I need to build a setup where I can only have one cable down from the roof. Since the access is very difficult, I want to keep the minimal electronics there and bring the raw RF signals down where I can split and use howhever I want.
My idea is to inject DC to the coaxial cable and power amplifiers and filters on the roof as per this diagram.
Question is: what’s the best way to combine the signals? I only found TV splitters around, nothing for 50ohms/SMA connectors. Considering I’ll use a LNA before the combiner, would be such mismatch loss noticeable? Any hint to build a combiner/DC splitter myself?
Nice! I’ll mod a CATV splitter, replacing the 150ohm resistor by a 100ohm and put on another box with SMA connectors.
How about the filter + amp setup? do you think I’ll need to put filters after the splitter (before the RTL-SDR) or these frequencies wont interfere each other?
Take a look at the Comet 413B duplexer – I use one with a discone antenna, using it for simultaneous receive of ADS-B, and receive/transmit on 144 MHz
Use of something like the Comet (probably one on each end of your coax) gets you the band splitting. The DC path is another issue. I’d go with a pair of MiniCircuits bias tees, one for injecting dc into the coax, and the other for picking the dc off the coax. You can also try rolling your own bias tees with a capacitor (10 nf, say) and an inductor, in a small case. You’re not going to have the impedance match that the MiniCircuits assemblies give, but you won’t be paying the MiniCircuits price, either.
You can usually find both the comet duplexer and mini circuits bias tees on eBay. Also some cheepie bias tees showing on eBay for under $10