On Sun, 20 Apr 2008 04:43:38 -0700 (PDT), Adam Chapman
<adam.chapman@student.manchester.ac.uk> wrote:

>I want to put this: http://www.radiolabs.com/products/wireless/waverv2.php
>on the ground,

That won't do much for you. Your airplane will be able to hear the
signals from the ground somewhat better, but your ground station will
not be able to do likewise. You would need such an amplifier at both
ends in order for it to be effective.

>I was planning to be sneaky on the fly-off day.

Cleverness and sneakiness are good things. However, I think the
judges are looking for good engineering.

>The
>camera system uses an http protocol (not perfect i know, but the
>easiest to interface with matlab for now).
>It's for a model sized aircraft (about 3m wingspan).

Duh. I forgot that you were the one building the autonomous model
airplane. I've been engaged in various email exchanges on the subject
of flying a camera in a much larger airplane, and was thinking along
those lines.

>Flying range has
>been specified as 500m, but also within visual range at all times for
>safety. Im not sure how 'visible' the aircraft will be over 500m, so
>we will probably be flying closer, and probably no higer than 100ft
>too.

Well, I can see you haven't bothered to do the math. I'll do a dry
run for you. See:
<http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>
I'll assume that you're using the "Waverider" gizmo, which belches
200mw of RF (+23dBm) and a 3dB power divider with a 0.5dB port loss.
I'm not sure what frame rate you're expecting so I'll assume that you
can tolerate the slowest OFDM speed (6Mbits/sec) as the 802.11b speeds
are generally not that useful for streaming anything. As I recall,
you're using this camera:
<http://trendnet.com/products/proddetail.asp?prod=110_TV-IP301W&cat=48>
Unfortunately, the data sheet does not offer RF specifications, so
I'll conjur my own. That basic idea is to see at what range you can
maintain a 20dB fade margin.

From the air to the ground:
TX power +15dBm
TX coax loss -1dB (includes connector losses)
Divider loss -3.5dB (half the power less internal losses)
TX ant gain 6dBi
Distance unknown
RX ant gain 2dBi
RX coax loss 0dB (assumes good RF amp in Waverider thing).
RX sens -88dBm (at 6Mbits/sec)
Fade margin 20dB

Plugging the numbers into:
<http://www.terabeam.com/support/calculations/som.php>
I get 0.12 miles or about 200 meters. Not very good.

The other direction, from the ground to the air is a bit different.
The power divider does NOT split the signal coming in from the two
ports. Therefore, the loss is less.

From ground to air:
TX power +22dBm
TX coax loss -0.5dB (connector losses)
TX ant gain 2dBi
Distance unknown
RX ant gain 6dBi
Divider loss -0.5dB (internal divider losses)
RX coax loss -0.5dB (connector losses).
RX sens -88dBm (at 6Mbits/sec)
Fade margin 20dB

Plugging the numbers into the calculator, I get 0.4 miles or about 650
meters. Much better but barely adequate.

Worse is yet to come. The above guesswork is based totally on the
assumption that your ground and air antennas are oriented for maximum
gain. This is obviously not a good assumption as the aircraft be off
axis for the majority of possible orientations. Therefore the 6dBi
antenna gain may actually be considerably less when oriented off axis.
For example, if you lose 3dB of gain due to orientation errors, your
range will be 0.707 times as far. 6dB loss is good for 0.5 times the
range.

Someone is sure to suggest that slowing down the system from
6Mbits/sec to 1Mbit/sec will dramatically increase the range. That
doesn't work. Look at the receiver sensitivity chart at:
<http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>
At 6Mbits/sec OFDM, the rx sensitivity is -88dBm. At 1Mbits/sec BPSK,
the rx sensitivity is -89dBm. For 1dB of additional gain, it's hardly
worth a 1/6th decrease in speed. That's also why I suggest you use
the slowed OFDM speed.

>Because the little laptop antenna i linked fits onto the screen
>of the laptop, i was hoping to tilt the screen so the high gain region
>points at the aircraft whilst a pretended to squint at the screen.

Get a tripod. Mount a fairly directional antenna on the tripod. You
don't need much gain, but you do need a wide beamwidth. Another 6 or
8dBi panel will do. Point it in the general direction of the test
range. Attach to your laptop. Put a photographers blind black cloth
over your head. Don't worry about looking weird.

>Because we will be flying pretty low and looking for targets on the
>ground with the camera, which looks directly down and is fixed to the
>aircraft belly, we don't really need a signal to be transmitted
>downward. (we won't need to identify ourselves as targets
>hopefully!).

Perfect. If you're not going to fly at much altitude, an
omnidirectional antenna (monopole) sticking out of the belly, will
work just fine. The radiation pattern is a horizontal donut. As long
as you maintain level flight, you're fine. Do aerobatics and you'll
have problems, but level flight up to about 30 degrees is just fine.

>I see what the alligator term represents, big mouth, little ears.

Yep. It's asymmetrical.

>A
>linear amplifier amplifies a signal in ONE direction and not in the
>other, but does it reduce the signal in its non-amplifying direction?

Most such RF amps have a receive amplifier. The problem is that it
doesn't improve the receive sensitivity in any useful way. The access
point (or laptop wireless card) already is running at the limit of
receiver sensitivity. Adding additional gain will only add additional
noise and reduce dynamic range of the receiver. What the receive
amplifier will do that's useful is compensate for the coax cable
losses between the amplifier and the receiver. However, for your
laptop installation, that's very little coax, very little loss, and
very little benifit.

>I guess I could use two alligators if they had big enough mouths.

Yep. Try to keep the tx power at both ends roughly the same. If
you're going to fly a power splitter, you'll need 3dB (twice) the TX
power output of the ground station in the airplane in order to
compensate for the splitter losses.

Good luck.

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