Antenna for RadioSonde RX

Last week I built my first prototype. Since last december I started tracking radiosondes in the 400 MHz band. I use the Diamond X50 antenna on the roof, connected to a cheap RTL-SDR dongle. I installed Ubuntu on the laptop and RadioSonde Auto-RX. Already 298 sondes were tracked, but I want a dedicated antenna tuned to this band. The Diamond antenna is a dual band antenna for ham radio use on 2m and 70cm. It works, but it could be better.

The goal is to build a 2x 5/8 wave antenna. I have no experience in building and tuning those antennas. But I think with the help of the NanoVNA it can be done. So let’s start with a normal single 5/8 first.

Some math… Online calculators enough on the net.
The radiator should be about 44,1 cm (5/8), the radials (1/4) about 17,7 cm. The impedance matching was made by trial and error, 2,5 or 3 turns and tapped at 1,5 turn.
The initial resonance was at 391 MHz, antenna a bit too long, but playing with the tap gave me a better result. Pictures say more than words.

I managed to get a decent result, but no real world tests yet. No coax available and we are currently in Corona Lockdown.

Some resources :