# Help understanding transmission loss of trivial circuit

I was trying to DIY a VHF/UHF diplexer, and got some weird readings from the VNA.
By debugging I reached this trivial circuit:

Which has the following frequency response:

I'm trying to figure out where the parasitic elements come from and why they are of such magnitude that I get -9dB in UHF. Any help or pointers would be awesome.

EDIT: To be more precise, I've calculated the stray inductance at around 60nH and capacitance at less than 1pF for this specific scenario (wire above ground plane). What makes me uncomfortable is that the simulated cutoff is earlier and less aggressive than what I measure.

Stray inductance calculations don't work when the circuit is a good fraction of a wavelength, but if you like, 60 nH is 170 $$\Omega$$ at 450 MHz, which is a lot more than 50 $$\Omega$$.