The "UHF" connector was originally designed for video applications and is certainly not considered suitable for use at UHF frequencies.  The centre contact was designed to be compatible with a 4mm "banana plug"  and as such, this 'over-size' connection results in a characteristic impedance of around 20~30 ohms for the connector.

While the original design specified the connector thread as 5/8" x 24 tpi, there are "metric" versions of the connector produced with M16 x 1 threads (16mm dia with a 1mm thread pitch.  While 16mm is very close to 5/8", the 1mm pitch is approximately equivalent to 25.4 tpi and damage to the threads will result if mixed type connectors are mated.