[Cerowrt-devel] Cerowrt-devel Digest, Vol 37, Issue 24
david at lang.hm
Sun Dec 21 03:32:38 EST 2014
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.
> 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.
>> Cerowrt-devel mailing list
>> Cerowrt-devel at lists.bufferbloat.net
> -- Sent from my Android device with K-@ Mail. Please excuse my brevity.
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