Hi George,
On Tue, Oct 21, 2025 at 01:41:35PM +0100, George wrote:
I have been wanting to build a TACS base station with Osmocom analog for a while now, and have decided finally to do it! To avoid regulations, I would like to do it over a cabled connection from the phone to the SDR. I have read that some form of load or attenuators are needed to stop the phone and SDR frying themselves. Unfortunately I have little knowledge on this subject, so I wondered if anyone here can tell be what sort of load I need.
You may find https://media.ccc.de/v/osmodevcall-20230719-laforge-wired-rf illustrative.
TL;DR:
You will need coaxial cabling with matching connectors as well as sufficient coaxial attenuators to resemble the typical loss of the radio interface. I don't know anything about TACS, but assuming the path loss of GSM, then something in the range of 70..100 dB of attenuation between phone and SDR should be in order. In case of doubt, start with higher attenuation and gradually reduce, but I wouldn't go below 70 dB in order to avoid any damage to the receiver side.
You can find coaxial attenuators e.g. from mini-circuits or pasternack. You can combine multiple in series to reach your target, i.e. you can use 3x 30dB to get to 90dB.
https://media.ccc.de/v/osmodevcall-20230719-laforge-wired-rf
Regards, Harald