Again we get tools and components and we get down to work. This time we propose to make a very simple and practical field meter at the same time. Lovers of electronic enjoy a lot with this assembly, who are encouraged?
All lovers of electronic usually start in this world of kits modules and assemblies precisely the most complex, it ie those that use high frequencies. Undoubtedly the construction of a receiver or a radio are among the most practical and entertaining productions.
In the following article we are going to explain how to make a field meter . But first, what is it ? It is a small instrument that allows us to know if a station is broadcasting and with what power does . The field meter can also be used to measure signals of radio frequency voltages at different stages of a station, even in a receptor to which we are injecting a signal via an external RF generator.
In Figure nº1 you can see the electronic diagram of this simple field meter.
The first element that can be seen is the antenna , which according to our needs, can be anything from a piece of wire conductor to a dipole, folded dipole, omnidirectional, etc.
Following the antenna we found a tuned circuit formed by the coil L1 and the variable capacitor CV1 .
The inductor L1 has a value of 0.22 uH (micro Henrys). If you do not find this type of coil in an electronic store, you can be constructed by winding three turns of wire on a diameter of 10 mm. This coil, with CV1 , to tune our meter in the field of VHF, ie any frequency between 30 and 300 Mhz.
The captured signal by the tuned circuit is rectified and doubled in tension by the two diodes D1 and D2 . These diodes are small signal and germanium, so can not be replaced by any other. The rectified signal is filtered by the capacitor C2 , appearing in extremes of potentiometer POT1 a DC voltage. This voltage depends on the power and distance of the transmitter or oscillator we are adjusting.
For very powerful or nearby stations, we have to turn the potentiometer POT1 to prevent the meter is out of scale. If our meter is used in the measurement and calibration of small oscillators will have to mount a very sensitive meter 50 to 100uA , for our circuit is completely passive (no power required), and therefore all the energy available for deflection meter needle comes only from the radio frequency signal captured by the antenna.
Installation and adjustment
For mounting the circuit will need a printed circuit board type islets .
The prototype boards of breadboard type, and printed circuit lines are fully deprecated circuits radio frequency, it has a lot of inductive and capacitive couplings between its conductive lines, all of which prevent proper operation at high frequency .
As assembly tools need only a welder 30w and fine point, some 1 mm and tin cutting pliers. The diodes D1 and D2 and ensuring be welded quickly be heated as little as possible, since they are quite fragile and are easily damaged by heat.
The adjustment gauge is very simple, just place it in the vicinity of an oscillator or transmitter and turn the capacitor CV1 through a plastic screwdriver (trimer). The rotation must be done slowly to be the maximum deviation of the gauge, then the circuit is tuned.
If the meter needle is outside the full scale, they can remove the meter station or act on potentiometer PT1, decreasing its resistance.
Being a totally passive circuit, which needs no power supply, it can be mounted inside a small plastic box , and place the meter and frequency adjustment and level in front of it.
We remind you that if I am ‘newbies’ in the electronics sector will come in handy the electronics course to learn and practice.
And if you want to share with us and our readers some of your projects to be published in the blog, do not hesitate to send it to the following email: firstname.lastname@example.org .