Overview
The Active Holder EFS 9219 in combination with the biconical elements form a probe for measurement of the electric fieldstrength. An antenna factor of 20 dB/m is obtained in combination with the BBUK-elements. For different antenna factors, elements of different shapes and sizes may be used if they fit in the clamps.
Unlike common, broadband E-field probes, this probe is qualified for the frequency selective measurement of very low fieldstrength. This is a common problem with data transmission via more or less perfectly screened cable. In this mode, the probe is connected to a receiver or spectrum analyser via a coaxial cable. The field sensitive "head" of common E-field probes can be perfectly "isolated" from the environment using resistive cable or optical fibre.
This method is nearly impossible when a complex frequency spectrum has to be transferred. Here a differential amplifier with very high common mode attenuation and a very symmetrical, low capacitance transformer are used for "isolation". Without these special techniques, the desired receiving effect of the biconical dipole would suffer from the parasitic antenna formed by the biconical elements and the coaxial cable. To avoid effectively all problems with power supply cables, a rechargeable NiMH battery is built into the holder.
After more than ten hours of operation, the automatic charger recharges the battery in 2-4 hours. Charging of a partly discharged battery can be done without risk for the battery's performance. An optional extension rod for convenient operation and enhanced isolation is available on request. Made of insulating material, it contains several ferrite braid-current chokes.
Its n-connector can be fixed directly to the holder. The antenna factor of 20 dB/m is good enough to give substantial noise indication using a qualified measuring receiver. Under these circumstances, more amplification is worth nothing, because it would only reduce the dynamic range.
To improve overall performance with receivers suffering from poor sensitivity, the optional amplifier BBV 9721 can be used. It combines high amplification of 20 dB with a wide dynamic range.
The EFS 9219 was designed because of the growing demand for a small, light weight receiving antenna to cover the frequency range 9 kHz-30 MHz with high sensitivity. PLC-measurement is only one example. Because of the fact that dipole length is very short compared to wavelength, symmetry has to be extremely high. So optimum symmetry was the main goal of this development. Above 30 MHz the active probe EFS 9218 can be used. Passive solutions are Biconical or Log.- Biconical antennas.
Application
Typical application is frequency selective EField-Measurement outside and inside of buildings and rooms using test receivers or spectrum analysers. The wide frequency and dynamic range covers the very low limits for data transmission on power lines as well as the higher limits for medical implants (heart pace-maker).
The high limits for human protection are not covered, however.
The antenna (conversion) factor is constant over nearly the complete frequency range giving a very natural fieldstrength image when spectrum-analysers are in use.
In order to eliminate power supply problems and stray coupling, the probe uses built-in NiMH rechargeable batteries. The state of the battery is monitored by a LED. After more than ten hours of continuous operation, charging with the automatic charger takes about 2-4 hours.
The biconical elements fit into clamps on the holder BBUK-elements are used as standard. Elements of different size result in different antenna factors. Using bigger elements will provide higher sensitivity in a restricted frequency range. More information on request.
Switching On
Put ON/OFF switch into ON-position. The red LED has to be on. At the end of the battery capacity the LED looses brightness and will finally be OFF, when the voltage is insufficient for safe operation.
Set-up
For high precision measurement a mast should be used. This is possible with the optional extension rod which is also useful for convenient hand-held operation. Changing height and polarisation the maximum can be found while the operating person at a distance doesn't influence the measurement.
Hand held measurement is also possible with restricted precision. Keep the hands to the n-connector side of the mounting tube and the cable straight off the probe.
E-field-measurement is more sensitive to environmental influences than H-field. For this reason the cable should be as short as possible.
Indication equipment
Only a very sensitive measuring receiver can utilise the high sensitivity of the probe. Common spectrum-analysers usually don't provide sufficient sensitivity, especially in the frequency range below several hundred kHz. There is a low noise preamplifier available to solve the problem. Broadband measurement of high fieldstrength can also be made with other equipment.
Care has to be taken to ensure that this equipment has a 50 W input. A BNC- or other coaxial input connector is no guaranty for that.
Oscilloscopes with a BNC-input- connector usually have an extremely high input impedance consisting of some MW in parallel with some pF. Some models have a switch to put a 50-W-termination in parallel. The situation is more or less the same considering r.-f.-millivolt-meters. Mismatch may be tolerated if only signal characteristics are monitored.
For correct measuring a 50-W-termination is a must.
