a longer portion of the path and be
more-affected by
air turbulence. It should be noted that such a narrow
divergence
would be self-limited by the atmosphere, anyway: A
rule-of-thumb
of 1 milliradian-per-kilometer is stated in some of the
literature - a
value that is widely variable, depending on many atmospheric
parameters.
Notice that, unlike the case with the Laser in the telescope,
there is
no severe distortion present at the apparent edge of the beam -
probably due to the fact that, while the Laser pointer's beam is
still
quite narrow, the edge-falloff of the pointer is much more
gradual than
that of the
telescope.
Using a high power LED with an acrylic Fresnel
Lens:
Figure 5: Waveforms
showing
the scintillation on a 4 kHz test tone from the high power
LED using
an inexpensive Fresnel lens. The top graph shows
scintillation over a 2 second period, the middle graph
shows
scintillation over a 0.2 second period, and the bottom
graph shows
scintillation over a 25 millsecond period. Note
that the vertical
and/or horizontal scales may be different for
these graphs.
Click on the image for a larger version.
|
It should be mentioned at this point that testing was done using
a
standard LED in the 8" telescope. Unfortunately, the
far-field
luminous flux output of the LED was much lower than that of
either of
the Lasers and insufficient signal level was obtained to be able
to
make useful measurements of scintillation.
The following tests were conducted using a 3-watt red LED (peak
wavelength of about 627 nM) with an acrylic Fresnel Lens.
To be
certain, some reduction of scintillation was observed simply
because of
the larger aperture, but the bulk of the reduction was achieved
through
the use of a noncoherent light source as indicated through
previous
experimentation.
It should be noted that the previous recordings on this page
(e.g.
those
prior to those exhibited in the above "planned experiments")
were obtained using a pair of similar optical transceivers, both
using
high-powered LEDs and plastic Fresnel lenses. What follows
is an
audio clip that contains the same music segment as the above
clips,
plus some general ragchewing between Ron and Dale over the
link.
As can be heard, the communications was easy, "armchair" copy:
- High-power
LED
with
Fresnel Lens (mp3, 1:58, 927 kB) Stereo
audio
file recorded at Inspiration Point
- The LEFT channel contains local
audio
transmitted from Inspiration Point.
- The RIGHT channel contains the audio received
at Inspiration Point, having been transmitted over 107 miles
via the
high-powered LED and Fresnel lens.
- 0:00-0:08: Left channel: 440 Hz
tone
with the tone from the audible S-meter in the
background. Right
channel: 1 kHz alignment
tone from Nebo, across 107 mile optical link.
- 0:08-0:35: Music clip. The brief
interruption in the audio channels was due to re-seating
of the audio
connectors. 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)
- 0:59-1:58: Ron and Dale, ragchewing over
the link.
Analysis:
Referring to
Figure 5, one can see a dramatic difference
in
both the amplitude and rate of the scintillation. These
particular waveforms are a selected showing of the worst-case
scintillation observed over a period of several minutes, with a
worst-case scintillation depth of about 22dB. Perhaps most
striking is the dramatically slowed rate "dA/dT" - that is, the
change
in amplitude versus change in time: The major
scintillatory
periods are, perhaps, 125 milliseconds in length, roughly twice
the
rate noticed with the telescope-collimated beam, and it is
unusual to
see a change of more than 15dB occurring in under 100
milliseconds. In the "zoomed-in" 25 milliseconds portion,
one can
see a rather weak scintillatory period of around 9 milliseconds,
but
with an amplitude variation of only, perhaps, 7 dB over that
time.
A few more audio clips:
Waving a green Laser pointer about:
In these sorts of outings, there is the irresistible urge to
shine
whatever lights one has at each other - and I happened to have a
low
power green Laser pointer on hand and because the eye is many
times
more sensitive to green than red, I had no doubt that it would
be
seen. What is interesting, however, is the sound that was
heard
at the receive end when the Laser swept across the receiver:
- Listening
for
the
green Laser pointer (mp3, 1:11, 557
kB) Stereo
audio
file recorded at Inspiration Point
- The LEFT channel contains local
audio
transmitted from Inspiration Point.
- The RIGHT channel contains the audio received
at Inspiration point.
- 0:00-1:11: This clip contains some banter
back and
forth between the two sites. During this clip,
however, the
receiver picked up the sound as the light from the green
Laser was
intercepted by the receiver. Instead of hearing a
"click" or
white noise, however, an odd, insect-like "whine" was
heard - most
notably around time index 0:50. This
particular Laser
pointer though to produce a continuous beam as noted by
having
"listened" to it on previous occasions but it is possible
that its current regulator may become unstable under
certain conditions.
