[Cerowrt-devel] Cerowrt-devel Digest, Vol 37, Issue 24

Mon Dec 22 21:16:29 EST 2014

Hi Sebastian -

So reading this chart, which is consistent with my reference materials: At 6 GHz, I see additional attenuation of water vapor being -0.002 db/kM, additional to the dry air attenuation of  10.0075 dB/kM already due to the atmosphere, at 5.8 GHz.

So at 5 kM (>1 mile in English units), a signal will be attenuated by about 0.01 dB by water vapour, in addition to 0.0375 dB of attenuation by the atmosphere.  But the attenuation due to path loss at distance d, which is typically log(d**k), or k log(d) - where 2<k<4 depending on a variety of factors - will be somewhere between -122 dB to ~ -190 dB (assuming the antennas are dipoles).

So the contribution of water vapor at 5.8 GHz is pretty insignificant.

On Sunday, December 21, 2014 2:20pm, "Sebastian Moeller" <moeller0 at gmx.de> said:

> Hi David,
> 
> 
> On Dec 21, 2014, at 17:45 , David P. Reed <dpreed at reed.com> 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.pdf it
> pretty much looks like there is higher attenuation at 5GHz compared to 2.4GHz
> (roughly 126 = % more attenuation @5GHz due to water in air), so there are some
> propagation differences at different frequencies, no?
> 
> >
> > 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.
> >
> > Think about acoustics. Higher frequencies from a tweeter propagate through
> air just as well as lower frequencies from subwoofers.
> 
> 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 “efficiently" 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…)
> 
> 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.
> >
> > On Dec 21, 2014, David Lang <david at lang.hm> wrote:
> > On Sat, 20 Dec 2014, David P. Reed wrote:
> >
> > Neither 2.4 GHZ nor 5.8 GHz are absorbed more than other bands. That's an old
> > wives tale. The reason for the bands' selection is that they were available
> at
> > the time. The water absorption peak frequency is 10x higher.
> >
> > well, microwave ovens do work at around 2.4GHz, so there's some interaction
> with
> > water at that frequency.
> >
> > Don't believe what people repeat without checking. The understanding of radio
> > propagation by CS and EE folks is pitiful. Some even seem to think that RF
> > energy travels less far the higher the frequency.
> >
> > I agree that the RF understanding is poor, but given that it's so far outside
> > their area of focus, that's understandable.
> >
> > the mistake about higher frequencies traveling less is easy to understand,
> since
> > higher frequency transmistters tend to be lower power than lower frequencies,
> > there is a correlation between frequency and distance with commonly available
> > equipment that is easy to mistake for causation.
> >
> > David Lang
> >
> > Please don't repeat nonsense.
> >
> > On Dec 20, 2014, Mike O'Dell <mo at ccr.org> wrote:
> > 15.9bps/Hz is unlikely to be using simple phase encoding
> >
> > 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).
> >
> > 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.
> >
> > 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.
> >
> > -mo
> >
> > Cerowrt-devel mailing list
> > Cerowrt-devel at lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/cerowrt-devel
> >
> > -- Sent from my Android device with K-@ Mail. Please excuse my brevity.
> >
> >
> > Cerowrt-devel mailing list
> > Cerowrt-devel at lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/cerowrt-devel
> >
> > -- Sent from my Android device with K-@ Mail. Please excuse my brevity.
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> 
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