DC Blocker to protect SDR using Y-Cable splitter

WRVS497 · April 7, 2023

I'm designing a cheap GMRS base station setup using a Baofeng GMRS-9R, a laptop with an SDR and SDR#, a Y-cable, and an antenna tuned for GMRS frequencies. I have these things already and figured I would play around with this while saving up for a real 50W base station radio. (Probably the Wouxun KG1000G)

My question is, would an inline DC blocker (normally used to block dc voltage from a bias-t) block the TX voltage from an HT and protect the SDR in this design? Or, is the TX voltage AC and the DC blocker would be rendered ineffective or damage both it and the SDR?

The DC blockers I have are cheapos from Amazon https://www.amazon.com/dp/B07P1KPQCX?ref=nb_sb_ss_w_as-reorder-t1_k7_1_10&amp=&crid=3R9W9HMRI3RAW&sprefix=dc%2Bblocker&th=1

I have not found anything equivalent for AC blockers either so my assumption is a DC blocker would be the way to do this. I know that duplexers accomplish this but they are frequency based and my intent is to transmit and receive on the HT and receive on the SDR to watch the GMRS spectrum for potential traffic as well as signal strength analysis. Would the SDR just be overloaded from the transmission regardless?

tweiss3 · April 7, 2023

No, you are looking for something more like an RF sensing disconnect switch: https://mfjenterprises.com/products/mfj-1708b-sdr

They also make things called RF limiters. I have them installed on the vehicle scanners to protect the front ends, but they can only handle/block 10W, so a 50W radio + antenna separation it keeps working. Once you exceed it's limit, there is a diode in them that pops and you would have to replace it, so it won't work well when you jump to 50W.

axorlov · April 7, 2023

DC blocker is not going to work for sure. Your radio already has this RF switch to protect it's own receiver. These switches are usually based on PIN diodes (here is more info: https://www.digikey.com/en/articles/how-and-why-to-use-pin-diodes-for-rf-switching). The practical solution would be to tap RF input for SDR after this RF switch in your radio. Search on the internet, there are solutions published for some popular radios. It may influence what are you going to buy. Below red arrow shows a good location for my radio (TK-880).

WRVS497 · April 7, 2023

Good thing I asked and I really appreciate the feedback! At $160 for the MFJ product and the amount of effort it would take to figure out axorlov's suggestion i think ill just make another antenna for the SDR side and separate the two. Back to the drawing board and thanks again.