Measurements of Insertion VSWR of UHF series connectors consistently show increasing Insertion VSWR with frequency, an issue that often impacts measurement accuracy.
My own article Exploiting your antenna analyser #12 is but one of many.
Measurements consistently hint that the defect is that the characteristic impedance is typically somewhere between 30 and 40Ω.
Above is a dimensioned drawing from Amphenol (https://www.amphenolrf.com/connectors/uhf.html).
Let’s focus on the right hand part, and in particular let us try to estimate the characteristic impedance (Zo) of the transmission line section formed by the outer surface of the centre female pin and the inner surface of the body with some solid dielectric occupying most of that space. This is the longest unavoidable transmission line section contributed by a connector pair with good (short circumferential) coaxial cable terminations on both connectors.
If we assume that the dielectric has a permittivity of 2.0, and dimensions scaled from the diagram, we can calculate Zo to be 30Ω.
RF Coaxial Transmission Line Loss Calculator
|Inner diameter||0.00539 m|
|Outer diameter||0.01098 m|
|Length||5.10 °, 0.014 λ, 0.0100 m, 4.718e+1 ps|
|Line Loss (matched)||5.73e-4 dB|
So, even with the best transition from cable to male pin, and female pin to cable there is an unavoidable 10mm of 30Ω line.
The Insertion VSWR of 10mm of Zo=30Ω, vf=0.707 line at the bottom of UHF (300MHz) is 1.10 , and 1.36 at the top end of UHF (1000MHz).
Observations that UHF series connectors have non-ideal Insertion VSWR, and that measurements hint that they act like short sections of transmission line with Zo between 30 and 40Ω are partly explained by calculation of the expected Zo based on a dimensioned drawing of the female connector.