2016-10-03, 16:53
Further noise chasing and reduction.
My 38 cm 20 turn loops from 7 x 0.5 mm dia stranded copper wire have a sharp and very deep null. Careful alignment can reduce the the extremely strong MSF 60 kHz carrier from Anthorn into the noise floor. It's a bit sensitive though the slightest movement and MSF comes back above the floor. I also found that the 20 kHz signal also comes from Anthorn, so for one loop both of those are sorted.
To aim the null of the loop accurately at Anthorn I connected an SSB communications receiver to one of the amplifier monitor ports. This receiver isn't supposed to go as low as 60 kHz but it does, it even goes down to 20 kHz but is beginning to get rather deaf by then. Using SSB makes the aiming dead easy just carefully rotate the loop until the whistle disappears.
Another "aiming aid" that is probably easier for most people to obtain is a spectrum analyser app for their smart device. I have Android and used Advanced Spectrum Analyzer PRO from Vuche Labs. Connect the smart devices mic in to one of the monitor ports. The mic in is probably on the second ring of the Tip, Ring, Ring, Sleeve 3.5 mm plug of a smart device headphone set with microphone. There will be a small voltage on that second ring to power the headsets electret microphone. A blocking capacitor might be wise if this connection is likely to be used for anything else. The Blue amplifier monitor ports are buffered and have a capacitor so aren't worried by this low voltage/current supply.
This app, and all other similar ones, are limited by the smart devices hardware to the audio band but this one will go up to 24 kHz. This showed me that the "20 kHz" carrier is actually 19.6 kHz and is 35 db above the noise on the loop looking at Anthorn. With the LP filter set to 16 kHz it's reduced to 10 dB above the noise. That's some sharp filter! It's not quite as easy to get the aim spot on but you can get pretty close.
The app also showed a smaller carrier around 16.4 kHz, that's not the frequency I was expecting from Skelton and anyway the bearing is wrong. Further research into the source of that carrier required. Skelton seems very quiet and is 26 degrees away from Anthorn so should be easy to tell apart.
The LP filters work very well, see above. For the loop looking at Anthorn I'll probably set the filter to 16 kHz to severely reduce the 19.6 kHz carrier. The other two set at 54 kHz to remove any residual 60 kHz signal but maintain as much bandwidth as possible to keep the rise time fast. On the loop with its null aimed at Anthorn I may try turning the filter off and see what Auto makes of that. With the 16/54 settings Auto sets the gain of the three channels the same (4000) to get the target 50 mV noise floor for each channel. In theory that means the loops will pickup fairly evenly from all directions.
Awaiting arrival of a few bits to build a power supply filter to enable the use of a TP-Link TL-POE10R Power over Ethernet splitter and a box to mount everything in. Once those arrive the lot can be properly installed and I can think about where to put the E-field antenna.
My 38 cm 20 turn loops from 7 x 0.5 mm dia stranded copper wire have a sharp and very deep null. Careful alignment can reduce the the extremely strong MSF 60 kHz carrier from Anthorn into the noise floor. It's a bit sensitive though the slightest movement and MSF comes back above the floor. I also found that the 20 kHz signal also comes from Anthorn, so for one loop both of those are sorted.
To aim the null of the loop accurately at Anthorn I connected an SSB communications receiver to one of the amplifier monitor ports. This receiver isn't supposed to go as low as 60 kHz but it does, it even goes down to 20 kHz but is beginning to get rather deaf by then. Using SSB makes the aiming dead easy just carefully rotate the loop until the whistle disappears.
Another "aiming aid" that is probably easier for most people to obtain is a spectrum analyser app for their smart device. I have Android and used Advanced Spectrum Analyzer PRO from Vuche Labs. Connect the smart devices mic in to one of the monitor ports. The mic in is probably on the second ring of the Tip, Ring, Ring, Sleeve 3.5 mm plug of a smart device headphone set with microphone. There will be a small voltage on that second ring to power the headsets electret microphone. A blocking capacitor might be wise if this connection is likely to be used for anything else. The Blue amplifier monitor ports are buffered and have a capacitor so aren't worried by this low voltage/current supply.
This app, and all other similar ones, are limited by the smart devices hardware to the audio band but this one will go up to 24 kHz. This showed me that the "20 kHz" carrier is actually 19.6 kHz and is 35 db above the noise on the loop looking at Anthorn. With the LP filter set to 16 kHz it's reduced to 10 dB above the noise. That's some sharp filter! It's not quite as easy to get the aim spot on but you can get pretty close.
The app also showed a smaller carrier around 16.4 kHz, that's not the frequency I was expecting from Skelton and anyway the bearing is wrong. Further research into the source of that carrier required. Skelton seems very quiet and is 26 degrees away from Anthorn so should be easy to tell apart.
The LP filters work very well, see above. For the loop looking at Anthorn I'll probably set the filter to 16 kHz to severely reduce the 19.6 kHz carrier. The other two set at 54 kHz to remove any residual 60 kHz signal but maintain as much bandwidth as possible to keep the rise time fast. On the loop with its null aimed at Anthorn I may try turning the filter off and see what Auto makes of that. With the 16/54 settings Auto sets the gain of the three channels the same (4000) to get the target 50 mV noise floor for each channel. In theory that means the loops will pickup fairly evenly from all directions.
Awaiting arrival of a few bits to build a power supply filter to enable the use of a TP-Link TL-POE10R Power over Ethernet splitter and a box to mount everything in. Once those arrive the lot can be properly installed and I can think about where to put the E-field antenna.