Schwarzbeck HMDA 1545 Sensitive Magnetic Field-Strength Meter with LCD Reading

Unlike low sensitivity passive meters, this active meter with a lower detection threshold of <46 dBmA/m (200 mA/m) operates also in the range of low fieldstrength.

In the insensitive range magnetic field- strength up to more than 120 dBmA/m ( 1 A/m) is covered. When many strong signals are present, or frequencies are of interest, the HMDA 1545 can be used as an active magnetic probe (loop antenna). Just connect the r.f.-output to a receiver or spectrum analyser.

The built-in tone generator and loudspeaker convert field-strength into variable tone frequency. This is very useful for radio direction finding with final reading of the 3 1/2 digit LCD.

The full metal cabinet and the electrically shielded antenna are very rugged. Operation via 3 toggle-switches is very easy. The built-in NiMH rechargeable battery gives 30 hours of operation . The optional Automatic Charger will perfectly charge the battery independent of its previous state.

1. Principle of Operation

A special operational amplifier using discrete components rather than integrated circuits converts the frequency independent loop-current caused by a magnetic field into a proportional voltage.

An r.-f.-attenuator can be switched between loop antenna and operational amplifier to get two amplitude ranges. This kind of switching results in improved measuring dynamic compared to switching at the end of the line by simply changing the DVM's sensitivity. The r.-f. output-voltage of the operational amplifier is fed to the input of a demodulating logarithmic amplifier. This amplifier provides a demodulated output proportional to the decibel value of the signal input over many decades. The dynamic range is in fact better than than that of the combination loop/operational amplifier, which is limited by internal noise and saturation effects.

The output voltage of the demodulating logarithmic amplifier controls the input of a digital volt-meter unit (DVM). The DVM shows the reading of magnetic field-strenght directly in dBmA/m. Furthermore the output voltage is converted into a variable audio frequency using a voltage to frequency converter which drives a small loudspeaker after further amplification. Increasing fieldstrength increases audio frequency. In contrast to the lcd-display, which is perfect for precise reading, the audio frequency information is fast and convenient.

Together with the directional behaviour of the loop, this feature makes direction finding of magnetic field sources easy. Magnetic field probes have to suppress electric fields as far as possible. This is accomplished using an electric shielding of the loop (tube) which is cut in the middle to avoid a magnetic short circuit of the winding.

2. Operation

2.1 Switch ON

Three toggle switches are located on the upper side of the metal box near the ending of the loop. Meter is ON in the left position of the toggle switch on the right side. The LCD-display is now active.

2.2 Switch Audio

The switch in the centre has three positions. Audio is off (mute) in the centre position. Left position is low, right position is loud audio.

The loudspeaker radiates a parasitic magnetic field of its own not far away from the loop antenna.

Under certain circumstances this may have negative influence on measurement. It is therefore good practice to switch off (mute) audio in the moment of LCD-reading whenever low fieldstrength levels are measured.

2.3 Switch Range

Switching Range will adapt the fieldstrength meter to the magnetic fieldstrength to be measured. In the right position the maximum fieldstrength to be measured is 100 dBmA/m, in the left 120 dBmA/m. There is a typical safety margin of 2 dB beyond these levels available, but it is good practice to switch to the higher range before the reading of 100 dBmA/m in lower range occurs. In the same way readings >120 dBmA/m in the higher range have to be considered as wrong.

For frequencies >30 MHz this margin is falling constantly. At 80 MHz saturation occurs 4.5 dB below full scale indication (see Appendix).

Inherent noise generated by the loop amplifier results in a basic reading, which is different in both ranges. This noise source is internal and must not be interpreted as external field-strength.

This reading is in fact the lower edge of indication.

2.4 LCD-display

The display has 3 1/2 digits. The MSD shows only "0" and "1". The unit of both ranges is [dBmA/m].

The decimal point on the right side of the MSD serves as an indicator for low battery.

When this decimal point is visible, the rechargeable NiMH-cells should be recharged.

Depending on many factors, there is a safety run-time of at least 15 minutes available.

Due to good internal voltage regulation and feedback techniques the meter will work even longer, but there is no guarantee for technical data under these circumstances.

