Greetings all:

I experimented on 30 meter today with different bandwidths for 
WSPR. I started with USB at 2.4 KHz and had LOTS!!! of signals,
and plenty of QRN. WSPR decoded but did have difficulty with
receiver desense. I turned on my 500 Hz filters, and the QRN
was gon, and the desense was no longer a problem. WSPR decoded
the signals easier, and QSB had less effect. I turned on my 
software DSP filter and set it to a 1500 Hz center frequency,
with a BW of 200 Hz. Again Less noise/QRN, and more signal
within the passband. QSB had very little effect on decodes.
The narrowest BW I tried was 25 Hz, and any signal in that
window was perfectly decoded. There is a problem trying this 
in CW mode, The Center Audio frequency drops to 800 Hz on most
rigs, and the highest frequency the would pass through on my
rig was 1200 Hz, which is not in the WSPR passband of frequencies.
The center Audio frequency needs to be at 1500 Hz.
If you don't have narrow filters in USB, the next best thing is 
to try a software DSP filter, or an outboard filter that is 
adjustable.
In sumary; my experiment today showed that for each time I 
narrowed the BW, I saw an increase in received signal level, and
improvement in the S/N, and QRN levels. 73:

Andy - KU4XR - EM75xr - Friendsville, TN.
LOWfer Beacon " XR " @ 184.322 KHz ( QRSS-60 )