From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.gmx.net (mout.gmx.net [212.227.15.15]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "mout.gmx.net", Issuer "TeleSec ServerPass DE-1" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 7537E21F36F for ; Sun, 21 Dec 2014 11:21:00 -0800 (PST) Received: from hms-beagle.home.lan ([217.247.215.91]) by mail.gmx.com (mrgmx003) with ESMTPSA (Nemesis) id 0MLujU-1XzRbY2pI4-007ojK; Sun, 21 Dec 2014 20:20:48 +0100 Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Content-Type: text/plain; charset=windows-1252 From: Sebastian Moeller X-Priority: 3 In-Reply-To: <628cf93a-2351-4e44-ab1f-f6b5fa8aae2f@reed.com> Date: Sun, 21 Dec 2014 20:20:45 +0100 Content-Transfer-Encoding: quoted-printable Message-Id: <29E1C390-9056-463D-885E-7587D71D6E98@gmx.de> References: <6764.1419109075@ccr.org> <68a9aec2-9a5e-4cc0-84d1-3fce8ccc0efb@reed.com> <628cf93a-2351-4e44-ab1f-f6b5fa8aae2f@reed.com> To: "David P. Reed" X-Mailer: Apple Mail (2.1878.6) X-Provags-ID: V03:K0:OAFjseawwip1LWDHMNKkaozPVBsO+tXhXBMVl40gG0DC2HZpICv xJ5H07EKEA3gWT2VPLzlpUDOHmmONIEUwtMk80Eeu5IJXZp7pfdJsTBX4+aMAeDCcvQcIUY jTqNqF+w8ELQgawEs1PPo7m9qIc42z4SV8NXJHVgUSA7Dhln3XEfGTQF2ihVM6GEQIIeUlJ tGyf+7RAEVYRH2Z3ED5Rg== X-UI-Out-Filterresults: notjunk:1; Cc: Mike O'Dell , cerowrt-devel Subject: Re: [Cerowrt-devel] Cerowrt-devel Digest, Vol 37, Issue 24 X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 21 Dec 2014 19:21:30 -0000 Hi David, On Dec 21, 2014, at 17:45 , David P. Reed wrote: > All microwave frequencies heat water molecules, fyi. The early ovens = used a klystron that was good at 2.4 GHZ because it was available and = cheap enough. But they don't radiate much. 5.8 GHz was chosen because = the band's primary was a government band at EOL. Looking at figure 5 of = http://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.676-10-201309-I!!PDF-E.p= df it pretty much looks like there is higher attenuation at 5GHz = compared to 2.4GHz (roughly 126 =3D % more attenuation @5GHz due to = water in air), so there are some propagation differences at different = frequencies, no? >=20 > Yes... higher frequency bands have not been used for broadcasting. = That's because planetary curvature can be conquered by refraction near = the earth's surface and reflection by the ionosphere. That's why power = doesn't help were we to use higher frequencies for broadcasting. But = data communications is not broadcasting. So satellite broadcasters can = use higher frequencies for broadcasting. And they do, because it's a = lot easier to build directional antennas at higher frequencies. Same for = radar and GPS. >=20 > Think about acoustics. Higher frequencies from a tweeter propagate = through air just as well as lower frequencies from subwoofers.=20 But look at https://ccrma.stanford.edu/~jos/HarrisJASA66.pdf = figure 5; air seems to attenuate sound waves as a function of frequency, = so high frequencies do not travel as far as low frequencies (but are = more =93efficiently" converted into heat). But that looks similar to RF = waves in air (see link above)... > But our ears are more directional antennae at the higher frequencies. True, once the inter-ear distance is down to 1/4 wavelength = there is no useable intensity and phase difference between the signal at = both ears, hence the inability to localize the subwoofer (that allows to = get a way with one subwoofer in a stereo system). But this depends to a = good deal on the inter-ear distance (e.g. elephants can reliably = localize sounds that humans can not due to the bigger head=85) Best Regards Sebastian > Similar properties apply to EM waves. And low frequencies refract = around corners and along the ground better. The steel of a car body = does not couple to higher frequencies so it reradiates low freq sounds = better than high freq ones. Hence the loud car stereo bass is much = louder than treble when the cabin is sealed. >=20 > On Dec 21, 2014, David Lang wrote: > On Sat, 20 Dec 2014, David P. Reed wrote: >=20 > Neither 2.4 GHZ nor 5.8 GHz are absorbed more than other bands. That's = an old=20 > wives tale. The reason for the bands' selection is that they were = available at=20 > the time. The water absorption peak frequency is 10x higher. >=20 > well, microwave ovens do work at around 2.4GHz, so there's some = interaction with=20 > water at that frequency. >=20 > Don't believe what people repeat without checking. The understanding = of radio=20 > propagation by CS and EE folks is pitiful. Some even seem to think = that RF=20 > energy travels less far the higher the frequency. >=20 > I agree that the RF understanding is poor, but given that it's so far = outside=20 > their area of focus, that's understandable. >=20 > the mistake about higher frequencies traveling less is easy to = understand, since=20 > higher frequency transmistters tend to be lower power than lower = frequencies,=20 > there is a correlation between frequency and distance with commonly = available=20 > equipment that is easy to mistake for causation. >=20 > David Lang >=20 > Please don't repeat nonsense. >=20 > On Dec 20, 2014, Mike O'Dell wrote: > 15.9bps/Hz is unlikely to be using simple phase encoding >=20 > that sounds more like 64QAM with FEC. > given the chips available these days for DTV, DBS, > and even LTE, that kind of processing is available > off-the-shelf (relatively speaking - compared to > writing your own DSP code). >=20 > keep in mind that the reason the 2.4 and 5.8 ISM bands > are where they are is specifically because of the ready > absorption of RF at those frequencies. the propagation > is *intended* to be problematic. that said, with > good-enough antennas mounted with sufficient stability > and sufficient power on the TX end and a good enough > noise floor on the RX end, one can push a bunch of bits > pretty far. >=20 > Bdale Garbee (of Debian fame) had a 10GHz bent-pipe repeater > up on the mountain above Colo Spgs for quite some time. X-band > Gunnplexers were not hard to come by and retune for the > 10GHz ham band. i believe he just FM'ed the Gunnplexer > with the output of a 10Mbps ethernet chip and ran > essentially pure Aloha. X-band dishes are relatively > small and with just a few stations in the area he had fun. >=20 > -mo >=20 > Cerowrt-devel mailing list > Cerowrt-devel@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/cerowrt-devel >=20 > -- Sent from my Android device with K-@ Mail. Please excuse my = brevity. >=20 >=20 > Cerowrt-devel mailing list > Cerowrt-devel@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/cerowrt-devel >=20 > -- Sent from my Android device with K-@ Mail. Please excuse my = brevity. _______________________________________________ > Cerowrt-devel mailing list > Cerowrt-devel@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/cerowrt-devel