RF switch for half-duplex sequencing on amateur UHF satellite ground station

Question

I need a 3-port RF switch illustrated below. The idea is to use this switch with a sequencer to enable half-duplex communication on a UHF antenna. The switch has three ports: port A connected to the HPA, port B to the receiver, and port C to the LNA. Ideally, the needed connections are A->C (transmit path) and C->B (receive path). However, signals on the A->C can leak into B, something that I don't want. Do switches with these requirements exist?

Answer 1

Your diagram is wrong - you need the LNA in the Receive path, switched out of the way when you're transmitting. (Unless the LNA has its own internal auto bybass). So the Tx/Rx switch is the first thing connected to the antenna.
For best receive sensitivity, EME hams use two coax runs. With the Switch and LNA mounted at the feed, the thick Tx coax from the PA to the switch, and then a thinner Rx cable from the LNA to the receiver.

Then you need a regular SPDT RF Relay, with the Antenna connected to the common port and the Tx and Rx to the selected ports.
You want what's called an absorptive switch which terminates the unused input - these have a second mechanism that connects the unused port to a load internally, this reduces the coupling between ports and ensures the LNA doesn't start oscillating or anything awful like that.

SMA relays typically have a power handling of up to hundreds of watts (depending on frequency, no hot-switching) and an isolation of >> 60 dB which should be enough to protect your LNA. Here are some from JFW, mini-circuits.

Teledyne was my go-to name but you know it will be expensive when the specifications include 500 G shock etc, I think they were a few thousand dollars.
But there are so many out there on ebay and at ham events. Try to find an older, solid case, made-in-USA/Germany/Taiwan model, rather than a brand new cheapie with the coil hanging out of one side. The quality of the plating is usually a clue, it should be perfectly smooth, slightly matt and slightly less bright than silver - the colour of solder, not chrome. (even if it's dirty). Thin cheap chrome or nickel plating looks perfectly shiny but is a little rough and wrinkly.

Answer 2

What you want is for path B to be terminated — to have a dummy load attached — while the switch is in transmit mode, rather than being an open circuit that can act as an antenna for high-impedance signal leakage.

I'm not greatly familiar with the terminology for RF switch products, but a quick bit of browsing suggests that you want a “transfer” switch. There doesn't seem to be complete agreement on what this means, but in the type I'm thinking of there are four ports, two pairs of which are connected, and activating the switch causes the other two pairs to be connected instead. In that way, you switch between the two configurations:

• TX (A) to antenna (C) and RX (B) to terminator (D)
• TX (A) to terminator (D) and RX (B) to antenna (C)

Another possibility is to use a conventional (as in, what non-RF electronics means by the name) DPDT switch (6 ports) or two SPDT switches (two of the type you already have in mind), such that the receiver can have a termination switched in. Using two switches could also allow the receiver switching to be physically separated from the transmit signal path, which might reduce leakage.

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Note that even with the best possible switching setup, you may also experience transmit-to-receive leakage that is not through the switch or the feed line at all, but from the transmitting antenna to the receiver around the feed line (i.e. through the receiver's enclosure and shielding), or even out of the enclosure of the transmitter and in to the enclosure of the receiver.

Answer 3

I think what you want is an RF circulator.

Circulators help direct the flow of microwave or radio-frequency signals in RF systems. Usually 3 or 4-port device that transmits input signal from 1 port to the next, Figure 1 shows a normal circulator symbol. The ports are either input signal, output signal or termination and typically have a line impedance of 50Ω. Common connectors are SMA or N-type coaxial. The signal will transfer from port 1 to 2, 2 to 3, and 3 to 1. The ports are connected to a symmetrical Y junction that is coupled with magnetically biased ferrite material. Ferrite along with a magnet creates a flow that helps push the signal in the circular path we see and ensure the port-to-port assignments are followed. Circulators are designed to have minimal loss when transmitting an input signal from one port to the next.

In your example, A is port 1, C is port 2, B is port 3. A->C, C->B.