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GSM in 70cm HAM band

This page exists to collect some information around the topic of operating a GSM/GPRS network within the 70cm (~430 MHz) HAM radio band. This is something being done in cooperation between DARC under and ARDC grant

Duplexers

The BTS side requires a duplexer as it is transmitting and receiving at full-duplex at all time. On the MS side, the traditional approach is to have a MEMS antenna switch to switch the antenna to either Rx or Tx. This is possible at least for GSM and most GPRS multi-slot classes. Until antenna switching via a GPIO is available from the Osmocom MS-SDR, a duplexer can also be used on the MS side

Traditional cellular duplexers for commercial cellular bands are designed as two band-passes with one common (antenna) part. This means they are multiple devices in one:
  • Rx band filter to block out-of-band signals from entering the receiver
  • Tx band filter to block out-of-band signals from being transmitted (including harmonics)
  • isolator to prevent strong Tx signals from entering the Rx chain

Existing 70cm duplexers

It appears that exsting duplexers in the 70cm band are not an arrangement of two band-passes (like commercial cellular) but rather two notch-filters.

This means they really only serve the purpose of preventing Tx leakage into Rx, but don't really provide the Rx/Tx band-filter functionality.

SGQ-450

Available from a number of sellers on Amazon, ebay, Aliexpress, ...

  • consist of 6 cavities
  • insertion loss around 1.5dB
  • 228 x 145 x 35mm
  • 1.2 kg
  • N-type connectors
  • >= 75 dB isolation between Rx and Tx
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Updated by laforge 8 months ago · 1 revisions

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