Considering that the SDR board has one TX/RX output/input and one RX input, I am wondering how could I use one antenna for a half duplex communications on the same frequency.
The idea is to use the TX/RX output for transmitting and the RX input for receiving. The problem is that the required output power is around 47dBm (which is achieved after a HPA) and if I use a fairly good circulator (for joining Rx with TX) as this one, it gives a 20dB isolation and that is not enough as it would burn the whole thing with the reflected power (max. input power of the SDR is -15dBm after the LNA!). Scheme: 
As an alternative, I have thought of using a electronically controllable switch that could open one path while closing the other. The problem with this option is that so far I have only found very expensive solutions and my budget is limited.
Can anyone give me some guides on building a PC controllable switch for this purpose (should withstand around 50W of power while offering the typical 50\Ohms of impedance)?
Otherwise, any different ideas/approaches on how could I solve the problem?
Best Answer
As mentioned by Sam, if you're doing half-duplex only, your problem is a lot easier, and you don't actually need a circulator; just a set of switches.
You don't mention which SDR device you're using, but a lot of devices, including the USRP series of SDR peripherals, have GPIOs for exactly that purpose:
Notice that at least in the case of USRPs (and if I judge by your nomenclature, you might have one of these), the SDR device itself supports switching the GPIO state based on the transmit/receive settings – i.e. you can automatically switch externally when disabling the transmit stream.
Please make sure that post-amplifier crosstalk doesn't fry your RX electronics – for example, the frontends available for USRPs are rather sensitive (ie. NF <<10dB, sensitivities below thermal noise level), and thus can only withstand free of damage maximum inputs of typically -15dBm. Hence, switching off the HPA before turning on the RX LNA might be a good idea – again, GPIOs might be helpful here; the modern USRPs support timed commands for addressing such operations in a hardware-timed manner.