Transmitter third harmonic received?

[Mike Watterson]

Simple frequency changer using a single OC170 transistor.

Which would easily internally generate a 3rd harmonic with a strong signal. Any single transistor mixer/oscillator is much less trustworthy than a DK96 or similar Octode which is much less trustworthy than a triode-hexode. It's why some better transistor sets had separate mixers and oscillators.

The question is how strong is the local station and if there is a 3rd harmonic, how many dB less?

I think the question can't easily be answered without professional gear, maybe a real spectrum analyser. I'd not trust the results of any cheap SDR as they have notoriously poor RF selectivity and dynamic range (less than 20 dB).

Somewhere on Ofcom there will be a spec for spurious emissions of a MW transmitter. I always thought Lisnagarvey is very close to Lisburn and most MW sets behaved strangely near it.