RE: [linrad] Linrad

[Leif Åsbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

Hi All,

Bob, N4HY wrote:
> I would like to hear
> experiences of people using the noise blanker
> in Linrad on HF.
-snip-

So would I ;)

I do not have any experience with HF at all myself.
My interest has always been 144MHz and I do not have
any reasonable HF equipment. I did some rx experiments
though. My general impression is that long distance
QRN will not be improved at all, multipath propagation
will smear out each pulse to a relatively long noise
burst that will be different each time. It can not be 
subtracted, blanking is the only possibillity. These 
pulses may be short enough to allow a blanker time of
only 100 microseconds or so which will require a bandwidth 
of at least 20kHz. A conventional blanker will not work
if there is a strong signal present within this passband
but the Linrad selective limiter will solve that problem 
and allows a conventional blanker in many situations where
a normal HF transceiver's blanker would fail.

Local QRN from powerlines, railroads, farmers electrical 
fences and so on are efficiently removed with the pulse
fitting procedure.

It is (of course) likely that Linrad does not work as 
well as it should because of programming errors and 
because of fundamental errors in the algorithms I have
implemented. To find out, I need recordings of situations 
where Linrad fails to remove QRN. There are tools included
for this, the timf2 oscilloscope (to look at pulses) and
the "S" command to save raw data on the hard disk.

The Linrad pulse removal can do "magic" things at narrow 
bandwidth. On a IC706-MKIIG which has a reasonably good 
audio dynamic range it works even a little better than 
the builtin blanker when there is no strong signal present.
With a strong signal just outside the passband the Linrad
pulse removal is unaffected (of course) while the builtin 
blanker fails completely. Linrad can even do pulse removal
with a rather strong signal within the SSB passband. The next 
QEX article will give the details. 

73

Leif / SM5BSZ

 



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