[RTTY] Greater than 600 watts for RTTY

[Kok Chen]

When all else is constant, a 10 dB gain (100 watts to 1 kW) can make a marginal signal turn into a perfectly printable signal when conditions are perfect (no fading, no selective fading, no Doppler spreading).

Take a look at the curves at Alex' web site:

http://www.dxatlas.com/rttycompare/

Notice in the AWGN curve that a perfect/good demodulator requires a SNR of about -12 dB (in 3 kHz noise bandwidth) to achieve 30% character error rate, and improving the SNR to -8.5 dB achieves an error rate of 3%.

So, if you are sitting right at the edge of being copyable, just a 3.5 dB increase in power (a little bit more than double) can make the difference between a garbled copy to a decent copy.

If the other side is using a less good demodulator, you would need to bump your power by another 3 dB; so lets hope DXpeditions use good demodulators.  

When propagation becomes poorer (scroll down to the Selective Fading curve at VE3NEA's page), the difference between good copy and poor copy becomes larger -- the better demodulators need something like 8 dB more transmit power, while the poorer demodulators require upwards of 10 dB extra transmit power to overcome propagation effects.  You need a bigger bump in power to go from marginal copy to good copy when conditions degrade..

That said, when the other side is on an unpopulated island, they can start with more than 10 dB SNR advantage; that is why they can copy you fine when you can barely copy them (if no one is QRM'ing you).   See the ITU-R PI.372-6 man-made noise chart at the end of this web page:

http://educypedia.karadimov.info/library/rsgb.pdf

Notice that "Residential" noise is more than 10 dB louder than the average atmospheric noise at 14 MHz, while "Quiet Rural" man made noise is dominated by the atmospheric noise. The advantage of a rural location is less on the lower bands because atmospheric noise is much higher at the lower frequencies.  The advantage of having lower noise floor on an uninhabitable island is not as great on 40m and 80m. 

Because of the lower atmospheric noise at high HF frequencies, the difference between rural and residential is very large on 15m and 10m.  At 20 MHz, the average atmospheric noise is lower than man-made noise in rural areas, so you end up with a whopping 20 dB improved SNR when you are away from population.   

If you can barely copy a DXpedition to an isolated island on 15m and 10m, they should have 100% copy on you, even if you run 20 dB less power than they do (e.g., they run 1 kW, you run 10 watts).

By concentrating the antenna lobe to near 0 degree elevation, a vertical over salt water that they often use also gives them better  SNR.  

As long as you can print a DXpedition on a remote island, the reason you need power to work them IMHO has more to do with being drowned out by the FSK keyclicks in the pile, rather than from not using high power.   Stay a full kHz or two away from loud stations that run FSK, and you should get through more easily.  A panadapter will show you who is running FSK and who is running waveshaped AFSK.

73
Chen, W7AY