On Apr 20, 5:23*pm, Jeff Liebermann <je...@cruzio.com> wrote:
> On Sun, 20 Apr 2008 04:43:38 -0700 (PDT), Adam Chapman
>
> <adam.chap...@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 * * je...@cruzio.com
> 150 Felker St #D * *http://www.LearnByDestroying.com
> Santa Cruz CA 95060http://802.11junk.com
> Skype: JeffLiebermann * * AE6KS * *831-336-2558

Thanks, I did do the maths before but admittedly I did it wrong. I
assumed that the Rx gain would be the same as the Tx power. Silly me!