iPhone/iPad/iPod app to decode NAVTEX marine transmissions
Undercover Radio was on 6925 kHz USB several times on Sunday, May 20, 2012, conducting some transmitter tests in the afternoon, and with a show in the evening. I noticed how, even with a relatively weak signal strength in the afternoon, the overall reception was still good, due to the low daytime noise levels on the 43 meter band. Transmitter power was around 20-30 watts PEP.
Here is a graph showing the signal level of Undercover Radio on 6925 kHz, as well as background noise from an otherwise unoccupied adjacent frequency for 4 minutes, starting at 1700 UTC May 20, 2012:
Undercover Radio’s signal strength was about -92 dBm. Bear in mind that this was a voice only program with Dr. Benway talking, with frequent pauses in speech. Since this was an SSB transmission, the received signal level falls to the background noise level during pauses in speech.
The background static at 6930 kHz was -100 dBm
The net result is a signal to noise ratio of 8 dB, which is certainly adequate for fair to good reception.
Undercover Radio came back on at around 1900 UTC. Here is another comparison of Undercover’s signal vs background noise on 6932 kHz:
(Sorry, this time the noise is pink and the signal is blue. Just to keep you on your toes)
Eyeballing the graphs, it looks like the signal to noise ratio was about 15 dB, better than before. The quality of the received audio was indeed very good. Here is a recording
Next, Undercover Radio came on again at 0212 UTC.
The noise levels were around -85 dBm. Undercover Radio’s signal started at just around the noise level. At the time, he was running 20-30 watts PEP. Later, around 0245, Dr Benway realized he didn’t have the amp on, and then switched it on, going to 500-600 watts PEP.
One reason for the much higher nighttime noise levels is that not only is 43 meters open to DX from distant stations, but also to distant thunderstorms and other noise sources. Think of every thunderstorm in the world as a transmitter (which it really is). There’s thousands of active thunderstorms at any time, transmitting RF energy over the entire radio spectrum. This energy is received at your location from whatever parts of the world propagation is open to, on a given frequency. So while your signal can get out further at nighttime, it also has to compete with a lot more QRM sources.
During the daytime, the D layer of the ionosphere attenuates low angle radiation on 43 meters, preventing DX reception. You’re limited to just a few hundred miles. This applies both to the signals from radio stations that we want to hear, and distant noise sources.
Also notice how much Undercover Radio’s signal varied after the amp was switched on – by around 30 dB. That’s five S units! This tells us that signal reports, or even recordings, can be very hit or miss. One minute, an op can be at the noise level, a few minutes later, he can be many S units above it.
Two recordings. First, one from 0222 UTC when he was running 20-30 watts PEP, and the SNR was just a few dB. And second, one from 0300 UTC during a signal peak, when he was running 500-600 watts PEP, and the SNR was about 25 dB.
For comparison, here are some plots of WWCR, 6875 kHz, showing their signal level last night:
You can see that when their carrier went off the air, the noise level was around -80 dBm. And the signal varies by about 30 dB during the transmission, during nighttime. Earlier in the transmission, while it was still daytime, the signal was slightly weaker, but there was a lot less fading.
Perhaps the main point to take away from this is that while a pirate can be heard much further at nighttime than during the daytime on 43 meters, the lower noise levels and lack of significant fading during the daytime generally make for better quality reception, for those listeners within the several hundred mile NVIS range, and allows reception by listeners with more modest receiver/antenna setups. This is especially true when using lower power (grenade type) transmitters. Nighttime DX reception quality will be poorer, and limited to those with more substantial receiving stations. By selecting the time of day for operation, operators can to some degree select their audience and target area. A pair of transmissions, one in the daytime and one at night, would reach both local and DX listeners.