- Note that much of the time, my voice is way
off-microphone,
which is
why it sounds like it is the background: Dale
occasionally relays
what I said to make sure that Ron had heard it.
A transition from Laser, back to LED:
As it turned out, I had been transmitting
to
Inspiration
point for nearly 20 minutes while I sent music clips, test
tones, and
even SSTV images - and they sent similar things back.
After this,
we got back to our normal back-and-forth ragchewing, so I
switched from
the Laser pointer back over to the high power LED and Fresnel
lens,
as can be heard in this clip:
- Laser
pointer
to
LED cut over (mp3, 0:29, 228 kB)
Stereo
audio file recorded at Inspiration Point
- The LEFT channel contains local
audio
transmitted from Inspiration Point.
- The RIGHT channel contains the audio received
signal at Inspiration point over the 107 mile path..
- At about 8-10 seconds into the recording, there are a
series of
clicks and thumps as the Laser is switched off and the LED
is turned
back on. As can be heard, the audio prior to this
point is
transmitted via Laser pointer over the 107 mile path and
after that, we
hear Dale and Ron talk back and forth, with Dale being
received via the
optical link. Gordon can also be heard in the
background, off
microphone.
Figure 6: Montages of
SSTV
images received via the 107+ mile optical link. Top:
Images
received
at Nebo from Inspiration point - at both 200 milliamps
and, later, at the full 1100 milliamps of LED current.
Bottom:
Images received at Inspiration Point via the optical
links, using both
LED and Laser pointer.
Click on an image for a larger version.
|
|
Switching to full power:
One of the experiments that we did was to measure our LED
current and
then begin to reduce it, noting the current, until we could no
longer
understand what each other was saying. At about 2 hours
and 10
minutes into our testing, we decided to do this just before we
concluded our tests - and at this time Ron noticed something
that he'd
not expected:
- Going
to
full power (mp3, 1:14, 583 kB) The
audio as received
at Nebo (on the south end)
is in the LEFT channel while the audio as
received at
Inspiration Point (on the north end) is in the RIGHT
channel.
- Into both channels - particularly into the LEFT channel -
there
is some crosstalk which causes the local transmitted audio
to be heard
in the receiver, so keep this in mind when you are
listening. You
may
want to mute one channel (or lift off an earphone) to hear
just one end
at a time.
- At the start of this recording, Ron had just connected a
voltmeter to the current monitor points on the modulator,
read the
current, and noticed that up to this point, it had been
set to 200
milliamps instead of the more-normal 1100 milliamps:
We hear the
exchange as this is discovered...
- For most of this clip, the audio gain for the audio
received at
Nebo (in the LEFT channel) was set at a
fairly low level
to accomodate the change when the LED current at
Inspiration Point was
increased.
After this, we continued with our "LED limbo dance" (e.g. "How
low
can we go?") and passed random words back and forth, gradually
reducing
the
current. As it turns out, we both began to have severe
difficulty
in understanding each other as we lowered the current below 60
milliamps
- a reduction of roughly 25dB in signal-noise ratio from our
"normal"
operating level.
Sending slow-scan television (SSTV) via the optical
link:
Another experiment that we decided to try was to send slow-scan
television (SSTV) signals over the optical link. Because
SSTV
images are frequency-modulated at audio frequencies, there was
little
doubt that this would work, but we wanted to try it just the
same.
While we
could have taken pictures on-site and
then used
a laptop computer to send them, we decided that we were going to
have
our hands full, anyway, just doing the planned activities, so I
prepared,
beforehand, audio files containing the SSTV pictures.
These were
generated using the MMSSTV program, recorded to a digital audio
file,
converted to MP3 format, and then loaded onto digital audio
players. I noted that the MP3 encoding caused some visible
degradation to the SSTV pictures - a sort of weak "solarization"
type
of noise, but I figured that the likely amount of degradation
over the
optical link would be greater than this.
In the case of the SSTV files to be used by those at Inspiration
Point,
I simply emailed the MP3 files to Gordon for later
playback. To
simulate some air of authenticity, I used pictures taken during
the
8/25 expedition in addition to simple computer-generated
graphics.
During the evening, the SSTV images were transmitted in both
directions
and recorded: While I managed to see some of the images at
the
time of original transmission by placing an open microphone near
the
speaker, it wasn't until
later that the recordings were played back and images displayed
and
captured. These images are shown, as-received, with
no additional noise reduction applied in
Figure 6.
As can
be seen, the images
transmitted from Inspiration Point with a 200 milliamp LED
current are
noisier than those transmitted at full power - but this isn't
unexpected.
