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From: Jeff Liebermann on 3 Apr 2008 13:07
On Thu, 03 Apr 2008 10:41:08 -0600, msg <msg@_cybertheque.org_> wrote:

>Adam Chapman wrote:
>
>> A helical antenna looks the best as it is the most isotropic,
>> although it is also pretty big for our aircraft. However even that has
>> +-45 degree 3dB lobe, so i might have to have the aircraft bank more
>> when closer to the centre of its circular mission area.
>
>How about taking a lesson from radiosonde antennas; I don't know about
>current models, but for a very long time the antenna at 1680 MHz was
>a 1/4 wave stub mounted at the center of an inverted conical reflector
>(about 60 degrees). When suspended from the balloon, the antenna points
>downward and illuminates the ground with a pattern formed from that
>of the stub combined with the reflector, giving horizon-wide coverage
>with no nulls for high inclination either.

Radiosondes have two basic assumptions. The transmitter is always
above the receiver and that antenna is always roughly vertically
oriented. Those assumptions don't work well with an RC airplane,
which can easily be inverted, at odd angles, and at the same elevation
as the controller. Incidentally, the usual RC screwup is losing
control on landing, where the ground (small hills, cars, buildings)
block the signal and the radio link loses control.

>Also, you could mount two such antennas on the fuselage, at 180 degrees
>top and bottom and use a splitter to drive them so that there is
>coverage during maneuvers.

Yep. That's what I'm thinking. Two cardioid patch antennas make a
fair hemispherical pattern. The receive sensitivity won't be affected
much by the splitter, but the xmit power will be equally divided
between the two antennas. Link calcs in each direction will need to
be independently calculated.

--
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 3 Apr 2008 13:26
On Thu, 03 Apr 2008 10:43:07 -0600, msg <msg@_cybertheque.org_> wrote:

>Jeff Liebermann wrote:
>
><snip>
>
>Sounds like a heck of a project you did ;)

Not "my" project. I originally got involved in bailing out a group of
arrogant engineers by cleaning up the antenna design. That grew
rapidly into cleaning up the control system, data link, and tracking
mechanism. The basic design was already done when I arrived. I just
made it all work.

One of the bad habits found in engineering is stopping when there is a
problem. These guys were running in circles around the problems
without actually attacking the problem. One had spent about a month
doing a computer simulation of a fundamentally flawed control system.
So, I got to jump in with both feet, turn the muddy waters into
quicksand, and kick those involved into action. Once the sacred cows
were slaughtered and sacrificed, and those involved were willing to
question their own assumptions, progress was rapid. I wasn't the only
one spending sleepless nights on this project.

>In days gone by, I had feeds from the MPQ-10A mortar tracking radar as well
>as the GMD-1 ground station.

2.7GHz, as I recall. Nice:
<http://www.primeportal.net/artillery/don/mpq10a_walk_1.htm>
It should work well at 2.4GHz for a Wi-Fi "shoot out". Got any more
sitting around?


--
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: DTC on 3 Apr 2008 17:57
Jeff Liebermann wrote:
> Radiosondes have two basic assumptions. The transmitter is always
> above the receiver and that antenna is always roughly vertically
> oriented.

Not so. The weather reporting radiosonde used in the 60s and 70s
aways drifted off to the horizon, driven by the prevailing wind.

The radio receiver was something like a ten foot diameter dish
under a plastic radome that tracked the radiosonde.

As i recall, they had a single tube transmitter that was frequency
modulated by the thermometer and air pressure was sensed by a
bellows that was mechanically connected to a rheostat that pulse
proportional modulated the signal. All this is going back over
thirty years ago when I played with the project.

