| From what I have been able to gather, the system currently going to
| market for parkflyers has 100 mW output. This is strictly a guess,
| but I suspect the design borrows heavily from modules developed for
| appliance (e.g. cell phone) markets with huge sales volume.
I was thinking WiFi would be a closer match. Or Bluetooth, which is
| Output of 1 W is allowed in the band
Hmm, I was thinking 0.250 watts, but it seems you are correct. Our 72
MHz radios are capped at 0.75 watts, so that's an (miniscule)
| As I read it (including some other accounts), the band is divided into
| 80 slots of 1 Mhz each. The Spektrum radio searches for and selects
| two unused slots.
... two slots that it thinks are unused. What's unused at ground
level may not be unused at some distance. But since it's really
spread spectrum, it shouldn't matter that much.
| UHF-Hi and above in vehicles that can quickly change orientation.
`Picket fencing'. Yes, it seems a good plan.
The FAQ I read covered their car system, which doesn't do all of this.
| Also, the permutions possible with pairs of channels from a group of
| 80 is 80^^2.
Technically it would be 80 * 79. Though assuming that the odds of a
problem are 1 / ( 80 * 79 ) also assumes that there's no other users
of the band. And hopefully the system is smart enough to not pick two
adjacent channels, and it stays away from the frequency used (2450
MHz) by microwave ovens.
| The 32- bit GUID is used for digital sequential spreading
... and since they are using spread spectrum (the new faq is a lot
better than the old one) then even if two transmitters are on the same
channel, it should still work.
Still, I'm surprised they chose 2.4 GHz. 5.8 GHz seems a better bet,
since it's not used so heavily, and the antennas could be even
smaller. FSL is larger, but not too much so.
Also, the antenna design on the transmitter (a whip just like existing
whips, but smaller) seems to be less than ideal. Since your TX is
almost always pointed in the direction of the car/plane when in use,
at least in the horizontal plane, it seems to me that a dipole antenna
(which is practical considering how small it is) pointing to the left
and right would be perfect, giving you even more range.
| There's a lot dB's to make up for when going from 72 MHz to 2.4 GHz
| due to atmospheric absorbtion; I get ~30 dB disadvantage for 2.4 GHz
| without resorting to a calculator.
The fact that you didn't give a dB/distance attenuation factor
suggests a mistake was made, as atmospheric absorbsion would indeed be
a function of distance.
Looking it up, at 2.4 GHz, it's approximately 0.01 dB/mile, which is
utterly insignifigant. (Cite: http://www.bbwexchange.com/glossary /.)
Now, there is free space loss, but that's totally different. And yes,
it's larger for 2.4 GHz than 72 MHz.
Their new FAQ also has some errors. For example, this one --
So when FCC devised the 2.4GHz rules, they set about a completely new
system: they mandated that all 2.4GHz devices will transmit at less
than 1 Watt.
... which is patently incorrect. For example, the ham band overlaps
the unlicensed 2.4 GHz band, and the power limit there is 1500 watts
(assuming it's not spread spectrum) with no limit on antenna gain.
Or this one --
Since all devices on the 2.4GHz band are required to avoid other users
(or else they wouldn't receive FCC type acceptance)
That would only affect spread spectrum uses. Not all 2.4 GHz band
equipment is spread spectrum. For example, non-spread spectrum 2.4
GHz cordless phones and wireless video cameras are known to wreak
havok with WiFi, and they could totally knock out one of these R/C
systems, especially the video system, probably using 6 MHz of
bandwidth. Hopefully the DX6 is smart enough to make sure the
channels it chooses are at least 6 MHz apart.
In any event, I was looking at the specifications for the old Spektrum
system for my original post. (Their web site could benefit from some
work.) The new plane system, DX6 looks a _lot_
better. And with a
$200 price tag for 6 channel TX, RX and 4 micro servos sounds like a
hell of a deal.
Of course, apparantly it's not even out yet. `Early December'.
Doug McLaren, firstname.lastname@example.org So many idiots. So few bullets.