It may also be noted that the SSTV images transmitted via Laser
pointer
look pretty much the same as those transmitted via LED, despite
the
extreme amount of scintillation from the former: Because
SSTV is
based on FM rather than AM, it is, for the most part, resistant
to
amplitude variations - plus, the SSTV decoder has a bit of a
"flywheel"
that allows it to "fill in" very brief periods where signal is
absent.
A number of image formats were used: The small
black-and-white
images use the 12-second monochrome standard while the small
color
graphic-only images used the 24 second Robot format. The
"real"
pictures are, as was mentioned, taken from the August 18
expedition,
with the small versions having been transmitted using the 36
second
Robot format and the large images with the PD120 (120 second)
format. I noticed, after the fact, that a complete 120
second
image had not been transmitted via the Laser pointer.
The noise present in the images is mostly a result of static
crashes -
some of them from very distant, unseen thunderstorms, while
others were
from strobes from passing airplanes, warning strobes on towers,
and the
occasional "pop" of noise from an unknown source. For
whatever
reason, the amount of extraneous noise (pops, crashes, etc.)
seemed to
diminish over the course of the evening: Because the
images sent
at 200 milliamps were transmitted early on, they would have been
"cleaner" had they been retransmitted near the end of the
testing,
possibly looking more like those transmitted using the full 1100
milliamps.
Trying out the "Cheap" transceiver:
The
second
of my
optical transceivers is one that had been
constructed
quickly and cheaply for the sole purpose of rapidly assembling
another
unit to take out in the field: After all, what good is
just one
unit if there isn't someone else with another one to talk
to?
From this need was born an inexpensive and quickly-built optical
transceiver constructed out of foam-core poster board, using
page
magnifier type Fresnel lenses. Its performance has been
measured
as being notably inferior to that of the "good" transceivers,
but we
wanted to know how well it would work, so I set it up.
- 2-way
exchange using the "cheap" enclosure (mp3,
1:14, 583
kB) The audio as received at
Nebo (on the
south end)
is in the LEFT channel while the audio as
received at
Inspiration Point (on the north end) is in the RIGHT
channel.
- Into both channels - particularly into the LEFT channel -
there
is some crosstalk which causes the local transmitted audio
to be heard
in the receiver, so keep this in mind when you are
listening. You
may
want to mute one channel (or lift off an earphone) to hear
just one end
at a time.
- Note: At the time of recording, the audio
level
being fed to the recorder at Nebo (the audio in the LEFT
channel) was extremely low, and much of the noise being
heard was from
the audio recorder itself: Over 50dB of amplification
was added
to the digitized recording in order to make its level
usable.
As can be heard, the signal-noise ratio is lower, but
communication is
still quite possible. It might also be noted that the
level of
the voice is noticeably lower than that of the tone - an
indication
that the speech compressor in the modulator (I was using the PWM
circuit for
this particular exchange) was not working to its full potential.
Figure 7: Pictures
from the
south end of the path at Nebo.
Top Left: Clint, doing final assembly of the
optical
transceiver. Top Right: Looking at the
distant LED
through the telescope. Center Left: The
optical
gear, set up on the table and operating. Center
Right:
Dale
and Clint, in front of the optical transceiver.
Bottom
Left: A view toward Inspiration Point from
Nebo, showing the
distant LED in the background on the left, the lights of
Provo on the
right, and a nearby fencepost in the foreground. Bottom
Right:
The
distant dot, as viewed through the 8" telescope.
(The nearby
ridge may be seen in this monochrome photo. (Photos
by Dale,
Clint, and the camera's built-in timer.)
Click on an image for a larger version.
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Lessons learned
Testing the gear:
One of the main reasons that we wanted to re-do this path was to
verify
that the gear was working properly, giving us hope that under
better
air conditions, we would be able to span even farther distances
than
this 107 mile path: While we were encouraged that we were
able to
complete a contact using this same gear under deplorable seeing
conditions on August 18th, we still wanted to know how well this
would
work under more normal, clear air conditions. We were
gratified
to find out that this did, in fact, work nicely.
Failure of the scintillation compensator:
One notable failure was that of the Scintillation Compensator
built
into the audio interface box. When it was switched in, an
effect
was noted at both ends - one that Ron described as being similar
to
"squelch clamping" - that would occur
in which audio peaks were muted with a loud click: Despite
having
tested this circuit in the field on two previous occasions, it
seemed
that this evening's combination of audio level, audio content,
and
scintillation was causing something else to happen.
Fortunately, the digital audio recordings contained what was
needed to
diagnose and
fix the problem after the fact. Because these recordings
contain
"un-processed"
audio, it was simply a matter of playing them back into the
audio
interface box to recreate that night's conditions and
problems.
