Saturday, December 22, 2012

EME Upgrade - Separate Receive Line

This last weekend I upgraded the EME / Satellite station by adding a separate receive line to the 2-meter antenna array. This means that the system will now use a dedicated RG-8U line exclusively for receive (the transmit side uses LMR-600). The new system block diagram is below.

Splitting the transmit and receive lines enables the incorporation of a second dedicated receive-only radio into the system. For this, I'm using the FunCube Dongle Pro Plus, a 160MHz to GHz wideband software-defined radio (SDR). This SDR is about the size of a USB thumb-drive, and directly translates radio signals from the SMA port on one end to the USB connector on the other. From there, all the subsequent processing is done within the computer: the IF stages, mixers, and demodulators are all done in software instead of hardware.   The SDR is connected directly to the output of the hybrid splitter/combiner, which in turn is directly connected to the CX520 relay which switches between the RX line and a 50-ohm load.  This relay is important to prevent overloading the radios with signals coupled during transmit.

The SDR allows for all sorts of interesting things. It can monitor an entire satellite sub-band at once. For EME, it can monitor all conversations in the 144.100 to 144.160 MHz EME sub-band simultaneously. Since EME signals are extremely weak and almost impossible to hear by ear, having the computer search the entire sub-band for signals will greatly speed up the time to find an active station on the band. I'm running the SDR with Linrad software do do the initial filtering and processing of the wideband data stream, then MAP65 for decoding the JT65B signals reflected from the moon.

With the split receive, the JT65B signals can be decoded both by the traditional FT-817ND radio running WSJT9 software, as well as the SDR running MAP65. An example screenshot showing both softwares running simultaneously is shown above.  It leads to a lot of open windows on the computer, but it's really interesting to watch!

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