# 40 GHz spectrum analyzer with N-type coax input?

We have an old Rohde & Schwarz FSEK 30 that is advertised as having a range from 20 Hz to 40 GHz, with an N-type input connector. My conundrum is that a type-N connector with air dielectric should have a maximum frequency of approximately 19 GHz before the primary TEM-mode propagation is joined by the TE11-mode. From MIL-STD-348A the inner and outer coax diameters are 0.12 and 0.2759 inches, respectively. According to Microwaves101 the cutoff wavelength for the TE11-mode in an air dielectric coax is $$\lambda_c = \pi\frac{D+d}{2}$$ where D is the outer diameter in meters and d in the inner diameter in meters.

This disconnect between the theoretical frequency limit of an N-type and the stated range of the instrument has me doubting the validity of measurements made above 19 GHz. Can anyone shed light on this mystery?

• Maybe the manufacturer can... – PlasmaHH Jan 27 '15 at 15:50
• You're probably meant to use a different cable with adapter immediately in front of the instrument's input at high frequencies. Also check the fine print to see what attenuation it is really promising at 40 GHz. – Olin Lathrop Jan 27 '15 at 16:08
• Is the coax air core? Maybe look up some RF cable manufacturer's and see what they spec for cable with N-type connector – George Herold Jan 27 '15 at 16:09