LOW-COST FIELD-STRENGTH UNIT USES SIMPLE BUFFER.

An inexpensive device for measuring field strength at frequencies from 150 kHz to 30 MHz is described. The device feeds the output of a standard flat-frequency-response antenna into a tuned voltmeter via a buffer amplifier. In the field-strength measurement, a vertical antenna that is effectively two meters long provides an output, measured in microvolts, numerically equal to twice the ambient field strength (measured in microwvolts per meter). This voltage cannot be applied directly to the tuned voltmeter because of the very high source impedance of the antenna at low frequencies. Instead, an impedance converter is used between the antenna and the tuned voltmeter (50-ohm or 75-ohm rfi receiver or spectrum analyzer). The 6-decibel loss resulting from the converter-to-voltmeter impedance match cancels the 6-db gain produced by using an antenna two meters long; therefore, the voltage applied to the tuned voltmeter is numerically equal to the field strength, permitting direct measurement without any corrections for frequency or for antenna gain. The heart of the device is a unity-gain buffer amplifier. The buffer uses a field-effect transistor and a p-n-p transistor connected to provide 100% negative feedback; the resulting stage has high input resistance, near-unity gain, and a frequency response that is flat to 85 MHz.