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Date: Mon, 23 Feb 2009 09:53:13 +0000
Message-ID: <fc7eccec0902230153o2fe739f7oebd2e1e9254c2362@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
DomainKey-Status: good (testing) 
Subject: LF: WSPRing and programmed ERPs
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Switched off at 0630 this morning, but unfortunately, according to the
database, no exotic receptions heard - just the usual culprits  'ISM
and XVL!

I see G4WGT was transmitting until around 2350 and note your quoted
power was 37dBm.

I thought we had agreed that for WSPR transmissions on the LF bands,
the power entered in the WSPR message would reflect (to at least a
reasonable approximation)  ERP and not RF power.    Hence my use of a
ridiculous figure (53dBm) when sending at very low power on Saturday.

Only certain power levels seem to be permissible - presumably those in
the pull-down menu in WSPR.EXE (windoze version) and there appear to
be 19.
While playing with WSPR.EXE (command line version) to generate teh
tone codes for my PIC /DDS combination, any value power level could be
entered, but when the subsequent symbol set was decoded, the software
threw up a {bad message] indicator instead of LOC and PWR. Presumably,
this makes for 20 stored power settings.  Its a pity that an illegal
power setting kills the LOC info as well, as the {bad message} code
would otherwise have been a good way to show a variable or non
specified power level.

Back to ERP vs. RF:  Legal values range from 0 to 60dBm, ie 1mW to
1kW.   While this is an ideal range for HF use where the power levels
are unlikely to be exceeded at either end (except by 'JNT on Friday on
10MHz with -2dBm, programmed as 0)  only a subset of values cover the
expected range of ERPs on LF -  0dBm to (now) 40dBm.     To allow for
QRP LF stations, can I suggest the values above 40dBm are used as
follows to reflect ERPs from -20 to 0dBm

Actual ERP = Stored value - 60
So if your ERP is below 0dBm, you programme a value of ERP + 60 into
your message.    This gives a discontinuity, and makes a nonsense of
the MEPTSpot website's ordering in terms of power or km per Watt

To save the discontinuity, another alternative would be to shift the
entire code-set by 20dB to reflect tens of microWatts rather than
milliwatts.   So now the total range would run from
-20dBm = "0"    to
+40dBm = "40"



Andy  G4JNT
www.g4jnt.com