A field strength meter is an electronic device which is used for detecting and measuring the RF radiation generated from any RF transmitter circuit.
As we all know that transmitter circuits invariably use an inductor/capacitor based LC resonating circuit, also called the tank circuit, which oscillates at a specified resonating frequency to transmit the frequency in the air with an optimum power.
A field strength meter picks up this frequency radiation from the transmitters LC antenna network, and displays its strength or power through the attached meter.
Using an FET
The first field strength meter circuit below can work with any frequency as high as 250 MHz, or perhaps even more if needed. Radio frequency energy is sensed by a short whip, rod, telescopic or other antenna, and the signal is rectified by diode D1 which gives a positive voltage for the FET gate across R1.
Because this FET is solely used as a DC amplifier, a 2N3819 or some other general-purpose transistor will suffice. The "Set Zero" potentiometer can be anywhere between 1k and 10k. When there is no RF signal present, the gate/source potential can be adjusted such that the meter only indicates a tiny current. In the presence of an RF field this current slowly grows in proportion to the intensity of the RF signal, which is indicated on the meter.
A 100uA metre may be used for increased sensitivity. An ammeter with a lesser sensitivity, for example like 250uA, 500uA, or 1mA, can also be utilized and might provide you with adequate information in most cases. A VHF choke RFC can be used if the field strength metre is exclusively used with VHF. On the other hand, a short wave RFC choke will be most appropriate for regular use with lower frequencies.
Another Simple Field Strength Meter Design using a BJT
Another simple, and tested field strength meter circuit that allows model RC fliers to confirm that their remote control transmitter is indeed transmitting. Any concerns about whether a problem is in the receiver or transmitter are immediately cleared up. The circuit's single active component is a transistor which serves like a regulated resistance in one of the bridge's arms of the meter.
The antenna wire which is a telescopic type antenna is attached to the transistor's base. As the HF voltage at the base of the antenna rises, the transistor is driven out of balance, causing the bridge to collapse. The current subsequently passes via R2, the mA meter, and the transistor's collector-emitter junction, indicating the field strength on the meter. Before turning on the transmitter for the testing, make sure that the meter needle us carefully adjusted to zero mark, using P1.
Transmitter RF Output Power Equation
The electric field strength generated by a transmitter with an isotropic radiator could be easily computed in an idealized free space. The following formula works well for determining the field strength emitted from a transmitter LC circuit:
E ≈ √30 x P / d
Here, the electric field strength, E, is measured in volts per metre.
The transmitter power output in watts is denoted by P.
d represents the distance in metres from the radiator.
The parameter √30 represents the approximate value of √Z0/4π
where Z0 denotes impedance value of the free space, which is equal to 119.9169832 πΩ
The spacing between the transmitter and the receiver is clearly inversely related to the intensity of the electric field. This relationship, unfortunately, is unsuitable for estimating the field strength generated by ground transmitters, because reflections and attenuation induced by structures in the vicinity of the transmitter or receiver can heavily impact the electrical field strength.