Sweptfrequency Swr Measurement With A Slotted Line

The equipment setup for the new SWR-measuring technique is illustrated in Fig. 1. The sweep oscillator output is connected to the input of the slotted-line sweep adapter, which is essentially a short piece of slotted line with a stationary detector probe. The output of the adapter's probe is connected to the ALC input of the sweep oscillator, forming a power-leveling feedback loop.

3 G. V. Sorger and B. 0. Weinschel, 'Swept Frequency High Resolution VSWR Measuring System.' Weinschel Engineering Company Internal Report 90-117, 723-3/66, March, 1966.

1 Hewlett-Packard Application Note 84, 'Swept SWR Measurement in Coax.'

The slotted line is placed between the slotted-line sweep adapter and the device whose SWR is being measured, and the output of the detector probe of the slotted line goes to the vertical input of the oscilloscope. The horizontal input of the oscilloscope is taken from the sweep output of the sweep oscillator.

To permit the slotted-line probe output to be displayed on the oscilloscope with sensitivities as high as 0.5 dB/cm, the sweep-oscillator output must be held reasonably constant as the frequency varies. The function of the slotted-line sweep adapter is to level the oscillator power output in such a way that the voltage output of the slotted-line probe remains constant with frequency, except for the variations caused by the SWR being measured. The adapter consists of a short length of slotted line, a wrell-matched

6-dB attenuator, and two matched detector probes, one for the adapter and one for the slotted line. Matching the two probes makes the frequency response of the adapter probe, which samples the oscillator power, exactly equal to the frequency response of the slotted-line probe. Thus the oscillator power is adjusted to keep the output of the slotted-line probe constant with frequency. The 6-dB attenuator improves the frequency response, probe isolation, and impedance match of the adapter.

The slotted line shown in Fig. 1 is a new precision 'slab-type' line with very well matched transitions at each end. Connectors can be either the precision 7-mm type or improved type N connectors which also operate up to 18 GHz but have slightly higher SWR. With the

7-mm connectors, the residual SWR of the slotted line varies from 1.02 at 2 GHz to 1.04 at 18 GHz. With

-hp- 141A Oscilloscope

-hp- 1416A Swept Frequency Indicator

-hp- 141A Oscilloscope

-hp- 1416A Swept Frequency Indicator

-hp- 448A Slotted Line Sweep Adapter

Slotted Line Probe Tuner

Fig. 1. Equipment setup for swept-frequency SWR measurements described in text. Method is highly accurate from 2 GHz to 18 GHz.

-hp- 448A Slotted Line Sweep Adapter

Fig. 1. Equipment setup for swept-frequency SWR measurements described in text. Method is highly accurate from 2 GHz to 18 GHz.

the type N connectors, SWR is 1.03 at 2 GHz and 1.06 at 18 GHz.

Although any oscilloscope can be used for displaying the slotted-line output, the variable persistence and storage feature of the one shown in Fig. 1 is particularly useful because it permits the unknown SWR to be read directly from the display. If a non-storage oscilloscope is used, the SWR data has to be photographed, using a time exposure. The swept-frequency-indicator plug-in is also a great convenience because it has a logarithmic vertical amplifier which makes it possible to read SWR directly in clB.

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