Re: Has anyone flown the Park Flyer 6 channel, SS system at Horizon?
- From: dougmc@xxxxxxxxxx (Doug McLaren)
- Date: Wed, 09 Nov 2005 16:04:28 GMT
In article <si8cf.16704$td.9151@xxxxxxxxxxxxxxxxxxxxxx>,
Red Scholefield <redscho@xxxxxxxxxxxxx> wrote:
| Wow! If I remember right -30 db is a 1000 fold reduction in whatever -
| signal strength in this case?
The frequency component of the free space loss formula is there
_strictly_ to take into account the effective receiving `area' of the
receiving antenna. It also assumes isotropic antennas, which do not
exist in the real world.
A 2.4 GHz antenna of a given type (like a half wave dipole) is 33
times smaller (in one dimension) than a 72 MHz antenna of the same
type, and so the received signal is 1100 times smaller, assuming that
the transmitted power remains the same. However, this is not a
problem for us at all -- as the noise received by this antenna will
also be 1100 times smaller, and it's the S/N ratio that really matters
here. The weaker signal is easily compensated for by simply setting
up the receiver's internal amplifier for more gain.
Since the channels are 1 MHz in size vs. 10 KHz in size as they are
now, Abel is right -- the S/N ratio needed should be very low, much
lower than what we have now. Really, with one watt to work with, and
a clear line of sight (which is hard to fly without) it shouldn't be
difficult to get all the range we're used to out of a 2.4 GHz system,
and more.
Also, since the antennas are so small, they could easily include a
dipole with some gain in exactly the right directions (on the TX
anyways), which would give you more range.
In any event, in the radio world, a 30 dB difference in signal
strength isn't _that_ big a big deal. And since the noise is reduced
by a similar value, it's pretty much a non-issue here.
| Sounds like the bird flu is a much bigger threat than 2.4 GHz. :-)
Is threat the right word here? I only see a few bad things here for
the hobby in general here --
- Spektrum appears to have `patent pending' all over their web site,
which suggests that they're patenting everything in sight, even though
what they're doing is really only natural (obvious) extensions of
existing technology, so most of it shouldn't pass the `obvious to the
layperson' requirement of the patent office. Unfortunately, this
requirement is very weakly enforced, and so it's entirely possible
that Spektrum may end up locking up the entire market via patents,
making it impossible for anybody else to make R/C spread spectrum
devices in the US.
- They also like to use the word `proprietary', which suggests that
other vendors won't be able to make equipment that interoperates with
this equipment. So it would be like PCM -- your RXs only work with
TXs by the same vendor.
- I'd have suggested the 5.8 GHz band instead of 2.4 GHz, due to less
noise, but ultimately it shouldn't be a big problem. Since Spektrum
is the forerunner here, other vendors are likely to make things that
work similarly.
(I'm guessing they've adapted WiFi or Bluetooth equipment for this,
and that's why they chose the 2.4 GHz band.)
- Giving users equipment that doesn't require frequency management may
help cause the users forget about frequency management if/when they
move up to large planes on the 72 MHz band.
- Having R/C gear on 2.4 GHz will mean that you can't have a 2.4 GHz
transmitter in your plane (unless it's spread spectrum as well, though
the small/cheap video systems we've used usually aren't.) Even having
a 2.4 GHz video transmitter in your plane with 72 MHz R/C gear may
crash _other_ nearby planes with 2.4 GHz gear. (It all depends on how
close the video TX is to the RX, how close the R/C TX is, and which
frequencies are being used by both. Since a video signal is usually 6
MHz in size, it could knock out both frequencies used by the DX6 if
you're not lucky.)
- 2.4 GHz is absorbed more by the human body than 72 MHz. I
personally don't think that cell phones cause cancer, but certainly,
2.4 GHz systems are going to be a bigger danger than 72 MHz ones
there. (And this may be why Sketrum hasn't added the dipole antenna I
was mentioning -- the gain of the antenna would increase the RF energy
directed at your plane, and decrease it aimed at the people standing
next to you, but it would also increase the RF energy aimed at you.
Though a fancier antenna could undo some of that, but you'd look sort
of dorky with a TX with a cantenna on it. Though perhaps we're used
to looking dorky?)
- If everybody starts using R/C equipment on other bands, the FCC may
decide to take the 72/75 MHz bands from us. Of course, this would be
many years away, and if it does come to pass, we're not likely to care
that much anymore (since if everybody's really on 2.4 GHz, that means
nobody's on 72 MHz.)
- With a 3" RX antenna, that may not be long enough to get it out of a
carbon fiber fuselage. (Though I guess you could extend it to 9"
which should work reasonably well.)
| > Yup, I estimated free-space path loss rather than what I incorrectly
| > stated. It's a function of 20 log (freq) + 20 log (dist). At any
| > arbitrary distance, the difference is reduced to a function of the
| > ratio of frequencies^^2, and that amounts to the ~30 dB given.
--
Doug McLaren, dougmc@xxxxxxxxxx
Madness takes its toll. Please have exact change.
.
- References:
- Has anyone flown the Park Flyer 6 channel, SS system at Horizon?
- From: Ed Cregger
- Re: Has anyone flown the Park Flyer 6 channel, SS system at Horizon?
- From: Doug McLaren
- Re: Has anyone flown the Park Flyer 6 channel, SS system at Horizon?
- From: Abel Pranger
- Re: Has anyone flown the Park Flyer 6 channel, SS system at Horizon?
- From: Red Scholefield
- Has anyone flown the Park Flyer 6 channel, SS system at Horizon?
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