On Aug 3, 2005, at 11:33 AM, WA4OSH wrote:
Jim Thompson wrote:
-==snip=-
To be perfectly blunt, I think the
"we operated under part 97" was a dodge.
Team PAD is composed of:
Andy Meng, N8MX
Ben Corrado, KC8RKO
Justin Rigling, KC8OIO
Brandon Schamer, KG4NVK
http://www.wifiworldrecord.com/team.html
Given they are all hams, it's very possible they were indeed
operating under part 97.
legally?
Here is their write-up:
http://www.wifiworldrecord.com/2005writeup.html
Violation of FCC part 97 rules? ---> "Both ends of the link were
using SSH"
Yeah, thats probably another violation.
"The connection had an astounding signal strength of -37 dBm,
bottoming
out at around -50 dBm throughout the 3 hours. The noise levels were
around -84 dBm"
With that kind of link margin, they should have been able to do this
with much more power. (6 dB = double the distance)
This is only true when the path loss coefficient is 2.0 (perfect
LOS). Jim's quick rule of thumb:
divide the delta link margin (in dB) by the path loss coefficient,
and the result is the difference in distanced, expressed in dB.
So, 6 dB / 2.0 = 3 dB, or 2X the distance.
Anyway, you're right, but only for LOS communications.
Further, something is ... spooky.
The 10' dish has a gain of around 35 dBi.
A 1 m^2 dish will generate about 26-27 dBi. 3 dBi more requires
twice the "aperture", or 2 m^2 for 29dBi 32 dBi would require 4 m^2,
and 35 dBi would require 8 m^2. There are 10.76 ft^2 in a square
meter.
(As an aside, consider just how bad a "Pringles can" antenna might be.)
A 10' dish has a capture area of about 78 sq ft, or 7.3 m^2. A 12'
dish has 113 sq ft, or 10.5 m^2. Dishes aren't 100% efficient, no
antenna is, but lets assume, for the sake of argument that the 10'
dish generates 35dBi, and the 12' dish approximately 40 dBi.
Now lets assume that the cards really were running below their 300mW
rating. Perhaps 24 dBm (250 mW), rather, 24.67 dBm (292.8 mW).
And, lets assume that the world is perfect, and we have ZERO losses
due to connectors and cables. ZERO.
LOS path loss at 125 miles is 146 dB. Due to reciprocity, it
doesn't matter which antenna gets the transmitter, but lets say that
we transmit
through the 40 dBi antenna. So, 24 dBm into 40 dBi for 64 dBm EIRP
(2511.89 Watts). We encounter a *MINIMUM* of 146 dB of path loss,
so the signal can't enter the remote antenna any higher than 64 -
146, or -82 dBm. The receiving antenna gets to add its gain, which
is 35dBi, so we're at -47 dBm.
This is a LONG WAY from -37dBm.
Note that if they turned the power down, the received signal level
would... go down. If the path loss was higher (it can't be lower!),
the signal level would go down.
The only possibilities left that I can account for:
1) my rough antenna gain calculations are wrong
2) they're lying, or their power meter wasn't calibrated, or ...
something.
What on earth do they need an amp for??
Violation of FCC part 97 rules? ---> "When the amp was used the signal
strength bumped up to -18 dBm at 124.9 miles!"
97.313 Transmitter power standards.
(a) An amateur station must use the minimum transmitter power
necessary
to carry out the desired communications.
also interesting...
"the base station end had FAA 2.4 ghz communications so we chose to
keep our ERP low on that end"
Why did they need an unbalanced link??
So as not to interfere with the FAA. I wonder what the back lobe on
the other side looks like. :-)
Further issues
97.307 (b) Emissions resulting from modulation must be confined to
the band or segment available to the control operator. Emissions
outside the necessary bandwidth must not cause splatter or keyclick
interference to operations on adjacent frequencies
What this says is "no out of band emissions" (from modulation, rather
than spurs). This *IS* the problem I keep pointing out.
Also, there would be a problem here, except they're under 1W of tx
power.
