Measuring Traps

A trap is a parallel resonant circuit network that can perform either of two functions in an antenna system, depending on whether the traps are resonant at the operating frequency. The first function of a trap is where the trap is parallel resonant at the operating frequency. Because this parallel resonance, the trap behaves as an isolator so it can be used to isolate sections of the antenna.

The second function of a trap, obtained when the frequency of operation is not the resonant frequency of the trap, is one of electrical loading. If the operating frequency is below of trap resonance, the trap behaves as an inductor; if above, as a capacitor. Inductive loading will electrically lengthen the antenna, and capacitive loading will electrically shorten the antenna.

With the SARK-110 Antenna Analyser is very easy measuring the resonance frequency and the impedance characteristics of traps. This application note describes basic measurement procedures using as example a homebrew coaxial cable trap.

“Grid-dip” meter method

A simple way for the determination of the resonant frequency of a trap, or any tuned circuit is to connect a small section of wire forming a close loop, connected to the analyzer test port extension cable and put it near to or around the trap; see the Figure 1.

The measurement procedure will be aimed to determine the frequency point of highest impedance |Z|. This frequency will be the trap resonance frequency. The measurement can be done from the Scalar Chart mode with one of the traces selected to measure the modulus of the impedance |Z|. For this example, the peak maximum of |Z| was found at a frequency of 8.79 MHz, which is the measured resonant point of the trap.

A step by step procedure will be the following:

The advantage of this method is that there is no need of connecting physically the trap leads to the test port extension of the analyser, which has influence in the measured resonant frequency due to the inductance of the leads.

“Direct” method

The trap resonance frequency and impedance characteristics can be measured by connecting the SARK-110 to the trap leads, as it is shown in the Figure 5.

The measurement procedure is the same as for the “grip-dip” method so it will not be repeated here. The only difference is that the trap leads are connected to the test port extension.

The Figure 6 shows the Scalar chart with the Span set to cover up to 32 MHz (Full HF Preset). In this chart it can be observed that at the resonance frequency of 8.5 MHz the impedance modulus peaks and the phase changes from a positive value (pure inductive) to a negative value (pure capacitive). The Figure 7 shows the same measurement as seen in the Smith chart mode. The resonant point is at the crossing point of the horizontal axis. The Figure 8 shows again the Scalar chart but with a smaller Span.

Compared with the previous measurement method, there is a slight difference in the measured resonance frequency. This is due to the influence of the connection leads of the trap to the test port extension. However, with this method it can be obtained the complete impedance characteristics.

Tuned circuits

These measurement procedures can be used for both parallel and series tuned circuits. The Figure 9 shows an example of a tuned LC circuit. The Figure 10 shows the impedance plot when the circuit is arranged in a parallel tuned configuration. In this case the impedance is maximum at the resonant frequency. The Figure 11 shows the impedance plot for the series tuned configuration. In this configuration the impedance shows a peak minimum at the resonance frequency.

The values of the inductor of the capacitor forming the tuned circuit, can be independently measured using the Single Frequency mode. Figure 12 shows the measurement of the inductor value at 1 MHz and Figure 13 the measurement of a capacitor value at the same frequency.

For a precise measurement it is important performing OSL calibration and that the leads to the component be the smaller as possible.