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From: reginal..._at_hotmail.com
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Subject: Low power devices - PART 2 of 2 PARTS
Date: Sun, 19 Aug 2007 11:41:23 -0700
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Continuation of article from "CQ" re low power transmissions.
PART 2 of 2 PARTS.
"As previously mentioned, QRSS signals are usually quite weak - so
weak they are masked by band noise. The key to hearing them involves
reducing the noise or improving the receive signal-to-noise ratio, and
this is accomplished by reducing the receive bandwidth so it is barely
wide enough to pass the (QRSS) signal but not wide enough to pass the
noise. How wide (or narrow) is that passband? It depends on the QRSS
speed and is calculated as: CW speed (wpm [words per minute]) x 2.5
(response time) = absolute minimum bandwidth (in Hz) for absolute
maximum S/N (signal-to-noise) ratio. As an example, 25 wpm x 2.5 62.5 Hz bandwidth, and 12 wpm x 2.5 = 30 Hz bandwidth. Likewise, 1 wpm
x 2.5 = 17.5 Hz bandwidth and a typical QRSS signal of .8 wph (that's
words per hour), or 0.0133 wpm x 2.5 = .03 Hz bandwidth.
Don't panic! A bandwidth this narrow (plus DSP noise reduction) can be
acquired at the AF rather than the IF level, and that is where your HF
transceiver plus computer equipped with sound card, QRSS software, and
a quick-brew interface enter the picture. You can tune your
transceiver to frequency, leave it, and the computer does the time-
consuming monitoring for you. Your transceiver's filter is also wide
enough such that mild drift is acceptable, because computer-based
filtering is used.
The details of where to find more information on QRSS(including times/
frequencies of QRSS test transmissions) and where to look for free
QRSS software are given in fig. 1 [omitted]. Bear in mind that at the
present time most QRSS activity is pre-announced as to time,
frequency, and power via e-mail (check the knights_qrss e-mail
reflector for details)[
http://www.cnts.be/knights_qrss/], and the Argo
and Spectran software (available through QSL.net) include construction
information for simple interfaces.
QRSS may or may not parallel your QRP interests. However, considering
it is a new pursuit, you deserve to know about it, and then draw your
own conclusions. That is why I covered it here. Would you like to read
more in future columns or prefer that we focus only on classic QRP?
Personally I feel that even if QRSS is not your cup of tea, writing
about it in this month's column again brought up some always useful
tricks for QRP success. How? Sending CW slower than usual and using
narrow filters plus DSP noise reduction improves a S/N ratio and
enhances copy under marginal conditions. Having problems in DX pile-
ups? Consider a distant station using a very narrow filter. If your
signal is not centered in that narrow passband, it will be attenuated
5, 10, 15 or greater dB boost.
As we think more about QRSS and also consider its 'non radiating'
abilities - such as the way prisoners of war have communicated by eye
blinks and wall taps - the possiblities of stealth communications with
QRSS boggle the mind. The FBI, CIA, Interpol, etc., should be
interested in QRSS!
......"
The End
Reg Curtis/VE9RWC
Received on Sat Mar 02 2024 - 00:57:24 CST