By doing so, it was
discovered that when the AGC in the scintillation compensator
was near
its maximum gain, the DC offset in the variable gain amplifer's
op amp
- combined
with leakthrough of the gain control voltage - would cause the
op amp
to smash into one of the supply rails. As it turned out, a
simple
resistor value change was all that was required to speed up a
time
constant and completely solve the problem.
Adding more microphone and headphone spigots:
As per Ron's suggestion, I copied his idea of making an
"octopus" box
that had two microphone inputs (with a selector switch) and four
headphone output jacks: This allows several bystanders to
all
wear headphones - something recommended to avoid feedback
between the
transmitter and receiver - and have the capability of quickly
and
easily switching between two microphones - which, in our cases,
were
built into the headphones. I also added a second "speaker
output"
jack to the audio interface into which one could plug headphone
without
muting the speaker - just in case others were nearby who wanted
to
hear, or if you wanted to connect an external speaker that one
could
place somewhat distant from the transmitter to avoid feedback -
a
useful feature if one is wandering around nearby, trying to
reconfigure
gear, but unable to be close enough to wear headphones.
A few final comments on the audio:
Using identical audio recorders made it much easier to
synchronize the
disparate audio tracks: Both devices had sample rates that
were
extremely close to each other, minimizing the drift over
time.
For synchronizing, I used the "Audacity" program - an
open-source
software package that is available for many operating systems
that has
a number of very useful features.
When synchronizing audio, I noticed that most of the time, a
"pop" or
crash would be heard at one end of the path but not the
other.
Interestingly, however, there were a number of instances were
strong
single "pops" were very audible at both ends of the path.
The
interesting thing about these single "pop" noises is that they
did
not
appear to be due to lightning: Experience gained on August
18th
shows that lightning strikes always seemed to have multiple
pulses and
were
not single-impulse events. While it is
certainly
possible that these common "pop" noises are from a particular
strong
portion of a lightning strike - most of which consists of
multiple
discharges - they seemed to be solitary in nature.
In addition to the pops and clicks, there was the expected "hum"
from
city lights, mostly consisting of 120 Hz and 360 Hz, both being
results
of modulation of lights on both sides of the sinusoid with
three-phase
AC power. Also related was a steady "hiss" which is the
strong
thermal noise component of urban lighting. A final test
was to
point the optical receiver skywards, at which point the hum and
some of
the hiss went away, the result being that the "zero-signal"
level as
measured across the bandwidth of 0-4 kHz decreased by about 6
dB.
Additional
details:
I'd like to thank those that
helped, including:
- Dale, WB7FID who was with me at the Nebo end.
- Ron, K7RJ, at the far end.
- Elaine, N7BDZ, Ron's
much better half.
- Gordon, K7HFV - also at the far end.
And, of course, Chris, VK3AML and Mike, VK7MJ, and the others in
VK-land.
Figure 8: Pictures
from the
north end of the path at Inspiration Point.
Top Left: Gordon and Ron, connecting all of
the many
cables. Top Right: Looking at the
distant LED to
the south. Center Left: Gordon and Ron
operating,
with the distant LED between them and the city lights in
the lower
foreground. Center Right: The
"business" end of the
LED transmitter. Bottom Left: Ron and
Gordon, in
conversation with those at the other red dot. Bottom
Right:
Elaine
in conversation over the optical link. (Photos by
Elaine
and Ron)
Click on an image for a larger version.
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At the south end of the QSO:
Present:
Clint,
KA7OEI
with Dale, WB7FID.
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
Gordon, K7HFV
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)
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
an Insignia NS-DV4G portable audio player. This device
can play
not only audio files (such as MP3, WMA, OGG, and WAV) but it
will also
play videos, show still pictures, receive (and record) FM
radio
broadcasts and, most importantly on this occasion do an
excellent job
of
recording audio from a stereo line-in connection.
Although
capable of recording compressed audio in WMA format, we used
the
(uncompressed) WAV format to yield a 2-channel, 16 bit audio
with a
32,000 sample/second rate. At this sample rate, well
over 5 hours
of recording is available on the 4 gig storage space - with
the caveat
that one must not allow any single audio file
to exceed
2 gig of file length - a precaution exercised by
occasionally stopping
and restarting the recording to a new file.
Optical transceiver used on the North-to-South link:
- This enclosure 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.
Optical transceiver used on the South-to-North link:
- The enclosure 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.
- For modulation of the Laser and the LED used in the
telescope -
and for the "Cheap"
cardboard
enclosure, the Pulse-Width
modulator 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.
Return
to the KA7OEI Optical communications Index page.
If you have questions or comments concerning the
contents
of this
page, feel free to contact me using the information at this URL.
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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2007-2010. Last update: 20101118