Hello, 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.
Any help would be greatly appreciated
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
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Your eagerness is palpable, but don't let technical hurdles deter you. I remember grappling with similar challenges when I dived deep into a Connections Game project, where I had to figure out the right components to prevent my setup from breaking down. Consult with experts or forums; you may find the right attenuators or loads crucial for your setup. https://connectionsgamefree.com
You’ll likely need a proper 50-ohm attenuator between the phone and the SDR to safely reduce the signal and protect the hardware. I came across this thread while browsing after reading something about https://soflowheelielife.org/, and the setup sounded interesting.
Hello there, I have got the setup working fine, I have a total of 90db of attenuation between the phone and the SDR and it works really well. I also have Osmocom setup so that the phone always transmits at its lowest power (a few milliwatts). Also I don’t think attenuation is measured with ohms…
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On Tue, 10 Mar 2026 at 9:27 am, soflowheelielife.org@outlook.com wrote:
You’ll likely need a proper 50-ohm attenuator between the phone and the SDR to safely reduce the signal and protect the hardware. I came across this thread while browsing after reading something about https://soflowheelielife.org/, and the setup sounded interesting. -- osmocom-analog mailing list -- osmocom-analog@lists.osmocom.org To unsubscribe send an email to osmocom-analog-leave@lists.osmocom.org
While keeping an eye out for any indications of distortion or overload, start with low power settings and progressively increase. Make sure the SDR is getting clear, distortion-free signals by using software tools to evaluate the signal quality. https://snowroad3d.io/
To safely connect your phone to the SDR for your TACS base station, you will need an appropriate load or attenuator to prevent damage. A typical approach is to use a 50-ohm load or an attenuator rated for the frequency range you are working with. Look for a 10-20 dB attenuator, as this should sufficiently reduce the signal strength to prevent frying the devices. Ensure that the attenuator can handle the power levels involved. It’s also advisable to double-check the specifications of both the phone and SDR to ensure compatibility. Good luck with your project! https://geometrydashmeltdown2.io
It’s great to see your enthusiasm for building a TACS base station with Osmocom analog! Ensuring proper attenuation is indeed crucial to protect your equipment. For more exciting projects and ideas, check out https://www.fivenightsatepsteins.com/
The idea of cabling directly to avoid RF regulations is clever but tricky. I once tried linking mismatched RF gear without proper attenuation and nearly cooked a front end, lesson learned fast. Crossy Road popped into my head because it felt like hopping across danger steps blindly. Definitely respect impedance and attenuation. https://crossyroadfree.com
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