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

["David P. Reed" <dpreed@reed.com>]

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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.

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 our ears are more directional antennae at the higher frequencies. 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@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@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@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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