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Wideband Field Strength Meter for Radio Frequency

2018-11-14 23:49  
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The circuit was designed to measure the electric field strength in radio frequency band where it can be applied during the modulation of AM and FM signals in theVHFrange.

Field Strength – the intensity of an electric, magnetic or electromagnetic field at a particular locationVery High frequency (VHF) – the radio frequency range from 30 MHz to 300 MHzWideband – contains a transmission rate from 64 Kbps to 2 MbpsField Effect Transistor (FET) – used for amplifying weak signals by controlling the current and the shape of an electric field where the flow of current or the conductivity of material is only through a single type of semiconductor materialRadio Frequency (RF) – operates within the range of 3 Hz to 300 GHz

The first stage of the circuit consists of a metallurgical bonded germanium diode OA91 which has low forward voltage drop as an advantage compared to silicon. Because of this, it produces higher efficiency and low power loss that makes it more suitable in low level logic circuits and low signal situations like the detection of signal in the audio to FM range. In the circuit, the signal received is converted to DC and amplified to be able to measure the strength or power density of radio field. This meter was intended to operate in theVHFfrequency range of 88 to 110 MHz.

To be able to have accurate measurement on the RF range, the meter should have lower voltage to achieve higher sensitivity. The power is supplied by a 9 V battery having a current consumption of 15 mA with standard batteries. It will cause the deflection of the needle. The frequency of the signal can be determined by the inductor and the capacitor. Using a standard wire gauge of 20, the inductor is wound 4 to 6 times with air space on a quarter inch ferrite rod. The frequency can always be varied by loosening or tightening the space between the turns.

The transistor provides RF signal amplification usingFET2N3819, where the bias of theFETcircuit is modified the preset resistor with 10k ohm value. The reading of the meter should be at zero if there is no transmitter present. Failure to be on the zero level, the preset resistor can be utilized for proper adjustments.

Attached to the circuit is a small whip antenna that is typically mounted in vertical position where the other end is close to the ground plane. The RF signal received from the antenna is converted to DC after rectification by the parallel combination of diode, capacitor and 3.3M ohm resistor. Any amount of the DC voltage, even for minimal, will cause deflection on the meter while upsetting the bias of the circuit. This is favorable when observing any slight difference in signal strength. Normally, RF signals are being transmitted vertically in the electric field, so to be able to measure the full strength of the signal, the meter is recommended to be held at vertical position.

The use of the field strength meter is to measure the capability of a transmitter if it is operating at its best performance. By doing this, it can track down any bugs, faults or other hidden transmitters. It can also be used to adjust and setup citizens band radio antenna and amateur radios. It may also be used to experiment with other types of antennas and for testing the operation of remote control and transmitters in the AM and FM bands with low power. In the RF fields, it can verify the interference from devices such as fluorescents, switching supplies, and light dimmers. In home appliances, it may be used to expose the danger of radio frequency from the devices such as microwave ovens where the excessive leakage can be checked even from a distance of 10 feet away.

Source:www.zen22142.zen.co.uk/Circuits/rf/fsm.htm


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