How to use the TekBox TBMA1 biconical antenna

Posted by gm77 11/02/2020 0 Comment(s) 37 Strumenti di Misura,

The TekBox TBMA1 biconical antenna was designed, targeting radiated noise EMC pre-compliance measurements.

To make optimum use of the TBMA1, a few details need to be considered:

The TBMA1 does not contain any filters at the output port. Consequently, high amplitude signals that appear at the RF output, especially when employing external pre-amplifiers might overdrive the spectrum analyzer and the resulting intermodulation will cause measurement errors. In environments with high ambient noise levels, using suitable filters may be of advantage.

The ambient noise level picked up by the antenna in an unshielded environment, combined with the base noise level of the analyzer may already cross the radiated emission limits of certain CISPR standards, even with no DUT present. Consequently, it may be very difficult to differentiate ambient noise and radiated noise from the DUT in an unshielded environment.
Even turning ON/OFF the DUT to identify the radiated noise from the DUT may often not be a solution, given the dynamic characteristics of contemporary sources of ambient noise.
A suitable procedure is first measuring the radiated noise of the DUT in a TEM cell which is placed in a shielded tent or shielded bag. This will give an excellent overview of the emitted noise spectrum of the DUT. You can easily identify the strongest emissions of the DUT and thereafter re-measure it in an open area test site (OATS) with the measurement antenna. You then don ́t need to confuse yourself with the entire ambient spectrum. Simply set the center frequency of the analyzer to the critical emission frequencies of the DUT, one by one. Choose a span as narrow as possible to zoom only at the frequency of the investigated DUT spurious. In case that the base noise is still too high, you can use suitable external bandpass filters, reduce the resolution bandwidth of the analyzer or move the antenna closer to the DUT until you can clearly identify the DUT spurious and measure its level. As long as you keep your antenna in the far field, you can easily convert from the actual measurement distance to the equivalent level in 3m or 10m distance.
In case that the DUT spurious exceed the limit of the standard, take it back to your lab and use near field probes to locate the origin of the spurious on your DUT PCBA. Take suitable measures to reduce the emissions of your product. Track the effect of the modifications by TEM cell measurements, until the relative improvement measured in the TEM cell matches the relative improvement required to meet the far field limits according to the relevant standard.
Then carry out another OATS measurement of the DUT to validate, if the DUT ́s radiated emissions are within the limits when measured with an antenna.

Use following formula to convert the measurement result from the actual measurement distance to the distance specified in the relevant standard:

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