Balun for long wire antenna

I would like to do some experiments with an end-fed long wire antenna (appr. 20 m. length). One needs a balun for impedance matching. For a longwire antenne a 1:9 balun is recommended.
I found a nice design on the website of PA3I: 
https://pa3i.jouwweb.nl/antenne

I have collected the components:

The ferrite core has Amidon partnumber FT157-2.
More info: http://www.amidoncorp.com/t157-2/

The box is IP55 water resistant.

Assembled it looks like this:

With 20 meter longwire attached to the balun, following first results on various frequencies (all FT-8 signals) popped up;