Over on his web site, Martin (G8JNJ) has uploaded a new article discussing how to optimize reception of VLF (<10 kHz) signals on a direct sampling modified RTL-SDR. He writes that the main problem with the RTL-SDR receiving VLF frequencies is that there is significant overloading caused by broadcast stations operating on the LF and MF bands.
So in order to optimize VLF reception, Martin built an external frequency equalisation network which has the following components and functions:
- 2MHz Low pass Filter – to minimise alias signals originating at 30MHz
- 20dB Variable attenuator – to set the overall signal level fed into the dongle
- Switched LF roll-off – to optimise the performance at frequencies around 10KHz in the presence of strong lightning surges
- -10dB notch at 198KHz – to reduce level of BBC R4 broadcast station in the LF band
- -10dB notch at 800KHz – to reduce level of local broadcast stations in the MF band
His screenshot results show that his filters work well and significantly reduce the effect of lightning pulse noise at 9 kHz. With the filters in place and properly optimized with the attenuator and various switches, he is able to receive Russian Alpha navigation signals at frequencies around 12 and 14 kHz and the 300 to 500 kHz aeronautical and maritime navigation bands.