SDRplay Community Forum

All,

I'm currently changing my antenna configuration on my RSP2 Pro. I intend to use one SMA port for ADS-B (with a dedicated colinear antenna and 1090 Mhz bandpass filter), and the second SMA port for general VHF/UHF frequencies (with a discone antenna).

I already have ultra-low noise pre-amps to use with each antenna. These pre-amps were made by SV1AFN (ham callsign) with PGA-103+ ICs, and they work best at 9-12v DC. These pre-amps each have an internal bias-tee, but I realize I am only able to provide ~5 volts DC through the USB cable and the RSP2.

Consider this, please...... I intend to use a "reversed" bias-tee at the two RSP2 SMA ports, connected so (what would usually be) the "antenna leg" would go now go to the RSP SMA port, and the normal DC output leg of the tee would now become the place to run my 12v DC power supply. The normal "cable side" of the tee would then carry the 12v DC up to the pre-amp, while the RF would flow from the antenna through the "reversed" bias-tee to the SDR. In a sense, I'd be using this reversed bias-tee as a power injector.

As another aspect, with the blocking capacitor in the reversed tee, the SDR would be isolated from the 12v DC power supply, and in addition, the reversed tee (and pre-amp) would be isolated from the internal SDR bias-tee 5v DC, if I accidentally "toggle on" the RSP bias-tee via my SDRUno software.

The bias-tees I intend to use are available from a number of sources for under $10 USD each. I intend to build cases for shielding, for each pre-amp and each "reversed" bias-tee power injector.

Comments, please--on this approach...... Since the bias-tees I'm considering are good to 3+ gHz, I would expect some insertion loss, but with the pre-amps this should not be insurmountable.

I don't understand your concept of reverse in this description.

It sounds like you are simply planning to use the bias tee the correct way. The tee has a DC supply input. This DC is fed to the coax/socket that goes to the preamp/antenna system. The remaining coax/socket has a DC block (capacitor) and goes to the receiver. I agree that this scheme would protect the internal bias tee in the RSP.

Looking at the circuit shown here https://en.wikipedia.org/wiki/Bias_tee

The left port goes to the preamp/antenna
The right port goes to the RSP
The bottom port goes to the DC power supply

I think you are on the right track

Perhaps this is just a personal epiphany for me..... I always considered the term "bias tee" to only apply to the tap at the pre-amp. In my past (albeit very limited) experience with providing DC through coax to a pre-amp, I used receivers with internal power injectors. As such, I was concerned I would have to modify the RSP to provide 12 vdc, as opposed to the (USB) 5 volts.

Thanks for your comments. I guess my thought using the bias tee "in reverse" to be a 12 vdc, RSP-isolated voltage injector isn't, in fact, that original(!).