I'm an electromechanical engineer in the middle of a side project and I stepped right in a big pile of RF. I've never really worked with RF signals before so I've done my best to spin up on the field but I wanted to run my design by others for feedback/guidance.

What I'm trying to accomplish in short:
Integrate a two separate VHF transmitting devices into one antenna, and bias the antenna with 3.7v DC.

The first RF device (a gps tracker) operates at 150-155 MHz, short pulses (data) every 5 seconds or so.
The second device (RDF pinger) operates at 145 MHz, short pulses (audio) every couple of seconds.
The antenna will be nitinol wire, 1/4 wave.
I need the tip of the antenna to be at 3.7v DC. I could just run a small wire up the antenna and heat shrink together, but I don't want the antenna coupling to the wire and feeding RF back into my DC supply (unless a simple choke would make that okay?)
I have a small RF power combiner, 2 way, 0 Degree from Mini Circuits.
The plan is to connect each VHF transmitter to the respective ports of the power combiner, and the antenna to the sum port, I have RG174 cable for the transmission lines.

As you can see in the picture, the GPS tracker has an 18inch which landed on a screw lug. There is a metal case around the smarts, which I pressume is an RF shield/ground plane. The first problem/question is that the antenna connector shows continuity to ground(battery ground and to the RF case on the pcb). Is there any issue with just landing the center conductor on the antenna output lug, and tying the coax shield/shell to the RF shield/case/ground?

Secondly, I need to bias the antenna so that there is 3.7v DC present at the tip at all times. I have ordered a 1nF capacitor and a 7uH inductor (with the highest self resonance I could find) to make a bias tee, which I plan to connect just after the sum port of the combiner. This signal (RF and DC) would then go through a short length (3-4in) of RG174 coax, to the top of the enclosure, where I would land the center conductor on the nitinol, and then use copper foil tape to form a ground plane and connect to the shield. Does this seem reasonable?

I purchased a VSWR meter and some unterminated connectors and I plan to temporarily solder it inline with the antenna for tuning, any advice there?

Any and all advice is welcome

 

I have never done anything like that myself, but the article on this page will describe RF Circulators.
https://www.everythingrf.com/browse/microwave-rf-circulators

 

Thanks for the reply, some good info there for sure. I spent the evening at the bench and doing more research. Looks like the most reliable way to do what I want is to swap the RF combiner for a dual coil latching relay. Then I can switch the two devices between the antenna at will and suffer no losses when receiving. Also makes my job easier tuning the antenna as I don't know what the transmitter impedance is, so I would have to match the two transmitters to the RF combiner and then the sum port to the antenna.

 

I only have a few minutes to reply, but it seems like you need to send DC from one module at the antenna one-way, but not from a second module also connected to the antenna. Take a look at cheap cable/satellite splitter/combiner modules. these will typically have one (or more) RF ports that conduct DC, but the other ports are DC isolated. Most of these modules are 75 Ohm, not 50 Ohm, but the mismatch may not matter depending on your application. Remember, depending on the splitter/combiner type (versus e.g., a diplexer which would be difficult to do to the close VHF frequency spacing you mention), you are going to introduce thermal noise into the receive path. A block diagram of what you are trying to do with cable length/types and links to the devices you are connecting would help us. You can buy cheap Bias-T's for the cable/satellite market if you do some searching. But again, without a block diagram, it is difficult to specify an optimal arrangement.