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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?

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    \$\begingroup\$ Maybe the manufacturer can... \$\endgroup\$ – PlasmaHH Jan 27 '15 at 15:50
  • \$\begingroup\$ 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. \$\endgroup\$ – Olin Lathrop Jan 27 '15 at 16:08
  • \$\begingroup\$ Is the coax air core? Maybe look up some RF cable manufacturer's and see what they spec for cable with N-type connector \$\endgroup\$ – George Herold Jan 27 '15 at 16:09
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The R&S instruments are equipped with female test port adapters.

Edit: archived pdf

enter image description here

You can replace the N type with a more suitable type for higher frequencies.

enter image description here

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  • \$\begingroup\$ Link is bad.... \$\endgroup\$ – rsaxvc Dec 29 '18 at 16:11
  • \$\begingroup\$ @rsaxvc archived link added, original retained \$\endgroup\$ – Spehro Pefhany Dec 29 '18 at 17:06

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