Tuesday, 8 September 2020

Just One More dB!


*** The following blog was originally published in 2016 but is still very applicable in 2020! ***

How often have you struggled to pull a weak signal out of the noise? "Just give me one more db", you tell yourself.

A recent posting to the Topband reflector by Frank, W3LPL, sent me to the interesting webpage of Dave, AB7E. Dave had been pondering two antenna systems, one of which would provide a 2db improvement in forward gain but at a much higher cost ... he wondered if the extra expense would be worthwhile and could he even hear the difference that 2db would make? He created a series of CW files, incrementing the signal level in 1db steps to see for himself!

Now I've always been told that you need to increase signal strength by at least 3db before your ears can detect any difference ... but listen carefully and you may be in for a surprise, as AB7E discovered.

It's probably best to listen to this signal with headphones but, even on my I-Pad's tiny speaker, the demonstration is clear. The first recording starts at "zero db", which is sent twice while the next signal is "one db", sent twice. See if you can hear the difference between each 1 db increment as he steps up to "six db":

Try going the other way, from "six db" down to "zero db":

The following recording has two signals, one of which is one db louder then the other. Can you hear the difference?

Although I was able to hear one call slightly better than the other, it was difficult. How about two signals again, one of them being 2db louder this time ... this one is much easier:

Lastly, AB7E demonstrates the problem with sending too fast when conditions are very marginal. Here, several signals are sent at 20, 25, 30 and 35 WPM. Sending calls at high speed can often seem effective, even under poor conditions but this seems to demonstrate that slowing down just a bit would make it somewhat easier:

One of the more interesting comments posted regarding these recordings was from Bob, N6RW who cited his work in satellite communications:

"I spent part of my engineering career designing satellite command FSK
demodulators - including the deep space Pioneer Venus orbiter. To test
the performance of them, we would mix the test signal with white noise.
When you look at the FSK Bit-Error-Rate (BER) curve (bit errors versus
signal to noise ratio in a bandwidth equal to the bit rate), you can see
the BER improves by a factor of 10 to 1 for every dB in S/N ratio. In
other words, for every dB improvement, you get one tenth the errors."

Now Dave never did tell us if he bought the bigger antenna or not but I'm betting that he did ... it looks like "just one more db" may really be just enough after all.


Terry Maurice VE3XTM said...

An interesting experiment.

I have found that the use of the BHI Noise Eliminating In-Line Module can greatly enhance the quality of weak signals. I tried it with these recorded examples and indeed the clarity of the signal was much improved after processing the signal through the BHI unit. The processing is applied to the audio signal coming from the receiver.

The BHI NEILM analyses the background noise and then subtracts it from the incoming signal to give better clarity. I especially find this helpful on weak voice signals SSB on the 2m and 70cm bands. The depth of noise reduction can be varied and the unit also has an audio preamlifier that gives some extra audio boost.


Photon said...

Or you could take a vertical down the coast and get a free 10dB, and often much more, from the sea (yes, I have the data to back it up - over many years).

Steve McDonald said...

The data regarding a sea-horizon has been well-documented and I've been realizing it here since setting up my antennas about 25' from the ocean... I'm guessing that the theoretical minimum of 6db of sea-gain is realized on any band that I am able to get a sense of it, such as the 2m EME work with a single 9el Yagi. Unfortunately most folks are not able to just travel to the coast and plunk down an antenna to get it!