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Old Jan 11, 2008, 05:44 PM
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900 MHz or 2.4 GHz what is better

I was just wondering what is better as XPS announced 900 MHz system for US and Canada.
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Old Jan 11, 2008, 06:01 PM
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Neither, both have their pros and cons.

There is however a major con of 868MHz for places like Europe. Their regulations are extremely restrictive not only to power but also as far as available bandwidth and duty cycle is concerned. At least on 2.4GHz, they have the same 83.5Mhz available to them as the US. On the 900Mhz band, the US (and Oz and Canada I believe) can use from 902-928MHz, so 28MHz is available. In Europe, only about 1MHz is available.
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Old Jan 11, 2008, 06:14 PM
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What he said, pros and cons for each. Unless you are doing video or need extreme range 2.4 is the best compromise due to the way ISM regulations are laid out. 900Mhz is better for range and if you are doing any type of 2.4 AP or something similar but other devices on 900Mhz are much higher power than what is allowed on 2.4.
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Old Jan 11, 2008, 07:31 PM
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Aren't there 900 MHz analog phones all over the place? I would be a bit worried about it if it's not really FHSS.
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Old Jan 11, 2008, 07:36 PM
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Apart from the regs and phone stuff , does not the 900Mhz have more range and less power requirements? but bigger antenna's. oh yer and heavier Rx's
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Old Jan 11, 2008, 07:45 PM
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More range but smaller antennae. There are some really small 900 chipsets.
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Old Jan 11, 2008, 07:50 PM
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For a given transmitter output and receiver sensitivity (with antenna gain of both sides also included in the equation), yes 900MHz does give longer range. Or to put it another way, for a given range, you would need less power. All else being equal, path loss is directly proportional to frequency (actually wavelength). So you theoretically have 2.667 times (2400/900) more range with 900MHz.

But as always, nothing is for free, eg. 900MHz is more affected by atmospheric conditions, etc.

Wavelength of 900MHz is about 13 inches compared to about 4.9in. for 2.4Ghz (this accounts for the 1.25in. long 1/4 wave whip on the XBees), so yes, antennas will be longer. However, this is not necessarily a bad thing. Less tendency for a receiver antenna to be shadowed and may allow one to poke it out of a carbon fuze.
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Old Jan 11, 2008, 07:55 PM
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Well since no one is currently flying R/C (maybe UAV but that is not R/C) on 900mhz........you can't compare yet.

900mhz will have its own issues just like 72mhz and 2.4 ghz have their own unique issues
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Old Jan 11, 2008, 08:09 PM
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Little tiny Plantraco receivers... Is 0.4 grams big enough?
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Old Jan 11, 2008, 08:10 PM
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Quote:
Originally Posted by sweetpea
Well since no one is currently flying R/C (maybe UAV but that is not R/C) on 900mhz........you can't compare yet.

900mhz will have its own issues just like 72mhz and 2.4 ghz have their own unique issues
Lots of Plantraco indoor stuff on 900.

Pat MacKenzie

edit - typing at the same time a v_i
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Old Jan 11, 2008, 09:03 PM
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dam that's some tiny snizle for 4 channels
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Old Jan 11, 2008, 09:20 PM
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Quote:
Originally Posted by pmackenzie
Lots of Plantraco indoor stuff on 900.

Pat MacKenzie

edit - typing at the same time a v_i

Indoor-short range vs 1-2 mile outdoor are 2 different beasts!

That would be like saying that 27mhz is fine because Tyco and Aerobirds use it on their planes. I would't trust 27mhz on anything but my R/C car that is only driven 100yds at most away

But I guess I have mispoken about no R/C on 900, for that I apologize
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Old Jan 11, 2008, 09:22 PM
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Most of the RF chips, whether 413MHz, 900MHz or 2.4Ghz are less than 1/4in. square (in fact the vast majority are just 5x5mm) and come mostly in leadless QFN packages. This allows some really small tranceivers (with bi-directional communications) to be designed around them. They are used in applications such as those ubiquitous Bluetooth wireless headphones that people stick in their ears.

Plantraco has some of the smallest around. Spektrum also has some in 2g. range. The reason that the XPS 6-channel (and the smaller ones as well) are so much larger and heavier, is that they are based on the XBee modules which are designed for completely different applications where size and weight are not as critical. This is one of the comprises that I alluded to in the other thread. There are also several other disadvantages in going the module route as opposed to the chip route.

However, as tlh101 has pointed out, XPS is awaiting FCC approval so it seems that for the 900Mhz product line, they have gone the chip route. Receivers will probably be much smaller and lighter as a consequence (and a whole lot less expensive to produce).
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Old Jan 11, 2008, 09:36 PM
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Quote:
Originally Posted by sweetpea
Indoor-short range vs 1-2 mile outdoor are 2 different beasts!
Purely a design choice on their part. Does not have any bearing on 900MHz, or the technology, or the RF chips that they use.

Remember, their systems are designed for airplanes where the wingspan is just inches with auw that in most cases is less than that of a micro servo. Longe range (>150ft or so) is not needed in this scenario.

With the addition of a PA to the front end, they COULD be used in full-range (1-2 mile) systems.
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Old Jan 11, 2008, 09:38 PM
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Jim did say that part of the problem with the 900 system, was trying to get it small, and not consume large amounts of power. It's supposed to be stable down to 1.5V.
Hopefully they recorded the interview, and we will be able to see it again. It's hard to recall everything mentioned.
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