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From: P.Schuman on 2 Apr 2008 16:33
Adam Chapman wrote:
> On Apr 2, 4:37 pm, DTC <m...(a)nothingtoseehere.zzx> wrote:
>> Adam Chapman wrote:
>>> My problem is that I don't know how to calculate the achievable
>>> transmission range from power and frequency. Is there a rule of
>>> thumb calculation for finding the transmission range?
>>
>> There are too many unpredictable variables. If your airplane tilts in
>> just the slightest, you'll loose the signal. All you can really do is
>> try it out and see how far it works.
>
> even if i have an omni-directional antenna?

remember that any omni antenna readiates as a donut sitting over the
antenna.
So - just as an example - imagine if the plane was directly overhead,
you might be able to see it, but not receive the video since your receiving
"donut"
and the plane's transmitting "donut" would not intersect.

As you can see - the name of the game is for both plane + ground station
to have intersecting donuts....
As the plane flies away, you may actually receive a better signal
as the donut beamwidth pattern increases with distance - but strength
decreases -
SO - ??


From: Jeff Liebermann on 2 Apr 2008 18:54
On Wed, 2 Apr 2008 08:00:06 -0700 (PDT), Adam Chapman
<adam.chapman(a)student.manchester.ac.uk> wrote:

>I've ordered a wireless webcam that will be out onboard an autonomous
>aircraft for a university project.

I've done that. You're about to have a battery problem.

>The camera has a "2dBi Diversity Antenna" and transmits video over an
>IP protocol in the 2.4-2.4835 GHz range.

Duz this camera have a manufacturer and a model number? A URL with
the specifications would be nice.

2dBi is a simple monopole antenna. It has a radiation pattern that
looks like a donut. There's a big null when in line with the antenna,
such as when the aircraft is flying directly overhead.

>Now the aircraft needs to operate in a 500x500m area, although I
>expect that my ground station computer will be placed some distance
>away from the operating area.

I have this thing about numbers. How far is "some distance"? How
high does the air thing have to fly? What's the MAXIMUM distance you
expect to see an image?

>My problem is that I don't know how to calculate the achievable
>transmission range from power and frequency.

Neither do I. You've supplied exactly one non-ambiguous number (the
2dBi antenna gain). What's missing is everything else that's on the
specification sheet.

>Is there a rule of thumb
>calculation for finding the transmission range?

If you can see your thumb, you can communicate.

Yes, there are calculations possible. They're actually fairly easy.
See example at:
<http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>
However, you're doing video which requires some tweaks to the
calculations (depending on bandwidth and modulation method). I can't
help without you supplying some numbers.

>UK regulations say that I cant have a power above 100mW- does anybody
>know what range is achievable with this power in the 2.4 GHz band?

Sorry, no answer without knowing the antenna gains, receiver
sensitivity, SNR required, and modulation method.

>Any help is heartfuly appreciated.

Take your video xmitter and receiver and connect to a portable TV.
Start walking. When the picture starts to look horrible, stop. Record
the distance. Let's say it's 100 meters.

If you increase the antenna gain by 6dB, you will get twice the range
or 200 meters. 12dB increase in gain will give you 4 times the range,
or 400 meters. 18dB gain will yield 8 times the range. Ad nasusium.
The problem is that every time you increase the gain, the beamwidth
and pattern of the antenna gets narrower and narrower. The airplane
mounted video camera I helped throw together had about a 1000 meter
range. However, we used a 24dBi dish antenna on the ground, that
tracked the aircraft (both automagically and manually).

--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: Jeff Liebermann on 2 Apr 2008 19:13
On Wed, 02 Apr 2008 15:37:48 GMT, DTC <me(a)nothingtoseehere.zzx> wrote:

>If your airplane tilts in
>just the slightest, you'll loose the signal.

Nope. A 2dBi donut pattern isn't the same as a hemispherical pattern,
but close enough. When we were tinkering with model airplane video,
the best antenna was a 3cm coaxial antenna (1/4 wave driven element,
1/4 wave coax sleeve) made out of a piece of RG-175, dangling roughly
downward. The only times when the signal disappeared were when the
plane flew overhead and when the body blocked the signal (i.e.
inverted flying). The null was rather sharp and the image returned
quickly. It's not as bad as it would seem. Some of the other model
airplane pilots were using patch antenna, which had less drag, and
were only useful in level flight.

What was really cool was flying the plane with the camera in the
cockpit and watching through video goggles. 640x480 for each eye:
<http://www.vuzix.com/iwear/products_vr920.html>
Nice for watching 3D TV when not flying. Say goodby to about $350 for
those.

