A single diode won't do it unless you know the pulse is always the same polarity which is highly unlikely in RF systems. The shottky diode does not 'open' as you mention. It will be reverse biased or forward biased depending on the polarity of the incoming signal. The diode will withstand some reverse bias and breakdown at some voltage from 20 to 400 depending on the diode specs. That 400 volt signal is being poked straight into your radio front end so you must eliminate this reverse breakdown by using two diodes where one protects the other and the voltage across the pair is never more than the forward voltage drop of the diode - about 250 mV

Your 'single wire' antenna has both an earth part (the outer of the SMA) and the active line (the centre pin on the SMA). Two shottky diodes in "anti-parallel" means one has the anode to the antenna with cathode to earth and the other is the reverse of that. The forward voltage drop of the shottky is about 250 mVolts so no matter what the polarity of the offending spike is, the diodes clamp the voltage to no more than 250 mV. RF, which is typically below 10 mV rides through unaffected.

HTH
BrianT