Cheap and nasty 50Ω SMA terminations

Above is the internals of one, it is a 51Ω 5% metal film resistor.

They often fail a DC test and tapping them gives erratic resistance readings up to hundreds of ohms, and of course they can be unreliable at RF.

They rely upon the resistor pigtail to make a spring contact with the inside of the barrel, and give that the pigtail is soft copper with little spring the contact is not very reliable.

The second major problem is that even with good contact, the VSWR is significantly worse than specification.

Above is a typical VSWR sweep to 600MHz, VSWR exceeds the specification above 230MHz and they are not really useful above 100MHz but for the crudest applications.

Why is the VSWR so bad?

Above is a resistor from one with the epoxy coating abraded away to reveal that it is a metal film resistor with a helical resistance element. It is an inductor with resistance. The Chinese copyists may have seen a design using a carbon composition resistor, or low inductance metal film resistor and substituted the cheapest thing they could procure, copied the specs and flog them to unwary buyers online.