Ken Hough's Website
Amateur Radio

Test Equipment

This section includes information about the following:

Multi-range test meters : SWR meters : Absorption wave meters : Frequency meters : Dip meters
Dummy loads : Oscilloscopes
Amateur radio licensing conditions require that tests are done from time to time to confirm compliance with the licensing conditions.

Perhaps the most important requirement is to ensure that transmissions remain within the frequencies set out in the band schedule and no spurious signals are emitted at other frequencies. eg harmonics or parasitic oscillations

It is also important that over modulation of transmissions is avoided as this is likely to generate widely spread sidebands that can cause problems for other users of the bands.

Most modern transmitters and transceiver are crystal controlled and should be accurately calibrated. They typically include reasonably accurate means of adjusting and setting modulation levels. However, faults can occur, so all radio amateurs should ensure that they have the means to carry out checks to confirm compliance with the licensing requirements.

Users of older transmitters that rely on the use of VFOs will need to be more careful about operating frequencies. If class C RF power stages are included, the possibility for (likelyhood of?) emission of harmonics should not be overlooked.

From time to time, all radio amateurs will need to check for circuit continuity, integrity of insulation, component values, etc, so some basic test instrumentation is essencial!

Many kinds of test equipment are available. Some are relatively cheap. Others are expensive and while being nice to play with, might not be neccesary for operation of a station.

Below, I have described my own approach to test procedures and equipment. I haven't spent much money on test equipment. Some of it is home made. The most expensive item was a 30MHz oscilloscope that I obtained via Ebay for only £75.
Multi-range test meter:
This is the very least that should be available and should be able to measure AC and DC voltages and currents, resistance and continuity. Many instruments can also determine capacitance and some basic parameters of semi-conductors such as transistors.

Most modern test meters use electronic circuitry to provide very high input impedance (good for measuring voltages).

Digital displays are now common, mainly because they are cheaper than good quality analogue meters. However, analogue meters can be better for following varying signal levels.
SWR meters:
ESSENTIAL! Described in the Antennas section.
Absorption wave meters:
Useful to detect presence of strong RF signals in the shack or on coax feeder lines. Simple versions do not include any form of tuning and can operate over very wide frequency ranges. Tunable versions can be calibrated for frequency and might be useful for detecting the presence of harmonic or parasitic radiations.

I have two home made wave meters as shown below.

The circuit on the left is about as simple as an absorption wave meter can be. It's not frequency dependent or particularly sensitive, but can be helpful in tracking down moderately strong RF fields. 50μA meters are not easy to find these days. I used a 500μA meter and still found useful sensitivity.

The circuit on the right has been taken from the RSGB Communications Handbook (11th edition, page 24.30). It can be calibrated (eg 30mV FSD). Depending on the input sensing circuit, it can used as a sensitive tuned or untuned field strength meter. For details of this design and how to use it, refer to the RSGB Communications Handbook.
Frequency meters:
It is ESSENTIAL that transmissions remain within the allocated frequency schedules. As a matter of courtesy, transmissions should also be kept within ranges set out in the generally accepted band plans.

Modern transmitters and transceivers use some form of crystal controlled oscillator, usually either a phase locked loop VFO system or direct digital synthesis. Once calibrated (eg against WWV transmissions) these systems can usually be relied on to remain accurately on the indicated/set frequency across the whole of their tuning ranges. For normal operation, no other frequency check is necessary.

Many older transmitters and receivers relied on free running VFOs (variable frequency oscillators). In these cases it was NECESSARY to be able to check operating frequencies against an accurate standard. Many older amateurs will remember the BC221 frequency meter that was often used for this purpose.

In all cases, it is advisable to have some additional form of accurately calibrated frequency meter. Presently, this is most likely to be a second crystal controlled radio or tranceiver.

I have two modern crystal controlled transceivers. These are the Yeasu FT897D and the Kenwood TS590S which cover all of the HF bands up to 6m and show exact calibrations for the 10MHz WWV tranmissions.

                            Yeasu FT897D                                                                Kenwood TS590S

The FT897D also covers the 2m and 70cm bands. The Kenwood does not cover these bands, so I need an additional means of checking these frequencies. An old Icom PCR1000 computer driven receiver which covers all frequencies from 10kHz through to 1.3GHz provides this means. Again frequency calibration is accurate.