Incidentally, almost all the ground based video receivers had some
manner of directional antenna attached. Ours was overkill with a
24dBi dish (because I had one handy). Others were using everything
from cantennas to big panels. There were a few high gain omnis, but
they didn't work too well.

>All you can really do is
>try it out and see how far it works.

Yep. I kinda prefer calculations first, but a sanity check is always
a good idea.

Thought 2.4GHz was kinda crowded? Futaba 2.4GHz radio control.
<http://2.4gigahertz.com/>
The wireless video and these should interfere nicely.

Ugh. Back to working on my taxes...


--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: msg on 2 Apr 2008 22:33
Jeff Liebermann wrote:

<snip>

> The airplane mounted video camera I helped throw together had about
> a 1000 meter range. However, we used a 24dBi dish antenna on the
> ground, that tracked the aircraft (both automagically and manually).

Did you build a nutating feed for the ground station, or perhaps a
phased array with electronic nutation?

Michael
From: Jeff Liebermann on 3 Apr 2008 02:43
On Wed, 02 Apr 2008 20:33:34 -0600, msg <msg@_cybertheque.org_> wrote:

>Jeff Liebermann wrote:
>
><snip>
>
>> The airplane mounted video camera I helped throw together had about
>> a 1000 meter range. However, we used a 24dBi dish antenna on the
>> ground, that tracked the aircraft (both automagically and manually).

>Did you build a nutating feed for the ground station, or perhaps a
>phased array with electronic nutation?
>Michael

Nope. There were numerous models of tracking antenna. That was my
job. The problem was that the spread spectrum signal does not have a
carrier, which made it difficult to throw together an easy tracking
circuit. The ability of the airplane to fly close and perpendicular
to the antenna made tracking even more difficult. Worse, the 24dBi
dish has a -3dB beamwidth of about 7 degrees. What seemed like an
easy project turned into a real mess.

One scheme that came really close to working required 4 additional
receivers. The center feed mount was extended beyond the feed and
made from an RF absorber (i.e. PVC pipe fill of water). 4 antennas
were arranged in 4 quadrants on each side of the center feed pipe
extension. With the signal source dead ahead, the signal levels at
all 4 receivers is (allegedly) identical. If the antenna were offset
in one direction, the center pipe casts an RF "shadow" on the one of
the 4 antennas. A differential amplifier runs a gimbal mounted pair
of motors to correct the direction.

This worked well fairly for tracking under ideal conditions, but had
acquisition problems that drove me into overtime and later panic.
There were just too many reflections that screwed up the direction. If
I had a deep solid dish (large f/D ratio), instead of a barbeque grill
dish, the reflections would have been minimized and it might have
worked better. However, once the direction antennas were hit by a
reflection, the motor would swerve the antenna radically, losing lock,
and not easily recovering.

The radio link was via 802.11b wi-fi so unlike the ATV link, there was
a transmitter involved at the dish. When it transmitted, the
directional receivers were instantly overloaded. I worked around this
problem by temporarily disabling the servos in transmit.

I eventually gave up on the RF approach.

There were several other attempts to build a tracker. I finally threw
together an optical system that worked. It was similar to the RF
tracker, but was immune to all but the most disgusting reflections.
The aircraft carried several green LED's that pulsed at about 100 Hz.
The 4 antennas were replaced by 4 security camera lenses and photo
transistors. The center pipe was replaced by a fiberglass tube, which
blocks light, but passes RF.

This system worked much better, especially at night. It crapped out
when pointed into the sun, when some dingbat shines his flashlight on
the antenna, and when I took a flash photo. Range was limited to
about 300 meters. I never did nail down the servo loop damping
factor, so it tended to either crawl across the traverse, or twitch
badly as it moved. Neither seemed to bother the RF data link, but it
sure made everyone around the antenna nervous. Like the RF solution,
it worked for tracking and sucked for acquisition.

I had thought of using a nutating (conical scan) feed (straight out of
the WWII SCR-584 radar):
<http://www.carnarvonspace.com/wiki/index.php?title=SCR-584_Technical_Description>
However, I didn't have time to machine the required components and the
corresponding control system. When I was much younger, there were
tons of those feeds available in the WWII surplus stores, but those
are long gone today.

I have some other ideas on how to do a wi-fi tracking antenna.
However, I keep seeing high skool and college project proposals that
involve the construction of such systems. I don't wanna ruin it for
the students.


--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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