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What Is an Antenna Analyzer?

Geisha A. Legazpi
Geisha A. Legazpi

An antenna analyzer is a type of radio frequency (RF) analyzer that measures the performance of the antenna feeder and the RF antenna itself. It is able to measure the quality of impedance matching of the electrical circuits made up of the transmitter or the receiver, RF feeder, and antenna or aerial. Impedance matching determines how much energy from the transmitter reaches the antenna and how much of the RF signal received by the antenna actually reaches the receiver. The antenna analyzer makes it easy to trim the antenna so it performs best at the given operating frequency.

Moreover, the antenna analyzer may also use a bridge circuit to compare the resistive and reactive components of antenna and feeder impedances at the selected frequencies. The RX bridge uses a receiver as the indicator of the balance in a bridge where one of the lower legs is connected to the antenna. Using non-reactive 50-ohm resistors for the top legs of the bridge, the other lower leg can be connected to known capacitive or inductive impedances or both. Given the frequency used for the test, the values of capacitance or inductance, the impedance of the antenna and its feeder can be calculated. In an RX noise bridge, a noise source becomes the main RF source.

Man with a drill
Man with a drill

An RF analyzer measures the performance of a transmitter and a receiver. The common transmitter performance includes the transmit power in watts (W), which is the electrical power that the transmitter is sending out. Other transmitter parameters are the frequency error in hertz (Hz) or cycles per second (cps), and the amount of modulation. In frequency modulation (FM), the amount of modulation is measured as frequency deviation of the carrier. For the receiver section, the RF analyzer measures the sensitivity and squelch level, or the amount of RF signal where the squelch circuit will unmute.

The standing wave ratio (SWR) is ideally 1.0 for a perfect match between the transmitter and the antenna and dependent on the reflection coefficient (RC). When the RC is low, the impedance of the antenna, and that of the transmitter and feeder, are about the same or together form almost purely resistive net impedance. An SWR level of 3.0 indicates an RC of about 0.5, which means that half the power is reflected back to the transmitter. Like in physics experiments, when a wave reflects and combines with the incoming waves, a steady pattern seen as a standing wave can be observed, which means the waves are not being conveyed to the right destination and they just create a power pattern that dissipates as heat in the transmitter and in the RF feeder. The SWR analyzer, which is another name for antenna analyzer, is able to measure the SWR of the antenna to verify maximum power transfer from transmitter to antenna.

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