What is low-frequency reception?
Well, it depends on your primary activity area. For regular High Frequency (HF) guys the lower end of the HF band is around 1.8 MHz or 160 m wavelength. Below the HF or ShortWave (SW) band we can find the Medium Wave (MW), the LongWave (LW), or even the very longwave bands too. These are usually called Low Freqeuncy (LF), Very Low Freqeuncy (VLF), Extra Low Frequency (ELF). Additional information can be found here: http://en.wikipedia.org/wiki/Frequency_allocation
Low-frequency reception limit of Quadrus SDR
The DRU-244A SDR digitizer hardware of the Quadrus SDR platform has AC coupled inputs. Thus LF reception on the platform is limited by the transformer at the input stage. According to the datasheet, the lower frequency -3 dB band edge is at 60 kHz.
I’ve used a simple inverted-V shaped 2×20 m G5RV antenna and bypassed the usual High-Pass Filter (HPF) in the HF receivers.
First, I looked at lower frequencies than MW broadcast bands, and I’ve found some interesting signals in the 300-400 kHz band.
There are a couple of Double SideBand (DSB) AM transmitters with strong carriers modulated by simple Morse code. These can be heard with a simple AM receiver as well. It is very simple to copy them, and you can visually decode the call sign from the waterfall diagram. Later, I’ve learned on one of the forums that these are Non-Directional Beacons NDBs for navigation proposes. More info here: https://en.wikipedia.org/wiki/Non-directional_beacon
Low-frequency reception of LW broadcasts
The lower part of the band, around 150-250 kHz, contains again AM modulated LW broadcast stations.
Low-frequency reception the DFC77 transmitter
Below 100 kHz we’ve found some interesting signals. I was only familiar with the DCF77.
This is a time-frequency standard from Hamburg. http://en.wikipedia.org/wiki/DCF77 It can be received very well with the wire antenna, and decoding the message is also possible with a special decoder program SpectrumLab by Wolf, DL4YHF.
The LF reception capability of the Quadrus SDR platform was introduced. NDBs were received in the 300-400 kHz band. The LW AM broadcast stations in the 150-250 kHz band as well. The well known DCF77 reference transmitter was received and decoded at 77 kHz and some signals were detected around 50 kHz. LF reception needs some modification on the analog input stage of the receiver hardware.
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