Additionally I have a small dedicated frequency counter that covers HF and VHF bands.
                            Icom PCR1000                                                      Dedicated Frequency Counter
Dip meters:
Back in the days of valve based amateur radio, "grid dip meters" were very popular. These simple devices could be used to determine resonant frequencies of tuned circuits such as those used in transmitter multiplier stages and resonant antennas such as dipoles.

Grid dip meters comprised a calibrated VFO with an exposed inductive element. The inductor was brought close to the resonant circuit under test. The dip meter frequency was then swept across the expected resonance of the test circuit. At resonance, the grid current of the VFO would show a pronounced dip in magnitude.

Very effective "dip meters" can be made using transistors instead of thermionic valves. Bipolar, FET, and dual gate MOSFET designs are available. In these cases, it is usual to detect the "dip" in current through the oscillator transistor. Sensitivity can be very high -- especially for MOSFET designs.

Most dip meter can also be used as tunable absorption wave meters.

Many transistor based designs can be found via the Internet. I chose to build the dual gate MOSFET design of I1FLC (Luigi Falconi) which was presented in English by G3PTO. The original circuit is shown below along with details of the tuning coils.
I made the following modifications:

1. The circuit was built on strip board (Vero board)
2. As the 40673 MOSFET is obsolete/expensive, I used a BF998. This is a surface mount device, so I carefully
   connected it to short stubs of wire that had been soldered to the strip board.
3. The BF998 required that the 2nd gate voltage was reduced so as to keep the maximum drain current below
   20mA. This was done by increasing the value of R3 to 150kΩ.
4. Additional tuning coils were made to provide continuous frequency coverage from 1.7MHz up to approx
   100MHz.

The high sensitivity of this dip meter allowed for very loose coupling to test circuits, which results in very little "pulling" of oscillator frequency. It was therefore worth spending time to provide accurate frequency calibration for this dip meter.

This design provides for operation as an absorbption wave meter (via switch S1), but my construction did not work at all in this mode. Even so, this is a very effective dip meter.

100MHz was about the maximum operating frequency of this dip meter. Given suitable circuits and layouts, higher frequencies are possible. The design shown below is an example.

This design has been copied from "Test Equipment for the Radio Amateur", by Clive Smith, published by the RSGB. It can operate between 29MHz and 460MHz. Details of the coils for L1, and a PCB mask are given in the original article.

I have found this design to be quite sensitive.
Dummy loads:
A dummy load is essentially a non-reactive resistor (typically 50Ω) that can be connected to the RF output of a transmitter in place of an antenna. This allows the transmitter to be tested without causing interference to other radio users.

Ideally, a dummy load should be capable of dissipating the full/maximum power of the transmitter being tested. In practice, it will often be possible to run a dummy load at perhaps three or four times it's rated power FOR SHORT PERIODS ONLY.

By measuring the voltage developed across a dummy load, the RF power level can be calculated.

Low power dummy loads can be made simply by soldering two 100Ω non-inductive resistors in parallel across a SO239 or BNC socket. Provided that leads are kept short, this arrangement will remain practically non-reactive up to 430MHz or so. Higher power dummy loads will often include large cooling fins. I have a dummy load that has a continuous rating of 75Watts at up to 1000MHz.

When used with an oscilloscope (see below), a dummy load can provide a very convenient means of checking modulation levels, audio compression levels, etc.
Oscilloscopes:
Oscilloscopes are not cheap, especially those that are capable of displaying signals of 30MHz or so. It is unlikely that an oscilloscope would be used very often, so why consider buying one?

There are certain tests that just cannot be done without an oscilloscope. eg looking at purity of waveforms (ie harmonic content), detection of parasitic oscillations, transmitter modulation levels and digital timing issues.

An oscilloscope is simply a means of producing real-time graphical displays of waveforms. Adjustment of time base scan speed enables stationary displays to be set up, thus allowing for detailed inspection of waveforms. Accurately calibrated time bases allow timing issues to be quantified. Dual channel 'scopes can compare amplitude and timing between two signals.

My own oscilloscope is a Hameg HM303 which I bought second hand via Ebay for only £75. This is a dual channel 'scope that is rated for operation at up to 30MHz. However, I have used it to "count" frequencies of up to 100MHz or so.

Return
29/08/13