Direction was resolved by the rotation coordinates of the dish.
From: Adam Chapman on 3 Apr 2008 19:13
On Apr 3, 6:02 pm, Jeff Liebermann <je...(a)cruzio.com> wrote:
> On Thu, 3 Apr 2008 08:12:00 -0700 (PDT), Adam Chapman
>
> <adam.chap...(a)student.manchester.ac.uk> wrote:
> >I do hope that Im not appearing to be lazy here, I've never had to
> >work in wireless systems as I am an aerodynamisist recently turned
> >image processing geek.
>
> If this is a skool project, you are expected to do your own research
> and calculations.  You don't learn much by having us doing it for you.
> If you have a *SPECIFIC* problem with some aspect of the project, feel
> free to ask.  However, the basic design in implimentation should be
> all your work.
>
> >I was expecting my question to have an easy
> >answer but as usual here wasn't so sorry and thank you for taking your
> >own time to help me.
>
> Welcome to engineering at its worst.  The URL I listed:
> <http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>
> shows a simplified wireless link calculation.  You are apparently
> using 802.11b/g instead of VSB modulation for video (and data) so the
> examples should work.  Use this as the basis for your calcs.  However,
> please note that the resultant numbers are the *BEST* case
> calculations.  Real world situations only make things worse.  For
> example, the 2dBi gain of the antenna is actually the MAXIMUM gain of
> the antenna.  If you are off axis from the antenna pattern, the gain
> (and range) will be less.  Receiver sensitivity is never as good as
> they claim.  This should help:
> <http://802.11junk.com/jeffl/rx-sens/receiver%20sensitivity.htm>
> You will need to specify the other end of your radio link (equipment,
> coax, antenna) before I can help with the numbers.  It's really quite
> simple.
>
> >The camera is a Trendnet IP-TV301W model, specification at
> >http://trendnet.com/products/proddetail.asp?prod=110_TV-IP301W&cat=48..
>
> OK.  802.11b/g.   Draws 7 watts (at 12V) and weighs 0.5kg.  Do you
> have any idea of how big a battery will need to be to supply 7 watts
> of continuous power?  4ea LIPO cell phone batteries will do the trick,
> but you'll only have perhaps 15 minutes of operating time.  To lift
> all this, the plane doesn't need to be huge, but it certainly won't be
> a styrofoam fly weight.
>
> >I don't physically have the camera yet because the university is still
> >processing the account, which in my experience can take a very long
> >time.
>
> The local university owes me some money from a project that is now 22
> years overdue.  Good luck.
>
> >If it is possible to estimate transmission ranges with certain
> >equipment, I would have an easier job justifying the expense to my
> >department.
>
> I don't see the connection between range and funding.  I suspect
> you'll do better with a feasability and finance study, than with a
> performance estimate.
>
> >The 2dBi antenna can be replaced, if there is a better type then I am
> >happy to use it. I would prefer an antenna that emits as spherical a
> >pattern as possible, otherwise we will have to limit aircraft
> >maneuevres.
>
> It really depends on what material you make the airplane from.  If RF
> transparent, a simple monopole (as supplied) will work.  When
> inverted, the signal will go through the aircraft.  However, if you
> make the airplane from reflective or absorptive materials, you'll need
> multiple antennas.  One 1/4 wave monopole on top and one on the bottom
> should be sufficient.  With a small ground plane under the antenna,
> the pattern should be mostly cardioid shaped.  If you have a wind
> resistance problem, you can use a small ceramic patch antenna instead.
> It is possible to model the antenna pattern using (free) antenna
> modeling tools:
> <http://home.ict.nl/~arivoors/>
> The sample files include a 747 aircraft (which will need to be
> scaled).  This is not exactly a trivial exercise, a potential time
> burning diversion, and is probably overkill for what you're
> attempting.  It's easy enough to construct a model, install the
> camera, and just play with the position of the antenna until it's a
> workable compromise.  
>
> Note that installing a bigger antenna on the airplane is not what you
> want.  Bigger antennas imply more gain.  You want hemispherical
> antenna pattern which requires a smaller or simpler antenna.  Here's
> an approximation of an isotropic antenna:
> <http://802.11junk.com/jeffl/antennas/isotropic/index.html>
> I think this is overkill and probably not practical on an airplane. If
> you're going to do work with the antenna, do it on the ground station.
>
> >I have just recieved more information from the competition organisers,
> >who tell me that all flying will be within 500m of the launch site and
> >flying is not permitted above 400ft (121.92 m) This gives the max.
> >Euclidean distance as 514.65m, although i expect we will be measuring
> >that distance on the aircraft with GPS, and with the civilian accuracy
> >limitations on GPS (~ +- 12m), I would suggest a target transmission
> >distance of 550m.
>
> >Due to safety rules we will never lose sight of the aircraft so we can
> >expect no obstacles between transmitter and reciever.
>
> If this is competition flying, it's also likely (but not guaranteed)
> that you'll also get minimal 2.4GHz interference.  It's traditional to
> confiscate the controllers during the competition to prevent sabotage.
> That works because RC has dedicated frequencies.  That's NOT the case
> with 2.4GHz, where there's a chance that some lunatic, with a portable
> wireless router, can crash your flying machine.  You can also lose
> control when the aircraft flys over a coffee shop wireless hot spot.
> (I'm assuming that you'll be controlling on 2.4GHz).  I sure hope this
> is going to be run in the middle of nowhere.  Testing it at skool,
> where there's lots of 2.4GHz interference, is going to be ummm...
> interesting.
>
> >I've been slowly writing this reply all day, and thinking a lot as i
> >do so.
>
> At the beginning of any project, leaps of faith, wild ideas, the
> shooting from the hip, are standard procedure.  Once the ideas are
> ossified, then you can be more studious and careful.
>
> >A helical antenna looks the best as it is the most isotropic,
>
> Too big, too much drag, too critical, poor gain to volume ratio.  If
> you're going to go to that level of complexity, think about two
> ceramic patch antennas with a -3dB Wilkinson combiner.  Two back to
> back cartoid patterns equal roughly a hemisphere.
>
> >although it is also pretty big for our aircraft. However even that has
> >+-45 degree 3dB lobe, so i might have to have the aircraft bank more
> >when closer to the centre of its circular mission area.
>
> Translation.  If you have a hole in the antenna pattern, you're going
> to lose control.  A helix also has no gain in the reverse direction,
> so you'll need two.  Hint:  Look at what's on a real airplane or
> guided missile.
>
> >The link calculator link from LR told me that if i put an antenna with
> >a 7dBi gain (like the one at _) on the UAV and at the ground station,
> >i will have a range of over 7km with a 100mW transmitter, which I
> >assume can be achieved using an in-line amplifier. Does this
> >calculator use the range in the direction of the strongest energy
> >emisiion from the antenna?
>
> I'm still missing some numbers.  See the example calcs on the URL I
> mentioned.  Copy the table of numbers and plug in YOUR numbers.
> Remember, those are the BEST case calculations.  Reality only makes it
> worse.
>
> >I guess that more than on antenna on the same aircraft with different
> >orientations would interfere with each others signals?
>
> If the two antennas cannot "see" each other, they will not mangle the
> combined pattern.  What other signals?
>
> Good luck.  Sounds like an interesting project.  Watch your weight and
> power consumption.  
> --
> Jeff Liebermann     je...(a)cruzio.com
> 150 Felker St #D    http://www.LearnByDestroying.com
> Santa Cruz CA 95060http://802.11junk.com
> Skype: JeffLiebermann     AE6KS    831-336-2558

