For those who want to use their 72kHz TX antenna as a 136kHz RX antenna
there might be an easy solution that does not require a 'many kV' relais :

Assume your antenna has 500pF capacitance. This means you need a 9.8mH coil
to bring it to resonance at 72kHz. On 137kHz the 9.8mH coils has an
impedance of 8.44kOhm, the 500pF antenna has an impedance og 2.32kOhm. So
at 137kHz the antenna has an impedance of 6.12kOm (inductive) and can be
brought to resonance by a parallel capacitor with the same impedance = 190pF.
So you can bring your 72kHz antenna to resonance on 137kHz using a parallel
capacitor (from the TX side of the loadingcoil to ground) in the 200pF
range. Fine-tuning can be done using a variable capacitor. As this
acapcitor is at the low-voltage side of the coil it can be switched with an
ordonary relais.

73, Rik  ON7YD