GB782450A - Improvements relating to microwave power measuring systems - Google Patents

Abstract

782,450. Microwave impedance networks. POLYTECHNIC INSTITUTE OF BROOKLYN. March 7, 1955, No. 6661/55. Glass 40 (8). [Also in Group XXXVI] Apparatus measuring microwave power comprising a main coaxial line connecting a power source to a load, a branch coaxial line connected to the main line, a probe section of reduced diameter carried by the centre conductor of the branch line to extend through an aperture in the outer conductor of the main line supported by a linear section of the centre conductor of enlarged cross-sectional area having a length small compared with the working wavelength and a diameter overlapping the aperture in the main line, and joined to the centre conductor by a short section of smaller diameter than the centre conductor; whereby the probe has coupling capacitance to the centre conductor and a shunt capacitance to the outer conductor giving an attenuation decreasing with frequency and the series attenuation of the branch line increases with frequency so that the output of the branch line applied to a bolometric power indicator means is independent of frequency. Fig. 1 shows a main coaxial transmission line 1 connected at la to a microwave power source and at 1b to a high power load 2 of broad frequency band width, which may comprise a coaxial section terminated in a short-circuiting plug 3 and filled immediately forward of the latter by a lossy plug 4 of tapered cross-section formed of plaster of Paris or like material mixed with finely-divided carbon. A probe line 5 extending at right-angles to the main line has its inner conductor 5a (Fig. 2) extended through a neck and a thin conducting disc 6a to a probe 6, passing through aperture 1c of the outer wall of the main line and coupled by its capacitance to the inner conductor thereof. Disc 6a and the neck are enclosed in a two-part dielectric shield 7, 7a which locates the probe and increases the capacitance between the disc and the outer wall. A buffer-equalizer attenuator element 8, which may comprise a high resistance metallized glass tube of the kind described in Specification 643,494, [Group II], in series with the inner conductor is connected between the probe and a known bolometric power measuring device 9. The capacitances constitute with the probe inductance a potentiometer (Fig. 3, not shown) whereby a known fraction of the power in the main line is tapped off for measurement by the bolometer while the buffer attenuator connects the high impedance mismatch between the probe and the line, and also compensates for variations of probe attenuation with frequency, so that the power in the main line is measurable over a wide frequency band. Design formulµ are given.