Measurement
The probe converts electrical fieldstrength into a voltage (across 50 W), which is indicated by a measuring receiver, spectrum analyser or r.-f.-millivolt meter. The fieldstrength can be calculated using the antenna factor (conversion factor, transducer factor) of the probe. This factor is constant over the wide centre frequency range with some minor changes at the edges. The main antenna factor is +20 dB(1/m).
A measuring receiver or spectrum analyser with a 50-W-input and dBmV- reading is used for measuring. Reading in dBmV is very common in the emc-field and available on almost every receiver, using 0 dBmV acc. to 1 mV. The voltage level on a certain frequency (f. e. an am transmitter on 1 MHz) is measured.
The antenna factor of 20 dB is added to the voltage level reading.
The result is the electric fieldstrength level in dBmV/m.
| Voltage level reading on the receiver | 60 dBmV |
| plus antenna factor | 20 dB |
| Electric field strength level | 80 dBmV/m |
Receiver reading in dBm (0 dBm acc. to 1 mW). The power level is measured and 127 db added.
| Example 2 | |
| Power level reading on the receiver | -50 dBm |
| plus antenna factor | 127 dB |
| Electric fieldstrength level | 77 dBmV/m |
Receiver voltage reading directly in V (mV, mV). The voltage is multiplied by 10 to get the electric fieldstrength in V/m
| Example 3 | |
| Receiver reading (voltage) | 0,1 V |
| multiplied by antenna factor | 10X0,1 V |
| Electric field strength | 1 V/m |
Measuring high fieldstrength
The linear range of the probe ends at 130 dBmV/m acc. to 3 V/m. Saturation begins "soft" and depends on the frequency. Higher fieldstrength levels lead to signal distortion and intermodulation products occur in the spectrum.
The same situation occurs with many weak signals. Whenever strange signals are recognised under high level conditions, increase the distance between probe and field source.
Intermodulation products then decrease faster than "real" signals
Simple receivers and spectrum analysers with no or insufficient front-end-filtering may cause intermodulation while the probe is still linear
In this case increase R.-F.-attenuation and decrease I.-F.-attenuation to reduce input saturation at the expense of noise.
Measuring very low fieldstrength
When a very sensitive receiver or spectrum analyser is used, the noise of the probe is the limit for low fieldstrength measurement. Making the receiver bandwidth smaller will reduce noise indication.
Under the condition that the signal to measure is a narrow-band-signal, smaller receiver bandwidth will give better signal to noise ratio. Choosing the average detector may reduce noise even more.
There will be improvements for narrow band signals (cw) without modulation, but not for broadband signals and pulse spectrum.
The following table shows the noise of the probe. Measurement was made with the EMIreceiver FMLK 1518 without any preamplifier. The bandwidths are standard emibandwidths. Smaller bandwidths give lower noise indication.
Inherent Noise:
Number without brackets: Voltage noise-level
Number with brackets: Calculated fieldstrength level
First steps with the probe
After connecting the probe to the receiver or spectrum analyser some signals can be seen.
In contrast to magnetic probes there will be a multitude of signals generated by electric and electronic equipment, but only a few broadcast transmitters. Radio am-transmitters which can be monitored with the magnetic probes FMZB 1537/1538 are covered by interference. In office rooms emission is dominated by PCs, monitors and data networks. Industrial environment shows high fieldstrength on ISM-frequencies (Industrial, Scientific, Medical).
Even some receivers or spectrum analysers radiate electric fieldstrength, which can be monitored with the probe.
Battery
The probe contains 6 NiMH-cells with 1,2 V/1000 mAh each resulting in a voltage of 7,2 V (nom.).
The batteries are built in the holder together with the other components. This avoids any negative influence on power cables.
The 10 hours of operation time is very conservative considering the current consumption of only 30 mA. Under these circumstances the reserve indication can occur very much on the safe side, providing sufficient time to complete the measurement.
Furthermore the operation time will still be available after ageing.
Batteries of this kind have a very low resistance. After a short current they will heat up in seconds. Burning out of the insulating material between the cells leads to permanent inner short circuit with even more heat.
The battery is protected by a thermal switch and a fuse. But once a short circuit between cells is established, switch and fuse cannot prevent severe damage to the probe.
For this reason never open the probe or charge it with inappropriate charging equipment. Avoid short circuits to the charging connector.
A standard charging connector is used.
Common chargers use a multiple connector system.
Be sure to select the correct polarity!
The optional charger ACS 410 (Ansmann) is a good choice because of the intelligent charging.
The battery may warm up while charging. This is normal.
Charging Connector
Inner conductor: PLUS
Outer conductor: MINUS (GROUND)
