Actually the 880 has a pretty decent reputation, given that it's a small portable radio - with multiple choices of bandwidth for selectivity (rare in radios like this), and at least in some units with certain firmware, things like sync detection and a lot more.
According to the Universal web site, the 880 comes with its own wire antenna. That's a reasonable place to start - but if you want to go longer, there are many good outdoor designs you can build yourself. Active indoor loops (such as the Kaito model) would be a decent indoor choice, since winter is fast approaching. You don't want to be outdoors doing soldering in a wind storm - been there, done that...
The problem with these coax-fed antennas is one of matching - the jack on most portables is expecting a wire input (which is, more often than not, high impedance) where a coax input is low impedance. Bottom line here is that a coax fed antenna may not work well with portables such as these - but there's no harm in trying it. You won't damage the radio in the slightest.
If it were me, I would get a spare 3.5mm phono jack, solder a short length of thin RG174U cable and terminate it with a SO-239 or similar. Keep the original antenna, and now you have some flexibility in which antenna you want to use. In addition it relieves potential stress issues on that little jack trying to feed coax, which is likely to be too heavy...
And of course, I would add the
first timers project to protect the front end of the radio from static charges and the like (by the way, links are always shown in blue).
I would toss a couple of questions around in the 880's
Yahoo group and see what others are doing. You're in Europe, where signal levels tend to be higher, so some awareness of the length of the antenna would be in order. Otherwise overloading - which is often evidenced by having stations show up where they don't belong - is a possibility.
Mike