Many thanks to SWLing Post contributor, Rolf Snijder, who writes:
I have added an external antenna capability for mediumwave on the XHData D-808.
Simply add a 27 pF capacitor on the backside of the PCB on C10 to the the ferrite antenna.
The mod causes no loss on FM or shortwave!
I also replaced the Ferrite antenna with a bigger one so I now have more performance on MW!
Longwave is not good in my radio, even with an external antenna, it is not performing well.
Thanks once again, Rolf! I must assume that the Digitech AR-1780 could be modified in the same way as the radios appear to be near identical. Perhaps someone can verify this at some point!