Tuesday, May 19, 2020

What WSPR signal to noise ratio is needed for sideband to be intelligible?

The end-fed antenna is working pretty well on 40m and I'd love to have a contact with Sydney friends. John, VK2ASU, kindly agreed to run a WSPR beacon so I could find the best times to receive him.

Over a 24 hour period, with him transmitting just 10mW, the best SNR was -21dB here.

My question, to the brains trust hopefully reading, is what WSPR signal to noise ratio would be enough such that a higher power sideband call would be audible?

I have read that a signal to noise ratio of 6dB is needed to be able to copy SSB.

John is transmitting 10mW which is 10dBm but he could run 100W on sideband or 50dBm. An extra 40dB.

If the bandwidths were the same then -21dB + 40dB = 19dB of signal to noise but while SSB is about 2.5kHz wide, WSPR is much narrower, perhaps only a few Hz?

Here is the SNR between VK2ASU and VK2TPM (portable VK3).


Spots from all stations to me looks like this:


Any insights or pointers gratefully received.

Kevin, VK2KB, commented: "My understanding is the noise power doubles every time the bandwidth is doubled while the power level of the received signal stays constant. I've verified this on my SA by changing the bandwidth of the SA response and noting the noise floor, you can see the floor decrease by 3db every time I halve the  SA  window but the amplitude of the signal I'm watching stays the same.  With WSPR you need to know what the effective bandwidth is. I had a look at this site:
https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)

They say the effective bit rate is about 1.5 baud  and its possible to recover a wspr signal at -34 db  on a 2500Hz bandwith rx.

Say that you are using an effective bandwidth of 1.5 Hz to recover the digital signal the ratio of 1.5/2500 =  .0006 or -32db  Because the wspr signal is coherent I think they get a few more dbs in the demodulation process.

The difficulty with this is however the power bandwidth of the wspr signal vs the power bandwidth of the ssb signal.  Anyway something to spend time on."

3 comments:

Unknown said...

Hi Peter,
https://www.qsl.net/kp4md/wsprmodes.htm
contains some interesting comparisons of WSPR vs other modes.

Don't forget the VK2RWI CW beacon on 3699 is a good Sydney beacon.
VK3ASE is over 59+20dB on AM into Sydney tonight on 3670!

Hope the move went well and you're re-acclimatizing to those cooler temps!
- Jules VK2EXT

Peter Marks said...

Thanks Jules, that's a great reference.

Chris VK2CJB said...

I've been told by another WSPR Tragic that there's a 500x/27dB advantage over SSB-- not that I've been able to rouse other VHF operators during above-average conditions. So.. during a WSPR 10mW/-21dB, 10W might get +6dB SSB.

(A while back I gave John my own code that cycles through the Si5351's power settings. Using that, I've managed a 2mW ping on 40 metres with a station in the States.)