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