2.5 Charging connector

The internal power supply of the meter uses six rechargeable NiMH-cells. When discharged, charging should be done with the automatic charger (option) ACS 410 (Ansmann). Combining NiMH-cells and automatic charger results in an optimum of operation time and lifetime expectation. You can even charge cells which are not completely discharged without side effects. Charging time for completely discharged cells is typically 3 hours.

2.6 R.-f.-output

The r.-f.-output may be connected to the input of a measuring receiver or spectrum analyser to monitor a frequency spectrum. In this way, radio broadcast transmitters can be identified according to their frequency.

The switchable range-attenuator is located at the input of the loop-amplifier. This means the conversion factor of the meter changes when ranges are changed.

In order to save battery power the r.-f.- output impedance is substantially higher than 50 ohms. This is no problem when measuring receivers or spectrum analysers with a 50 ohm input are in use. High impedance equipment such as oscilloscopes give a higher indication because of the insufficient load. A 50 ohm feed-through-terminator solves the problem.

As mentioned in 2.2, the loudspeaker may cause negative influence on measurement under certain circumstances. This is also true for the r.-f.-output. If problems occur switch off audio.

2.7 Loudspeaker

The loudspeaker is located behind the holes in the metal box. The volume is affected by 2.2.

3.1 Influence of internal noise

The noise of the amplifiers result in a basic (idle) reading, which is different in the two ranges, because the ranges are switched directly at the input i n front of these noise sources.

Because of the fact that the noise source is internal, it should not be considered as field-strength. In fact it must be considered as the lowest possible edge of measurement.

Measurement near this edge comes with a high error caused by noise. This error decreases with increasing signal/noise factor.

It is good practice to measure fieldstrength <90 dBmA/m always in the lower range, because internal noise is 20 dB lower than in the upper range, when the 20-dB-attenuator is between loop and loop amplifier.

3.2 Influence of saturation

Saturation of the loop amplifier is typically 2dB beyond upper range limit. When higher field-strength is applied, output voltage of the loop amplifier will not rise proportionally and clipping will occur.

If only one unmodulated signal at a time is present, considerations are quite simple.

If there are two or more strong amsignals present, as it is the case near a short wave transmitter area, these signals have to "share" the limited resources the loop amplifier. In this case for one single signal the upper range limit of 120 dBmA/m is no longer provided, but only a fraction.

The situation is even worse caused by the envelope of the am-modulated signals. Worst case consideration shows that there are moments, when all signals add up when they have maximum amplitude of the envelope.

Saturation has also negative effects considering receivers or spectrum analysers connected via the r.-f--output. In this case spurious signals occur, which are not present in the original (clean) spectrum. Due to the attenuator used in the upper range, intermodulation is lower, but noise is higher as explained in 3.1 (Low Noise-Low Distortion).

3.2.1 Influence of saturation >30 MHz

Up to 30 MHz saturation occurs beyond full scale of ranges.

At higher frequencies saturation restricts full scale indication. At 80 MHz, saturation occurs 4,5 dB below full scale of 120/100 dBmA/m. So be sure to switch to the higher range when you read 90 dBmA/m in the lower range. Saturation has no negative influence on smaller signals.

3.3 Frequency range

The frequency range is mainly limited by loop and loop amplifier. The loop amplifier has to load the loop with a very low impedance over a wide frequency range. Any component in this path, in this case the switchable attenuator, will cause some frequency dependent attenuation.

The frequency range is determined by a upper and lower limit. Within these limits the transducer factor is practically constant. In this case the frequency range is 50 kHz-80 MHz. Below the lower limit there is high-passcharacteristic, above the higher limit there is low-pass-characteristic.

As these characteristics are relatively smooth, field-strength outside this frequency range still gives a substantial reading. This reading may be used when the frequency is known and a correction table is used.

Furthermore, the attenuation of the switchable range-attenuator is optimised for the specified frequency range, not for outside frequencies.

Whenever field-strength with frequencies outside the specified frequency range is measured, or when the strongest of a couple of signals is outside the specified frequency range, there will be substantial differences when the ranges are changed.

A good example are computer-monitors. Their deflection coils radiate fields with frequencies near or outside the specified frequency range of the meter.

An example for signals above the specified frequency range are strong fm-transmitters in the range from 88 MHz-108 MHz.