97.311 (d) The transmitter power must not exceed 100 W under any
circumstances. If more than 1 W is used, automatic transmitter
control shall limit output power to that which is required for the
communication. This shall be determined by the use of the ratio,
measured at the receiver, of the received energy per user data bit
(Eb) to the sum of the received power spectral densities of noise
(N0) and co-channel interference (I0). Average transmitter power over
1 W shall be automatically adjusted to maintain an Eb/(N0 + I0) ratio
of no more than 23 dB at the intended receiver.
By their own admission, the Eb/No is > 23 dB. Quoting:
"The connection had an astounding signal strength of -37 dBm,
bottoming out at around -50 dBm throughout the 3 hours. The noise
levels were around -84 dBm"
Jim,
To be perfectly fair about it, I think that DefCon should amend
their rules for the shootout to limit it under FCC part 15 regs and
any stock
Wi-Fi card. Without this restriction, the DefCon shootout is
meaningless.
The rules should probably state that all equipment, and the resulting
system must be demonstrably able to pass the FCC Part 15
requirements, or the participants should get a special operating
waiver from the FCC. Heh heh.
They should probably also make the contest more difficult by
dividing by the measured area of the antenna(s), or at least the
stated gain.
Perhaps DEFCON should change the rules so you have to communicate
with two stations at the proposed range with a minimum angular
separation of 30 degrees.
Then it might become a bit more about engineering, and
experimentation, and less about big dishes and external amps.
I don't have any problem with enhancing the drivers do deal with
the distance,
for a prism-based card, its more difficult than you think. (And
really, I am aware of iwconfig <interface> txpower NN.) "Full" and
"off" work well, but these are the only positions where you have any
accuracy or repeatability on a Prism2/2.5/3.0 card.
For an Atheros card, its straight-forward. (Thoughput goes to hell
anyway.)
but it will just become a my dish is bigger than your dish contest.
Oh, size isn't everything. I considered taking a pair of the Vivato
"prototypes" to the first contest. These *were* FCC-legal, and
*did* have 27dBi arrays. If anything they would be easier to aim,
and there is a bit of array gain as well.
Yes, I know some random Vivato distributor took their first-
generation (production) Vivato "WiFi Switch" to a shootout and failed
miserably. Anyone who's read through my weblog knows why.
Frankly, if I wanted to win, and didn't care about FCC rules, I'd
just go purchase 2 of the current-generation Vivato units. These
can (if you know what you're doing) generate 30dBm (and the tx power
is adjustable in 0.5 dBm increments) into a 23dBi array, but they've
got **excellent** receiver sensitivity. (About 9 dBm better than
the rated spec on the cards these guys used.)
This means I'm fighting their 35dBi antenna with an effective 32dBi
(23dBi antenna, and 9dBm better receiver sensitivity). Yes, I give
up 3dB to their 10' dish, but my unit is 1/2 m x 1/2 m, and it won't
rattle apart while I'm driving around the desert. (Did I mention
that I'm a Las Vegas native, *and* I have a LandCruiser
(www.smallworks.com/~jim/LandCruiser) under construction in Las
Vegas, *and* I probably understand the insides of Vivato's products
as well as anyone?)
This way I could:
a) run under Part 15 or Part 97 at will. (Perhaps DEFCON will have a
class for each.)
b) actually tune the transmit power down to the minimum required to
maintain the link. (DEFCON should buy a power meter.)
c) use less power during transmit, due to a superior receiver structure.
d) very likely win, even if they cheat.
In a perfect world (true line of sight, very low noise and
interference, and high-enough mounting points), assuming that I stay
inside the FCC limits, I can generate 24dBm into that 23dBi array,
have a 23dBi array at the other end, and can decode 11Mbps down to
-92 dBm. So, in theory, I can tolerate 162 dB of path loss. On
paper, I can do 800 miles. <---- I am not proposing or promising
this in the real world.
But I'd have plenty of fade margin available on their 125 mile link.
And I'd be Part 15 legal.
...I wonder if doppler is too much to make an EME contact using Wi-Fi?
maybe, (I don't want to have to calculate the coherence frequency for
CCK right now), but Just the path loss is 211 dB, never mind the
losses due to bouncing off the moon, and the (approximate) 2 second
round trip means there is no way you're using "Wi-Fi" (the 802.11
MAC) to close the link. You might use 802.11 gear, and some huge
antennas, but it won't be running the Wi-Fi / 802.11 MAC.
Jim
