I'd made up a cheap and cheerful antenna adapter using a spare connector for the RTLSDR dongle, but I hadn't been overwhelmed with its performance. I suspect it was a bit lossy!
When I was reading up on something the other day, I discovered that the antenna socket on the RTLSDR dongle is an MCX connector, of which, I confess, I had not heard previously. In the course of my investigations, I discovered that MCX male to SO-239 adapters are available readily (and pretty reasonably priced) on Amazon
The adapter arrived yesterday and I hooked the RTLSDR up to the 50/144/432MHz collinear. I ran up SDR# and it worked well. Happily there was a 50MHz Es opening going on at the time and I heard several LA stations on CW and SSB. On 144 and 432MHz, the performance was definitely better than my rather lossy arrangement. Airband too was working much better than when I'd tried it before and several transmissions were easily identified on the display.
A small but worthwhile investment.