wireless range vs. power

Hi,

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

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

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.

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?

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?

Any help is heartfuly appreciated.

Adam

Reply to
Adam Chapman
Loading thread data ...

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.

Reply to
DTC

even if i have an omni-directional antenna?

Reply to
Adam Chapman

Communication is a 2 way operation and you would need to know the Rx sensitivity, Tx o/p, Antenna gain... of both the devices to get even a vague idea of the Link distance that would be achieved by 2 staic devices. If one of the devices is moving in 3 dimensions that would make it extremely difficult to calculate. You say it would operate in an area

500x500 metres...not true as it would be 500x500xheight metres. If you were
Reply to
LR

Bleeding cats..grrh. How do you intend to keep the Antennas in the same plane.Depending on the gain of the omni on the ground unit you may find that if your aircraft goes to high the camera's antenna may not be in the beamwidth of the groundstaions antenna.

formatting link
You could perhaps get a rough idea of the max distance of a static link using a link calculator:-
formatting link
adding movement in will just just reduce the workable distance. You will also have to try and factor in the Data transfer rate you need to use for an acceptable video quality. The higher the rate the less distance.

Reply to
LR

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 - ??

Reply to
P.Schuman

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

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.

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?

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.

If you can see your thumb, you can communicate.

Yes, there are calculations possible. They're actually fairly easy. See example at:

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.

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

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).

Reply to
Jeff Liebermann

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:

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.

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.

The wireless video and these should interfere nicely.

Ugh. Back to working on my taxes...

Reply to
Jeff Liebermann

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

Michael

Reply to
msg

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):

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.

Reply to
Jeff Liebermann

Sounds like a heck of a project you did ;)

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

Michael

Reply to
msg

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.

2.7GHz, as I recall. Nice:

It should work well at 2.4GHz for a Wi-Fi "shoot out". Got any more sitting around?

Reply to
Jeff Liebermann

Do you think an 11.1V battery would do the trick? Im looking at this one

formatting link
^currency=GBPAt the cost of another 40 grams i could use a 14.8V battery and put some resistors in.

Reply to
Adam Chapman

Which battery were you thinking of? I'm not going to pick one for you as there are quite a few on the site. Is this going to also power the servos, receiver, propellers, etc or just the camera? You do the numbers, and I'll check the results. I'm not doing your homework for you.

The specs say that the camera at:

show that it has a 12v 1.5A wall wart. That's not what it draws in power, but that's what the power supply delivers. Your first step is to *MEASURE* what it really draws. Don't be suprised if you get wide variations depending on what the camera is doing, whether the IR illuminator is running, frame rate settings, etc. Use and adjustable voltage power supply and see what voltage range the camera can tolerate. My guess(tm) is that the camera will run down to perhaps

8VDC or maybe even lower if it has an efficient switching regulator inside. Measure the current at various points and calculate the efficiency. I wouldn't be suprised if the camera can run on a 2 cell battery pack (7.4VDC).

Once you know how much energy (i.e. watts/time) is required. LIPO batteries have a maximum current draw spec which should not be exceeded or you kill the battery. Watch out for so called "burst" current ratings on the batteries. It's the battery that will burst if you go over. Be sure to allow for temperature variations as it has a huge effect on battery capacity. I'm not sure what percentage of the capacity of the battery you can discharge down to, but if you go too far, you also kill the battery. You'll need to know this number. How long were you planning on draining the battery, which really means how long is this thing suppose to be in the air? If it powers just the camera, you can turn it off for the takeoff and landing, thus saving energy.

Anyway, do the numbers. No Mathcad model needed for this one. Just some measurements and arithmetic.

Reply to
Jeff Liebermann

Hey Jeff,

Do you have the Mathcad 4.0 "Signal Processing Function Pack" or the smaller "EE Electronic Handbook"? From the ad literature (way back) the filter design tools appear to provide a level of interactivity beyond the average (probably much better than the old tools that I use).

Michael

Reply to
msg

Nope. I think you mean the Signal Processing Extension Pack. I'm using a "borrowed" copy of Mathcad 3.1. A former employer has the real copy and some of the add ons. We're not currently on good terms so I have no access to the nifty software or test equipment.

Mathcad 4.0???? The current version is 14.0. Only $1200 for Mathcad and $345 for the Sig Proc Exten Pack. Ouch.

Dunno. It's been a while since I did anything with Mathcad and don't recall the details. I'm also not very good with using it.

Reply to
Jeff Liebermann

Sorry i didnt realise the link was for the list rather than the specific battery i chose. This battery will only be powering the camera and onboard wireless stuff. I just wondered if 11.1 volts would be enough. Your view was similar to mine, that the camera has a tolerance. I should be able to get 13 minutes out of the camera, and if we run out of power for the camera in flight, we can just land and plug a spare in.

Thanks Adam

Reply to
Adam Chapman

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.