I have found a commercially available patch antenna, here at
http://www.cisco.com/en/US/docs/wireless/antenna/installation/guide/ant2485.html
and a groundstation antenna at http://www.radiolabs.com/products/wireless/waverv2.php
.
Now the cisco site I linked for the patch antenna does not say what
the Tx power is, althoug at a another site (http://www.cisco.com/en/US/
prod/collateral/wireless/ps7183/ps469/
product_data_sheet09186a008008883b.html) I read that the Cisco Aironet
2.4 GHz Bridge transmitter power is 20 dBm. I cant seem to find a
defintion for a bridge transmitter though, so im not sure if i know
the Tx power of the patch antenna or not. Does 20 dBm seem reasonable
for this antenna?


If the patch antenna power is 20dBm (which is the legal maximum in the
UK), then over a 550m range the SOM should be just below 28 dB
according to my calculations. Although I was aiming for 28 or above
the PER does not appear to be too bad.

Thanks again
Adam
From: msg on 3 Apr 2008 22:34
DTC wrote:

> Jeff Liebermann wrote:
>
>> Radiosondes have two basic assumptions. The transmitter is always
>> above the receiver and that antenna is always roughly vertically
>> oriented.
>
> Not so. The weather reporting radiosonde used in the 60s and 70s
> aways drifted off to the horizon, driven by the prevailing wind.

Indeed, the antenna was vertical, but the pattern permitted
horizon to horizon coverage but without nulls overhead.

>
> The radio receiver was something like a ten foot diameter dish
> under a plastic radome that tracked the radiosonde.

The GMD-1 dish often was exposed and not in a radome.

>
> As i recall, they had a single tube transmitter that was frequency
> modulated by the thermometer and air pressure was sensed by a
> bellows that was mechanically connected to a rheostat that pulse
> proportional modulated the signal.

There was a baroswitch which selected the sensor to read based on
increments in altitude; the modulation was PRR. The oscillator
was a pencil triode in a cavity resonating at 1680 MHz and tuned
with a screw probe.

Michael
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