Digital Spread Spectrum in FM Bands

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Digital Spread Spectrum in FM Bands

Postby skyhigh » Thu Jun 24, 2010 5:28 am

So spread spectrum technology is ubiquitous now in the 2.4GHz band. Is the technique specific to these higher frequencies due to some physical limitation?

That is, is it possible to have DSSS or FHSS in, say, the 72MHz band? I fly FPV using 2.4GHz and never fly more than a mile -- just too nervous about it. But I hear that 3 miles or 5 miles is a regular flight for the guys doing FPV in 72MHz band. I do realize that 2.4GHz is in the microwave band and it is highly suspectible to scattering just from moisture in the air -- we do use EM radiation in this band to cook food of course.

So the folks flying at 72MHz enjoy the benefits of super long range just from the physics of their radio waves. But I've always thought that if I were to fly miles away over some area where a random kid just gets his new RC aircraft toy for his birthday and just happens turn it on right on my channel then I'd get shot down for sure since his transmitter's location is much closer to my FPV plane and would be exponentially stronger than my transmitter's signal 3 miles away.

I don't understand why folks do not worry about getting shot down using FM radios for FPV. FPV flights go well beyond the boundaries of a normal flying club's field so frequency control is not available out there. But if we had DSSS or FHSS spread spectrum in the FM bands then it would be FPV nirvana.
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Re: Digital Spread Spectrum in FM Bands

Postby RCModelReviews » Thu Jun 24, 2010 6:31 am

skyhigh wrote:So spread spectrum technology is ubiquitous now in the 2.4GHz band. Is the technique specific to these higher frequencies due to some physical limitation?

It's not so much a factor of the frequency but the amount amount of space we're allocated.

The 72MHz band is just 1MHz wide whereas the 2.4GHz band is over 80MHz wide and each "frequency" used by 2.4GHz RC systems is usually a MHz or more in width. So, as you can see, there's just not enough room on 72MHz to run spread-spectrum technology effectively.

So the folks flying at 72MHz enjoy the benefits of super long range just from the physics of their radio waves. But I've always thought that if I were to fly miles away over some area where a random kid just gets his new RC aircraft toy for his birthday and just happens turn it on right on my channel then I'd get shot down for sure since his transmitter's location is much closer to my FPV plane and would be exponentially stronger than my transmitter's signal 3 miles away.

Most 72MHz radios are claimed to have range of "at least a mile" and most 2.4GHz systems are "1.2Kms to 1.5Kms" or just under a mile.

In reality however, many RC systems have been tested and found to work at ranges of 5Kms (3 miles) or more -- and that's from ground to ground. Once the receiver is elevated (such as in a flying model) the range can be further.

I intend to use my UAV to properly test the actual ground-to-air range of popular RC systems but would not be surprised to find that it is 6-8Kms (4-5 miles)in the case of good systems such as Futaba, Hitec or FrSky.

I don't understand why folks do not worry about getting shot down using FM radios for FPV. FPV flights go well beyond the boundaries of a normal flying club's field so frequency control is not available out there. But if we had DSSS or FHSS spread spectrum in the FM bands then it would be FPV nirvana.

2.4GHz FPV is quite practical -- one advantage is that you can use hi-gain antennas to get extra range from the same power of transmitter (if your country's laws allow it). You can also use hi-gain antennas on your video receiver to really extend the range and there's no (legal) limit as to the power of the receiver antenna.

I'll be testing and reviewing 5.8GHz video equipment very soon, since that's the most practical frequency for FPV users who have 2.4GHz RC systems.
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Re: Digital Spread Spectrum in FM Bands

Postby skyhigh » Thu Jun 24, 2010 7:23 am

Thank you for a straighforward reply. It makes perfect sense now. I didn't know the 72MHz band was so narrow.

Looking forward to your UAV test results. One thing I would want to see, even if not frequently encountered, is how 2.4GHz would handle atmospheric moisture. For instance, fly at 3000ft above low level clouds or on a pea soup foggy day at 2 miles.

The most incredible FPV videos I've ever seen to date are from a guy in Austria. He flies his wing over the alps at long distances. This must be seen!

http://www.youtube.com/watch?v=njFTZh_e8o8&fmt=22

He has plenty other videos like this and says he's had too many troubles with 2.4GHz. But his flying style takes his FPV wing around tree tops up there! He uses 35MHz FM and with a very long antenna as you could calculate.
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Re: Digital Spread Spectrum in FM Bands

Postby RCModelReviews » Thu Jun 24, 2010 7:38 am

I've flown my own models into cloud and retained control.

I think one of the problems they have in Austria (as in many parts of Europe) is a limitation on the maximum power output that you can use on 2.4GHz. In the USA (and many other countries) the allowed power levels are somewhat higher which will give you more range.

Also, when flying at extended range it becomes increasingly important to use a sensitive receiver with true antenna diversity, something that not all 2.4GHz systems offer.
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Re: Digital Spread Spectrum in FM Bands

Postby skyhigh » Thu Jun 24, 2010 8:36 am

True, I forgot about the EU standards. I believe they max out at 10mW while N.America can have 100mW. Thats 10x difference but only 1dB in radio units. Not sure if 1dB is a big difference but I'll take the 100mW anyday. I might be tempted to go 10x more and get a 1000mW 2.4GHz L-Com amplifier if I'm flying the alps!

http://www.l-com.com/item.aspx?id=22138

If I were insane I might get their 25-watt(!) amplifier at $2,200 USD per unit.

http://www.l-com.com/item.aspx?id=22145

Forget about legal, would 25 watts of microwave radiation next to your eyeballs even be safe? :!:
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Re: Digital Spread Spectrum in FM Bands

Postby RCModelReviews » Thu Jun 24, 2010 10:18 am

skyhigh wrote:True, I forgot about the EU standards. I believe they max out at 10mW while N.America can have 100mW. Thats 10x difference but only 1dB in radio units.

Actually, 10x is 10dB, so it's quite a significant difference that would increase the range by about two and a half times that of a 100mW transmitter.

Forget about legal, would 25 watts of microwave radiation next to your eyeballs even be safe? :!:

Prolonged exposure would not do you much good.

A better option would be to use a hi-gain antenna that tracked the model using telemetry fed back from onboard.

You could use a hi-gain antenna (up to 20dBi) and that would provide a very significant increase in range. Of course most countries have a limit on the effective radiated power (the actual power times the antenna gain) so you'd still likely be illegal if you did that.
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