Measuring instruments


FIELD STRENGTH MODULATION AND SWR TESTERS
These Micronta and Midland SWR testers were found as the other old radio equipment in Salesian's Catholic Mission's scrapyard in Lungi, Sierra Leone - (9L) .
In addition to Heathkit radios and SWR testers, have found there eight 10 meters band transceivers.
As I was told this stuff was sent to Catholic Mission by hams from the USA. It was a great gesture guys. However, the permanent lack of electricity, lack of technical skills, and everyday hardships deprived this project of any chance for implementation.

micronta On the box, you can read:
- Measures the amount of signal transmitted (field strength) by your antenna system.....
- Covers 3 to 30 MHz
- Handles up to 1 KW
- Measures efficiency of your antenna system in SWR (standing wave ratio) - indicates the ratio of power transferred to your antenna (signal to power reflected from the antenna (loss)
- No need to disconnect - can be permanently coupled.

3 range micronta 3 RANGE POWER/ MODULATION/ SWR TESTER
CAT No 21-522. In addition, you may monitor the modulation of your RF signal using earphones. Detailed information there are in this manual.

On top of the tester, there is the "Pick up Box" -(measuring line) which can be placed just on the back of the meter or at a distance app.2ft for easy use.
MIDLAND TESTER
midland 23-138 2 RANGE POWER (10 and 100W), SWR, MODULATION and FIELD STRENGTH TESTER. Model: 23-138.



Q-meter
q meter
Once I decided to build a detector (crystal) radio there was a need to measure the Q-factor of coils which I planned to use. So, this simple Q-meter as a measuring attachment to the NWT was designed. In cooperation with NWT you will read the Q factor (QF) of the coil for desired frequency direct on the PC screen.

On the left, you can see the MW coil for an old fashion radio.

Any coil may be soldered to the posts for a measurement as well...


The q-meter circuit is quite simple. The signal from the output of the NWT goes through the transistor emitter follower to the transformer's 40 turns of the primary windings. Since the secondary winding is represented by a rod the transformer ratio is 40:1 it means that the resistance transformer ratio is (40/1)squared = 1600, (40/1)² so the loading resistance is increasing by 1600 times to 1600 x 50 Ohm = 80 000 Ohm. Then the signal is traveling to the resonant circuit - Lx coil and variable capacitor. This circuit must be tuned to the desired frequency since a Q factor is a frequency sensitive matter - the same coil would have other QF once the frequency is changed. After that signal is sent do the "IN" socket of the NWT.
On the NWT side, in a "Sweep" mode, check the 3dB/Q box, then tune the LC circuit to the proper frequency, and NWT will show you a value of the Q factor.


Talking about measuring coil's Q factor (QF) one may use a method described below:
- prepare a circuit for measurement as on the sketch above
- set signal from the generator to desired frequency, note it as F0
- tune the resonant circuit to the resonance - max level at the oscilloscope
- read the signal amplitude on the oscilloscope screen: eg.08V
- calculate 3db level of signal eg. 0,8V at the oscilloscope multiply it by 0.707 = 0,57V
- change the generator frequency below F0 up to time the signal level at the oscilloscope reaches 0,57V- write down this frequency asF2
- change the generator frequency above the F0 up to time the signal level at the oscilloscope reaches again 0,57V- write down this frequency as F1
- now calculate the formula: Q=F0/(F1-F2)

Example: F0=225kHz, F1=227,1kHz, F2=222,9kHz

Q=225/(227,1 - 222,9)= 225/4,2= 53,57
Q = 53,57


Dip meters





dip meter
This dip meter was built based on "Dip It-the ultimate Dip-Meter" designed by Peter Solf DK1HE. The original description of this sophisticated Dip meter you may see here.
Before starting any works on my version I asked the designer for permission to use his method of resonance indication - just for my personal, noncommercial use. Having a positive answer I started my construction. To make things simple I used a commercial 4 -digit frequency meter. This construction is limited to an indication of the circuit resonance - as in the original for this a light-emitting diode (LED) is used.
The following circuits from the original project were used:

-VFO + Demodulator
-Display Amplifier
-Saw-tooth 400Hz Oscillator
-Buffer

Since my dip-meter is powered by an external 12V power source to get 10V and 8 V a chain of integrated stabilizers LM7810 and LM78L08 is used.
Another Dip meter I built is a classic design with an analog resonance indicator and a digital frequency meter.

Click for a high resolution scheme

The last dip-meter I have built uses, in principle, the so-called tunnel diode effect. A tunnel diode is a P-N junction device that exhibits negative resistance - increasing the voltage causes the tunneling current to decrease.
Instead of a tunnel diode, a pair of FET and PNP transistors is used. In this combination, their negative resistance makes them useful in creating an RF oscillator.
The rest of the dip-meter is typical: coils, meter, tuning capacitor, digital frequency meter followed by buffor, and, in addition, a tone-generator.

The procedure how to use it is as follows:
Description:

P1- 22kOhms - determines the BIAS of the lambda diode and hence the sensitivity of the circuit.
P2- 2,2kOhms - keeps needle of the meter within the scale range.

Set of the BIAS (P1)
-Turn the wiper of the BIAS resistor fully towards the D1 cathode (counterclockwise). The oscillator is not oscillating, and the meter shows maximum deflection – using P2 (sensitivity) keep the needle of the meter in the middle scale area. Turn P1 slowly down up to the point when needle deflection starts decreasing – a sign that the generator starts oscillation. Continue turning P1 to the minimum deflection. It is a point of the proper BIAS and maximum sensitivity of the circuit.

Measurement of the LC circuit.

Close the Dip meter’s coil to the passive LC circuit and adjust the generator’s frequency. The point when the needle “jumps up” is the resonance frequency of the measured circuit. For more accurate measurements, decrease the coupling of the dip meter’s coil with the circuit under test and tune the oscillator again.

Note: Due to the lambda diode effect (negative resistance), the behavior of the meter is the opposite of the common grid-dip meter, where oscillator frequency is adjusted for minimum deflection (dip). With a lambda diode, we read the resonance frequency when the meter shows MAXIMUM deflection.




The buffer and a tone generator followed by a switch are connected to point A. A tone generator modulates the RF signal if needed.



buffer reduces the influence of the digital frequency meter on the oscillator. The commercial digital meter is used to make construction simpler.

The Tone generator is a typical Wien-bridge configuration based on an operational amplifier.


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