Why 3db beamwidth

The 3 dB, or half power, beamwidth of the antenna is defined as the angular width of the radiation pattern, including beam peak maximum, between points 3 dB down from maximum beam level beam peak.

Azimuth and elevation beamwidths are calculated through measurement of the antenna co-polar radiation pattern.

Radiation patterns are measured by rotating an actively transmitting antenna inside an anechoic chamber, and measuring power levels at set intervals. The angle from which power level is -3 dB down either side of the maximum power level i. IEC defines appropriate test methodology. A second approximation is an assumption that the total power is within the half-power beamwidth of the antenna and evenly distributed within it see Figure 1, the green colored area.

On the other hand, it is assumed that no power can be measured outside the half-power beamwidth. If it is necessary to correct the effects of this approximation in the calculation of distances or signal amplitudes then the Beam Width Factor can be used. Because of these approximations, it is possible to use the gain of the antenna and the power generated by the transmitter directly in the radar equation as a parameter.

Beamwidth Figure 1: Beamwidth of an antenna. In addition to beamwidth, factors such as its resonant frequency, bandwidth, polarization and gain will play a role. No single antenna design can cover all aspects, so selecting the best antenna for each test is necessary. Log periodic antennas have wide-frequency bandwidth while at the same time can also be designed to have a fair amount of directionality.

Typically, the beamwidth of a log periodic is related to the boom length, element spacing, taper and number of elements. Beamwidth is typically used for testing the half-power. This can be calculated based on the antenna characteristics and is often obtained from the manufacturer.

The half-power beamwidth of the antenna, along with the distance to the device under test DUT provides the information needed to set up a test environment. An online beamwidth coverage calculator is available. Horn antennas have reasonable directivity, are used at microwave frequencies and are fed by waveguides.

They have a wider bandwidth than some other microwave antenna solutions, which makes them a good antenna choice for testing. Among the different kinds of horn antennas are the pyramid, conical or corrugated horn.

The different types affect the radiation pattern and beamwidth. One such example is a horn antenna designed to have polarity by flaring the sides so that it can be aligned with the E or H plane. Testing procedure Testing products for RF compliance, output, susceptibility or other criteria is necessary to meet FCC requirements and to assure quality and design. Tests are done in an anechoic chamber designed to absorb RF energy so that only the intended signals are present.

An antenna or antenna system is placed at various points around the device to determine its characteristics. A number of details must be taken into account, such as the test frequency, type of antenna, position, orientation, beamwidth as well as its distance to the DUT, all of which are important criteria to achieve repeatable and reliable results.

A spurious emissions test is designed to identify non-intended or non-primary transmissions from the DUT. This test is often done to comply with FCC part 15, but it may also be completed to understand how a device is operating or how it could influence devices around it.

The test setup involves placing the DUT at a specified distance from a receiving antenna of a known beamwidth and observing the received signals at various frequencies, as well as antenna placement. The combination of this data creates a picture of all RF emissions from the DUT and determines if it is in compliance. RF susceptibility testing determines how an outside RF signal will affect the normal function of a device. Like spurious emissions, testing is done in an anechoic room where a radiating antenna is placed a specific distance from the DUT.

The antenna transmits a signal of a specified strength and frequency although multiple frequencies are often tested. A receiving probe may be used to ensure the field strength is correct near the DUT.

The device is then operated to determine when or if the outside RF interferes with normal operation and at what level. Devices such as radio receivers tend to be affected the most due to overloading the receiver, but devices such as computer equipment are impacted at some level due to interference with clock frequencies and other circuitry.

Here is a Field Intensity Calculator to help determine field strength. Repeatability is important for any test and this applies to RF testing.

Tests should be repeatable utilizing the same equipment and facility, and also should be repeatable at multiple places and with different equipment, provided the testing procedure is set up correctly and the equipment is calibrated.