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Antenna Fundamentals part. 2

Antenna Fundamentals part. 2

crease the signal-to-noise ratio. The thermal noise floor of the environment, if detected by a 9-kHz bandwidth, is approximately -27dB?μV -134 dBm. However, when signals are processed and amplified to useable levels, noise is introduced. The noise figure of an amplifier is defined as the difference between its noise floor and the ambient noise of the environment. Consider an antenna picking up a signal that is only 0 dBμV as shown in Figure 11a.

The signal may be 27 dB above the ambient but to a receiver with a 24-dB noise figure, the signal is only 3 dB above the noise floor. Thus, the signal-to-noise ratio is only 3 dB. A good amplifier can be used to increase this margin as shown in Figure 11b. Here, a 20-dB amplifier raises the signal level from 0 dBμV to 20 dBμV. The amplifier also raises the ambient by 20 dB to -7 dBμV. Since the amplifier has an 8-dB noise figure, it then adds another 8 dB to the ambient making it +1 dBμV. The noise floor of the receiver -3 dBμV is below this figure and thus does not affect the result. The new signal-to- noise ratio is 19 dBμV.

Figure 9. a VSWR and b ratio of reflected to forward power as load resistance varies in 50W system.

Figure 10. a Common-mode current due to ground plane coupling and b use of simple balun.

Figure 11. Signal to noise ratio a without and b with amplification.

SUMMARY

A proper understanding of antennas requires familiarity with electromagnetics, circuit theory, electronics, and signal processing. Such knowledge is indispensable to the EMC engineer who must interpret test results, improve accuracy and sensitivity of tests, and suggest ways to eliminate unintentional antennas from product designs.

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