I put my RF hat on last night and had a think about how the grounding box cables are made.
The construction will effectively create a balun that can reflect an impedance connected to the end of the cable.
The length of the cable (balun) will determine where impedance nulls and peaks will occur across the frequency range. This is the theory so the test is to find out if the grounding box actually has an impedance.
This impedance is relative to free space (RF/antenna engineers will understand this). The impedance of free space is 377 ohms, though this could be thought of as a construct as it is calculated from the theoretical capacitance and inductance of free space, or a vacuum if you will.
What is really needed is a Network Analyser to measure the reflected impedance but I don't have one.
The next best thing is to use the calibration signal of the Spectrum Analyser which is a 50MHz square wave, so displays frequencies at 50MHz intervals across the screen.
I quickly made an adaptor so the grounding box cable could be connected directly to the Spectrum Analyser.
The spectrum was monitored with and without just the lead connected, no difference was found. When the grounding box was connected to the other end of the cable a difference was observed, but only to the bottom 4 frequencies, namely 50/100/150 and 200MHz.
The differences were small but repeatable so the Analyser was set to display to 200MHz and at its maximum vertical resolution of 3dB per graticule line.
Display with/without just the cable connected:
Note the respective levels of the four spikes.
Display with cable and grounding box connected:
Note the 50 and 150MHz responses have increased and the 100 and 200MHz responses have decreased.
OK what does this tell us? It doesn't tell us what the impedance is as that is beyond the scope of the test.
It does confirm that the grounding box has an impedance at RF.
Originally Posted by Firebottle
I put my RF hat on last night and had a think about how the grounding box cables are made.
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