What is an Antenna Meter?

A whip antenna is a flexible vertical rod antenna, and its length is generally 1/4 or 1/2 wavelength. Car antennas are generally 4 meters or 6 meters, and backpack type is 2.4 meters.

Whip antenna is also called "whip antenna", a simple

An equivalent diagram of a whip antenna can be represented in FIG. 1. It can be seen from the equivalent diagram that the antenna is actually an LC loop. Different from the general LC circuit, the inductance and capacitance of the antenna are distributed in various parts of the antenna, and the general LC circuit inductance and capacitance are concentrated, so the antenna is also called an open LC circuit. When the high-frequency voltage is connected to the antenna, high-frequency current flows in the antenna, and an alternating electric and magnetic field will be generated around the antenna. Since the antenna is an open-circuit LC circuit, the change of the electromagnetic field is exposed to space. The alternating changes with the magnetic field will propagate outward from near to far, forming electromagnetic waves.

Like the general LC resonance circuit, the antenna also has its natural resonance frequency. Resonant frequency wavelength of whip antenna

It is equal to 4 times the antenna length L, that is, = 4L. In actual use, the length of the antenna must be equal to 1/4 of the wavelength of the transmitting frequency in order to obtain the maximum power from the output stage and also play a good filtering role.

Where

Is the wavelength;

for

In order for the antenna to obtain the maximum power from the output stage and have a good filtering effect, the antenna must be debugged. The debugging work is similar to the output stage of the power amplifier, which is divided into "tune" and "adjust". "Tuning" is to make the antenna loop resonate at the transmitting frequency. Its means is to adjust the length of the antenna or add a coil to achieve the purpose. "Adjustment" refers to adjusting the coupling degree between the output stage LC loop and the antenna loop so that the equivalent resonance resistance of the antenna and the critical load resistance of the output stage

Match.

The whip antenna is very sensitive to changes in the surrounding environment, such as the different attitudes of the operator holding the transmitter, the height of the transmitter from the ground, and the difference between the antenna's adjacent objects (such as someone or nobody), etc., will cause the antenna to be equivalent The change in impedance causes detuning of the antenna loop and reduces the transmit power. Therefore, when debugging the antenna, it is best to be as close to the use environment as possible to get close to the actual effect. When debugging, at least two indicators should be monitored to observe the debugging effect. One is to insert an ammeter in the power circuit to observe the collector current of the transmitting tube, and the other is to place a field strength meter near the antenna to observe the emission. Intensity (only for relative comparison). The specific debugging steps are as follows.

(1) First reduce the coupling between the antenna and the output stage LC loop. The specific method is to increase the distance between the primary and secondary sides of the LC loop.

(2) the length of the word antenna (if there is a sense, pull the antenna out the longest, adjust the inductance), observe the changes in the field strength meter and the ammeter indication. With the resonance of the antenna, the ammeter indication should increase, and the field strength meter indicates the peak point. At this time, the antenna length (or the inductance of the induction coil) is considered to be appropriate.

(3) Increase the coupling between the antenna and the output LC circuit of the power amplifier stage (move the primary stage distance of the LC circuit closer). As the coupling gradually increases, the power amplifier stage moves from an overvoltage state to a critical state. The indication increases again until a peak is indicated by the field strength meter. If you continue to increase the coupling at this time, the radiated power will decrease, and the field strength meter will not increase but decrease.

Before the sensing antenna is connected to the circuit for debugging, if possible, the frequency sweeper can be used for preliminary debugging. The output signal of the frequency sweeper is connected to the bottom of the antenna through a resistance of about 100, and the Y-axis input of the frequency sweeper is also connected to the bottom of the antenna through the detection probe. Resonate the antenna to the transmitter operating frequency. From the display of the frequency sweeper, the deepest part of the curve is the frequency of the antenna resonance. ^[2]

figure 2