Friday, January 22, 2010

WSPR vs CW - what's the signal advantage?

I was chatting to Justin, G4TSH last night. I told him about my experiments with WSPR on 7MHz and of course, couldn't resist telling him that my 1 watt WSPR signal had been heard in Tasmania. Justin wondered whether a 1W CW signal would have been heard in the same circumstances.

My feeling, at least based on the propagation at the time was that it probably wouldn't have been possible; it was a typical noisy evening on 7MHz, so my guess was that any QRP CW transmission would have been wiped out. Justin felt you could have probably made a rubber stamp '599/599' type QSO! Maybe, in the right conditions, but I think he might have been a little optimistic!

Thinking about it, I wondered if there's been any comparison about the advantage of WSPR over CW for reporting. A quick Google found me an interesting link on Roger, G3XBM's blog, who'd considered this very point.

Roger had asked Joe, K1JT, father of WSPR and the 'WS modes' for his comments and I've taken the liberty of reproducing Joe's reply to Roger;

"For an answer to your question about relative sensitivities of CW, WSPR, and some of the other modes implemented in WSJT, let me suggest going to the "References" link on the WSJT web site, ,
and select reference #11. Table 3 on page 9 gives the information you asked about. The difference given there is about 11 dB, in favor of WSPR over ear-and-brain CW. For most operators, the difference is more like 15 dB."

If we assume 6dB per S-point and that a 1 watt CW signal was S1 (or so), then a WSPR signal could be very weak and still detected. And of course, the repetitive nature of WSPR transmissions means that they may be heard where a 'random' CW transmission might not be.

Of course, in some ways, this is an unfair comparison. WSPR is purely a reporting mode whereas CW is a conversational tool, but it certainly suggests considerable advantages in using WSPR for propagation testing.


g4ilo said...

I have always assumed that the redundant nature of WSPR transmissions with error detection etc. was included in its theoretical benefit.

Many people have been using QRSS since before WSPR was invented, using "grabbers" (i.e. very slow waterfalls) to detect the signals and read the code from the pattern of dots and dashes.

The main advantage of WSPR is that it is automatic so you can leave it running and catch openings that would otherwise be missed because no-one would sit listening to an empty band for hours and hours. But when propagation is obviously present I'd rather try and make some actual contacts.

Laurent said...

Mmmh, i've always been reluctant on such comparisons.

I haven't read the mentionned reference, but i'm wondering what's compared here: One of the advantage of WSPR is to be repetitive and to work unattended. A fair comparison would be with a 24/24 CW signal repeating the same callsign and locator..

I may have a less-than-optimal setup, but it SEEMs to me that "if i can't hear it, WSPR won't decode it".

Laurent said...

My previous comment hasn't been published. Was there a problem ?

Tim said...

Hi Laurent,

No - the only problem was that I hadn't had time to logon and check the comments! I've been busy! Sorry.

As you say, if you can't hear something, then it can't be detected.

On the other hand, if you can narrow a bandwidth right down or look for specific tones etc... then you can probably detect much weaker signals than a voice or CW signal in a wider bandwidth. And then if you can do interpolation on weak or incomplete signals - that could help too.

CW is pretty good in conjunction with a brain, but I think when you get to look at VERY weak signals it could be a challenge to beat WSPR. Could be a fun event to try!


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