# Combine ADS-B and VHF (Airband) signals on the same cable

#1

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?

Thanks

#2

Each transition from 50 ohms to 75 ohms (or 75 ohms to 50 ohms) results in a mismatch loss of 0.177 dB.

If you have a total of 4 transitions in series, the total mismatch loss will be 4 x 0.177 = 0.708 dB

*Calculations for attenuation loss in single transition 50 to 75 ohms (or 75 to 50 ohms).

Reflection coefficient Γ = (75-50)/(75+50) = 0.2
VSWR = (1+Γ)/(1-Γ) = (1+0.2)/(1-0.2) = 1.2 / 0.8 = 1.5
Mismatch Loss in dB = -10 log (1 - Γ²) = -10 log (1 - 0.2²) = -10 log 0.96 = 0.177 dB*

#3

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?

Thanks

#4

Sat TV (>1GHz) +Air TV (<1GHz) diplexer

#5

Regarding the diagram in the OP, ‘usually’ (I use the term guardedly) you will have a lower noise system if you put the preamp before the filter.

The exception is when local transmitters would otherwise saturate either the preamp or the the receiver.

#6

That’s my case, without the filter every amp I tried was saturated

Thanks for the diplexer hint, much better solution indeed.

#7

#8

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

Let us know what you try and how it works out.

bob k6rtm