R: [TowerTalk] best coax/receiving

[Ian White, G3SEK]

K6LL wrote:
>
>
>
>On Wed, 12 Apr 2000 20:16:40 +0200 "Maurizio Panicara" <i4jmy@iol.it>
>writes:
>> 1) Forget the S-Meter, noise, AM mode, etc etc..
>> 2) Ask a friend to transmit a carrier signal on a clean spot of 
>> the10m band
>> and receive it in SSB mode, AGC off  (if possible).
>> 3) Ask him to reduce power (or to do something) until it's barely 
>> audible
>> for you.
>> 4) Switch on your 3 dB attenuator.
>> 5) Evaluate results.
>> 6) Repost your impressions to "towertalk".
>> 
>> 73,
>> Mauri I4JMY
>
>I just finished running this test with W7WW, who lives about 15
>miles away. When a 6dB attenuator was switched in, there was absolutely
>no degradation in signal to noise ratio.
>
>I had W7WW send some slow qrp cw on 28390. The band was
>pretty quiet, no LED's illuminated on the s-meter. I turned my
>antenna so that I could copy just about half of what he was sending.
>SSB mode on my TS-850, no agc.
>
>With 6dB of attenuation, copy was not impaired at all.
>
>With 12dB of attenuation, I could still hear it, but the percent
>copy went down to about 25 percent.
>
>With 18dB attenuation, I could just about hear him, but couldn't
>copy what he was sending.
>
>The band was a little noisier than last night. Unlike last night,
>I could hear the background noise go down a little when I went from
>12 to 18 dB of attenuation, but the s-meter still read zero with no
>attenuation.
>
>I conclude from this little experiment that my ability to copy
>a weak signal would not have been impaired, even if my coax cable
>loss had been 6dB for sure, and maybe even as high as 10 dB. 
>

This seems to be an example of the Avis Effect - as the task gets
harder, "we try harder" too, so we still keep on copying. You can't
prevent yourself from doing this.

I think the point we were trying to make was that although we always
*can* dig deeper to copy weaker signals, it slows down the QSO rate
and also adds greatly to operator fatigue.



At a moonbounce convention several years ago, someone produced a CW
tape with about 30 well-known callsigns, each sent at 1dB lower s/n than
the one before. The s/n ranged from +15 to -15dB in an SSB bandwidth. 

Almost everybody tried this, and there was a surprisingly wide range of
ability to copy as signals became weaker. People started to drop out at
s/n as high as 6dB, most dropped out at around 0dB, but some could still
identify the callsigns at significantly negative s/n.

With multiple repeats instead of just once per callsign, most people
could have edged down by a few dB more. On the other hand, the more
successful people were probably correlating what they were actually
hearing with a mental "intelligence database" of what they might be
likely to hear - but all the best operators do some of that.

OK, it was fun, not science - but it did show that people's abilities to
read weak CW can differ very widely. 


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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