Choeje Morchen (?) John,
"wait and see": an external antenna, situated high!!! and its signals beeing transfered by a low loss cable!!! will improve your "seeing conditions" may be even more than you expect.
"Nether"-lands, by its "name", are optimized for signals travelling according to their "line of sight" behavior, which proves for the 1090 MHz transponder's frequency.
I myself am located close to Colognes "EDDK", and WIMOS GP1090 allows me to track the atlantic traffic departing out of EDDF until it reaches the sea, after having passed your country between about Rotterdam and Amsterdam, ideally via the SPY VOR, which gives me the best range. My eastern directions are shaded by hills, southwest beeing the best in terms of wave propagation, almost seeing aircraft in the vicinity of Paris, more than about 200 NM away. If you want, I can send you a screenhot. (I am going to release my e-mail adress in the meantime, so you may contact me) Unfortunatly the “shots” originate from BaseStation rather than RadarBox, (awfull sorry for that, AirNav) but finally, it’s "almost the same".(Again my apologies AirNav and John)
Get your antenna out as soon as possible John !!!.
Then we'll see.
Regards
Karl.