Tutorial on making a simple field strength meter

Field strength meters play a very important role in the installation of antennas and the testing of wireless transmitters, but professional field strength meters have complex circuits and are expensive. They are beyond the reach of ordinary enthusiasts. The author has designed a simple field strength meter (circuit shown in the figure), which can be used for the debugging of general wireless transmitting and receiving equipment.

Generally, most professional field strength meter circuits use a special wide-band amplifier to amplify and then detect, and then send it to the indicating instrument through a series of circuits. This method has a high indication performance, but the circuit structure and adjustment are very complicated. The simple field strength meter designed by the author adopts the scheme of first detecting and then performing DC amplification. The circuit is simple and its principle is as follows:

The circuit composed of antenna TX, diode D1 and capacitor C1 receives the high-frequency signal, and the DC voltage generated after detection by D2 and C2 is amplified by BG1 and BG2, which drives the LED to light up and adjust W to control the base voltage of BG1, thereby changing the light intensity of the LED to achieve the purpose of adjusting the sensitivity.

Although the sensitivity of the detector (the ratio of the DC voltage output after detection to the high-frequency AC voltage input to the diode) decreases rapidly as the amplitude of the high-frequency voltage decreases, for a simple field strength meter, it is still possible to perform detection by selecting a detection diode with good frequency characteristics and adjusting W. The upper limit of the frequency test of this field strength meter depends on the operating frequency of diodes D1 and D2.

When making it, just select the components as shown in Figure 1. W can be a linear potentiometer with a power switch, which makes it easy to draw a uniform attenuation scale on the panel. The power supply can be a 6V laminated battery. The antenna can be a small telescopic antenna of a pocket radio. After the production is completed, the circuit should be placed in a shielding cover to avoid interference from external electric fields.

When using it, pull out the antenna, get close to the appliance under test, unscrew the W button, and adjust it left and right, while observing the luminous intensity of the I.ED. Combined with the scale number of the W button at this time, you can estimate whether the appliance under test is working and the magnitude of the field strength. For example, whether a cordless phone is working, whether the remote control handle of a wireless remote control electric toy is transmitting radio waves normally, etc. It works well, is simple and practical.