VHF receiver (monitor) Harry Litall. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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It seems that many QRP devices are usually transmitters, but I believe receivers are also quite important. In this project you can make a SIMPLE VHF receiver, the only active component of which is a diode. Yes, it is a detector receiver!

This project was originally intended as a transmitter monitor for my VHF station, but one day I found that I could listen to fairly powerful stations on this receiver with very little effort. Here you can see the construction of one of the prototypes I built for the 150 - 250 MHz band:

The drawings show the mechanical design of the resonator, which can be made using household "garbage". A coffee can, milk can, or even an old galvanized dustbin can be used in this project. A 20dB commercial amplifier in the antenna will increase the sensitivity, and an audio amplifier will generate a low frequency signal. You can connect the resonator output to the "PHONO" input of a stereo amplifier. In the RF loop, the antenna input. AF - audio output to a stereo amplifier. D1 - germanium diode. I used a Hewlett Packard microwave diode.

APPROXIMATE resonator dimensions are given below. Note that the 900 MHz band is enabled, so you can also listen to cellular communications if you live close to a base station. If you get caught, don't mention my name :-).

The above is given as a START for experimentation. The length of the resonator is very critical and should be within +0 -10% of the specified values. All other dimensions can be changed by as much as +100 -50%. The input and output communication loops can be changed. If the loop is too big, then Q will decrease. If too small, then the output signal will decrease. A good compromise is 5 to 10% of the resonator length. I made my hinges from straightened copper tubing from a car's brake system. The prototype shown in the figure below uses only copper wire. In the left loop you see a microwave diode soldered in to make a detector receiver.

The resonator is a long tube that is shortened according to the table and soldered to the lid of the jar as shown in the figure. The screw is screwed into a nut soldered to the base of the can so that it enters the copper tube of the resonator. The screw must not touch the copper tube resonator. In my designs, I usually used plastic tubes from a felt-tip pen for insulation.

The cans can be anything from coffee and the like that are of suitable size and can be soldered. You can increase the length of the can by using a can opener to remove the top of one can and the bottom of the other, then solder two (or more?) cans together to form one large tube. For soldering, it is advisable to use a powerful soldering iron or preheat the cans on a gas stove.

I have built about twenty or thirty of these receivers for various bands with 100% success every time. I even took a steel dustbin up to 28 MHz, but the resonator was composite: a 1.5 meter pipe - as a resonator, into which a plastic drainpipe (insulator) was inserted. The capacitor was found to be a variable type rather than the screw type mentioned above. The receiver was able to receive FM modulation because the cavity resonator has a high quality factor, which gives a very steep slope in the frequency response. If the setting is slightly away from the carrier frequency, then FM will appear as AM and be detected by the diode. No decoupling capacitor is required because the coaxial cable for the low frequency output has more than enough capacitance.

Below is a photo from the analyzer (please excuse the fuzzy quality), at a frequency of 220 MHz and a 200 kHz horizontal section. This only shows you how steep the resonant response of the resonator is. But it must be taken into account that solder drops and irregularities on the resonator can significantly distort the shape of the frequency response.

The picture from the spectrum analyzer above probably allows you to use a similar resonator as a bandpass filter (of course, the diode is removed). The slope of the frequency response changes quite a bit, so that the reconstructed speech is quite loud and of the highest quality, if the system is slightly "detuned".

I have only offered you a description of the resonator along with pictures, and I think you can develop the idea of ​​using this design. With a little work, making everything symmetrical and neat, you can improve the slope characteristics and make them more linear. One "interesting" (but so far useless) effect is if you receive an FM signal with a 1KHz tone and the resonator is set to dead center, then the restored modulation becomes 2KHz but at a lower AF level of the signal. This effect can be used to tune the resonator to exactly (for example) 145.7375 MHz so that you can receive 145.750 (and 145.725 MHz) with the highest possible sensitivity and quality.

This design can also be used as a TX/RX filter using 2 identical IN/OUT connection loops, but you will need to experiment with the loop size. The analyzer picture shows a signal loss of about 5dB, but with great care, silvering the resonator can significantly reduce the loss.

Author: Harry Lythall, Sweden, translated by Nikolai Bolshakov, rf.atnn.ru

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