The high speed switching and square waveforms inside a switching power
supply can often interfere with a radio.
There are several ways to fix this.
Put the switching PSU inside a sealed metal box. I have made some out of sheet copper and soldered it together. This vastly reduces the radiated noise.
Keep the PSU physically as far from the radio as possible.
Filter the leads.
There are toroids in a junked computer PSU that can be used. Take the output
leads, both of them, and wrap them a few times around a toroid. This will
stop the noise travelling along them both. There is a double wound filter
on the mains input to a computer PSU that is used to stop noise travelling
out through the mains cord. Use this on the battery leads (both of them)
and on the HT leads. The computer PSU also has several small filter chokes
smoothing its DC output. These consist of some turns on ferrite formers.
Use these for DC filtering.
The waveforms present at the primary and secondary of the transformer are not perfect square waves. They are nosiy waveforms with lots of ringing. Place a series Resistor and Capacitor across both primary and secondary to dampen this nosie and improve efficiency. On the high voltage side, use 100 ohms 2 watt resistor in series with a 102P 2kv capacitor. On the low voltage side use 15 ohm 1/2 watt resistor in series with a 0.01 200v capacitor.