You will read in the article
The advantage of a voltmeter with a needle meter
In RF technology, the phrase "tune to the maximum" often appears. A needle voltmeter with a diode probe is suitable for this.
Using a digital meter is often not possible, or at least not convenient. Only a meter unaffected by the VF field will do (which can be verified, for example, by locking a hand-held FM station to the meter when measuring the voltage. The data on the display must not change) and a bargraph.
Even so, the display on the needle measuring device is more convenient. And just such a device is described in the following article. It is intended for beginners in VF technology who are expanding the ham Shack o other measuring devices.
Large input impedance of the voltmeter
The connection uses a large input impedance of field-controlled transistors - FETs. The bipolar transistor connected behind it increases the gain and at the same time serves as a branch of the bridge, which has a measuring device connected diagonally. The voltmeter is characterized by high internal resistance (approx. 10 Mohm) and good sensitivity.
Voltmeter connection
The connection does not hide any intricacies and must work on the first connection. For now, we do not fit the input divider with fixed resistors, but with resistance trimmers so that we can calibrate the voltmeter. The tolerance of T1 and T2 amplification is usually huge.
After switching on, the components begin to heat up unevenly, causing the bridge to become unbalanced. We remove this by setting the potentiometer so that the needle of the meter points to zero. After a few minutes, this phenomenon stabilizes. Power supply is possible from a 9V battery (pay attention to the current drawn, the life of the battery is only a few hours of measurement), or from an external 9-15V source. However, the battery is more advantageous.
Calibration
Now we can start the calibration. We will need an adjustable DC voltage source of 0 to 25V and an accurate voltmeter. We gradually adjust the voltages on the source and by turning the trimmers we try to achieve the same voltage display on both voltmeters. We start with the 50V range, but turning one trimmer spreads out the other ranges. It requires patience. Finally, we will replace the trimmers with fixed resistors. Unusual values can be made up of several resistors.
HF probe for measuring HF voltage
A VF probe is a very useful addition to the voltmeter. It can be conveniently built into a dry marker. Don't forget the short ground cable with alligator clip from the probe to the device being measured. Otherwise, we will only measure the induced voltage.
However, the measurement of HF voltages is rather indicative. The probe is frequency and voltage dependent. Accurate calibration would require a separate scale for each range and multiple probes per frequency. This probe is sufficient for the KV range, for higher frequencies it would be appropriate to reduce the input capacitor. The accuracy is reduced especially when measuring small voltages, the accuracy increases at higher voltages. For most measurements, the accuracy is sufficient.