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RLC parallel circuit

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Hello. I am planning on using a design for a field strength meter from the electro-tech:

https://www.electro-tech-online.com/threads/simple-field-strength-meter.58/

I was wondering what effect the resistor has on the parallel RLC circuit, and why it is used. Also is it necessary.

Thanks very much in advance.

This is not an RLC resonant circuit. The resistor provides a load whereby the diode can conduct current. The capacitor holds the peak level of the voltage across the resistor between cycles of the RF waveform. The coil inductance combined with the antenna form the tuned part of the detector. The resistor value will affect the sensitivity of the detector. For a field strength meter, I would get rid of the resistor altogether. The capacitor, alone, is sufficient to conduct current through the diode. The resistor is used in AM envelope detectors to allow the voltage across the capacitor to vary with the amplitude modulation.
 
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Okay. Thanks for that explanantion. I do not understand the relationship between the antenna and the inductor though. How do they form the tuned part of the circuit. And do the inductor and capacitor also share a relationship in the circuit?
 
The inductor (and its parasitic capacitance and resistance) combined with the capacitance and inductance of the antenna form a crude tuned circuit. Obviously, the length, shape, and position of the antenna will affect the impedance of the antenna, and the resonant frequency of the tuned circuit. This form of tuning is "iffy" at best, but if adjusted properly (through a good deal of experimentation) does offer some measure of selectivity from other surrounding EM fields at different frequencies.

This detector circuit is not very sensitive to begin with, so I question the need for the inductor and would go without it (and the resistor). Due to its poor sensitivity, it is not likely you will find use for the detector anywhere other than where the field strength from the source you are measuring is much greater than that from any other source. The inductor will only tend to attenuate the signal at the antenna connection and this detector can use all the sensitivity you can give it.

Don't plan on making accurate measurements of field strength with this circuit, but rather, relative measurements between various locations in the field.

The tuned portion of the circuit does not have much relationship to the rest of the circuit. Ideally, you would use a good antenna, which is already cut for the frequency of interest (providing selectivity), with a known real impedance and replace the inductor with a resistor to match that impedance.
 
I am going to use this Field strength meter to test three antennas connected to a vhf 5 watt hand-held.

1/4 Wave Vertical

3/4 Wave Verticla

Directional Yagi Vertical

These are all self-built. I am going to measure field strength every 30 degrees around the transmitter at a distance of at least 3 wavelengths on an oval at school. The differences in field strength will be plotted on an x,y coordinates and then plotted on polar coordinates to show the directivity of the beams.

Do you have any suggestions. Also this is a year 12 physics project so the equipment can't be proffesional, and crudeness and anomalies are good because this provides alot to talk about (recommended by teacher).
 
Let me first say that I am very glad to see a high schooler doing such work! You have my greatest encouragement.

The detector should work fine for your application. I have used a very similar device and have obtained usable readings at distances father than what you plan to do. My suggestion is to do one set of measurements as far away from the test antennas as possible, yet still obtain a reading from the detector, in a circle around the test antenna. Three wavelengths should get you into the far field, but just into it (relatively speaking). It may be interesting to see the difference in readings between three wavelengths and as far out as measurable.

I would think you would do your measurements in a circle, not an oval, around the test antenna so that you know that your measurements are not affected by a free space loss.
 
Thankyou :). Your idea of measuring from far away, as well as the three wavelength vicinity is a very good idea. That way I can can compare the results and write a more comprehensive discussion. I am just worried about the mathematics of this experiment still. I need to include fairly decent amounts of maths, so I am hoping that the antenna theory will be sufficient.
 
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