The narrowing of the band only works on AM. On FM that button only switches between Mono and Stereo.
This evening I tried another radio with a detachable aerial, a Roberts Stream 83i Internet radio, which also does FM and DAB. Detaching its antenna turned out to be an all-evening task, so for anyone planning on connecting an aerial to this radio (or I'm sure any other Roberts radio), be sure to have a variety of tools handy.
After some initial frustration getting it off, I spent quite a while searching online and mainly got unanswered threads on how to detach it, other than a useful clue that the telescopic aerial has an an F-connector. As I really wanted to try it with an external aerial to compare with my Tecsun, I started trying various methods.
Long needle pliers couldn't get a grip on it, so I tried a socket 11mm wrench which F-connectors generally use. It wasn't deep enough. So I tried what I thought was a 12mm deep socket wrench, but turned out my brother often doesn't put things back where they belong and ended up loosening the entire aerial socket with a 13mm deep socket! :doh: At that point I thought i ruined the aerial socket as while the proper 12mm deep socket does grip the f-connector, the whole socket still turned, making it impossible to remove, so I thought I ripped it off the circuit board.
So the next task I tried was take the entire back off the radio, i.e. all the screws marked with an arrow. It turns out that the aerial socket is actually just a double-ended F-plug joiner. Basically, the internal wire F-connector ended up unscrewing and detached. With that exposed, I was able to grip it with a 11mm socket and use the 13mm deep socket at the other side to screw the inner nut tight, then use the 12mm deep socket to finally unscrew the F-connector of the aerial.
I used a VHF TV aerial as a temporary FM aerial. Basically, to turn it into a dipole, both telescopic antennas need to be extended to about 73cm (for centre of band) and then aimed in opposite directions to each other. I then turned it so the entire aerial was vertical and tied it to a camera tripod, again as a temporary set up. I used an F-connector to coax adapter to attach it and saw a nice boost on the Roberts radio. The BBC stations came in weak, but listenable, compared to nothing at all with the Roberts built-in aerial.
I had to order a 3.5mm to F-Plug adapter for the Tecsun as it has a 3.5mm aerial jack. So once it arrives, I can do a good comparison of the two.
One other thing I did discover is that some LED bulbs cause terrible interference on the weaker FM stations when using either radio's internal aerial while inside the house. The LED bulb in my bedside lamp appears to have no affect, so one workaround is to turn off the main room light.
Another more serious shortwave interference problem I came across is something my ham radio friend told me about years ago - Homeplug units are an amateur radio nightmare, i.e. those units that run Ethernet over the mains electricity. Sure enough, the pair in my house obliterate the shortwave band to the point where I only pick up 5 stations across the entire band! When I head outside away from the house, the shortwave band becomes pretty clear and can easily tune over 100 stations. The homeplug units don't appear to affect Medium Wave or FM, unlike the LED bulbs (and most CFLs).