wireless range vs. power [Wireless Internet]

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From: Adam Chapman on 3 Apr 2008 11:12

Thanks to all of you, I replied to this particular post because it had
the most questions for me to answer.
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. 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.

On Apr 2, 11:54 pm, Jeff Liebermann <je...(a)cruzio.com> wrote:
> On Wed, 2 Apr 2008 08:00:06 -0700 (PDT), Adam Chapman
>
> <adam.chap...(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.
>

The camera is a Trendnet IP-TV301W model, specification at
http://trendnet.com/products/proddetail.asp?prod=110_TV-IP301W&cat=48.
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. If it is possible to estimate transmission ranges with certain
equipment, I would have an easier job justifying the expense to my
department.

> 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.
>

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.

> >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?
>

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.

I've been slowly writing this reply all day, and thinking a lot as i
do so. 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.

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 guess that more than on antenna on the same aircraft with different
orientations would interfere with each others signals?

Adam

From: Adam Chapman on 3 Apr 2008 11:15

From: msg on 3 Apr 2008 12:41

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.

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.

Michael

From: msg on 3 Apr 2008 12:43

Jeff Liebermann wrote:

<snip>

Sounds like a heck of a project you did ;)

>
> 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.

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

Michael

From: Jeff Liebermann on 3 Apr 2008 13:02

On Thu, 3 Apr 2008 08:12:00 -0700 (PDT), Adam Chapman
<adam.chapman(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 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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