Radio frequency circuit is impossible to get started quickly. This thing needs a lot of experience, unless you can find a good teacher or master who is willing to teach wholeheartedly. But you can also do some work.

1, review advanced mathematics, since there is a research foundation, I'm afraid there should be some accumulation in mathematics. Mainly some integral transformations (Fourier, Laplace, Z), ordinary differential equations and so on. Maxwell equation is the most basic equation in the field of microwave and electromagnetic wave. If you deal with each other frequently, you will have a good foundation in mathematics, which will save a lot of effort and make it easy to write a paper.

2. If you have time, review the distributed parameter circuit in the circuit foundation, the basic amplifier circuit in module electricity and the high-frequency electronic circuit. In addition, you have to take up courses in microwave. I remember that when I was an undergraduate, I had three courses on microwave, namely, applied electromagnetism (which should correspond to your microwave principle), microwave and optical guided wave technology, and radio frequency electronic technology (which belongs to the enhanced version of high frequency, but it is not high frequency, and high frequency is a separate course). You can also read the book "High Speed Mathematical Design" again, which may be useful.

The field of RF circuits is very difficult and the progress is slow, but few people do it, so it is easy to find a job.