iPhone/iPad/iPod app to send and receive Hellschreiber
In a previous entry, How many watts do you need?, I discussed how transmitter power affects the received signal, and touched on the concept of the SNR, Signal to Noise Ratio. Seeing numbers expressed in dB is one thing, but actually hearing the difference between a station with an SNR of 10 dB and one of 20 dB is far more enlightening.
I created some simulated Signal to Noise Radio recordings. They were produced by mixing a relatively constant noise signal (actual static RF from a Software Defined Radio connected to an antenna) with a software generated AM modulated signal. One difference between these recordings and an actual station is that there is no fading, so real world conditions are likely to be somewhat worse, depending on the amount of fading the station is experiencing.
I’ve produced five recordings, with SNR’s of 0, 6, 10, 20 and 40 dB. A SNR of 0 dB means that the signal and noise levels are exactly the same. This is essentially the weakest signal that you could possibly receive. On the other hand, an SNR of 40 dB represents excellent reception conditions, say that of a local high powered MW station. The others obviously fall in between.
Remember that every 6 dB (voltage) of SNR is equivalent to 6 dB more signal (with the noise level held constant), in other words, doubling the transmitter power. Conversely, a drop of 6 dB is the same as cutting the transmitter power in half.
Let’s make up a crude example. A very strong pirate signal may have an SNR of 30 dB, somewhat weaker than a local station. Going from 30 dB to 10 dB, or 20 dB, is a change in transmitter power of a factor of 10 times. Going, for example, from 200 watt transmitter to a 20 watt transmitter. A 10 watt transmitter, half the power, would be 6 dB lower, or around 4 dB. It would be slightly weaker than the 6 dB simulated recording below.
Listen to the simulated recordings below to see the effects of various Signal to Noise Ratios: