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
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 sideTraditional 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.
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