What does the radiation resistance of an antenna mean?

What does the radiation resistance of an antenna mean?

In a receiving antenna the radiation resistance represents the source resistance of the antenna, and the portion of the received radio power consumed by the radiation resistance represents radio waves reradiated (scattered) by the antenna. Electromagnetic waves are radiated by electric charges when they are accelerated.

What is the real part of the antenna impedance Ra?

The real part of the antenna impedance RA is due to radiation loss and dissipation, R A = R r + R l. If the current at the antenna terminals I0 is known, these partial resistances can be expressed as

How is the impedance of an antenna measured?

Typically, the quality of the antenna impedance match to the feedline (or the system impedance) is evaluated through the reflection coefficient | S 11 | = | Γ | or the standing wave ratio (SWR). 4 These are calculated from the antenna input impedance ZA and the system impedance Z0 as (18) SWR = 1 + | S 11 | 1 − | S 11 |.

What happens when the length of an antenna is decreased?

As the length is decreased the loss resistance, which is in series with the radiation resistance, makes up a larger fraction of the feedpoint resistance, so it consumes a larger fraction of the transmitter power, causing the efficiency of the antenna to decrease.

How are radiation resistance and loss resistance calculated?

In order to give a meaningful value for the antenna efficiency, the radiation resistance and loss resistance must be referred to the same point on the antenna, usually the input terminals. Radiation resistance is usually calculated with respect to the maximum current in the antenna.

How is the power dissipated in radiation resistance?

The power dissipated in the radiation resistance is due to radio waves reradiated (scattered) by the antenna. Maximum power is delivered to the receiver when it is impedance matched to the antenna.

How is radiation resistance related to ohmic resistance?

Radiation resistance is sometimes called a “fictitious” resistance, or a “virtual” resistance, but it represents an actual loss of voltage in the antenna feed circuit, proportional to the current, just like the Ohmic resistance.

How to measure the resistance of a UHF antenna?

The easiest and most accurate method of measuring the resistance of a UHF antenna is to use a digital multimeter. Measuring the resistance gives you the level that the electrical current is being impeded or hindered as it travels down the cable or circuit.

What should I do if my antenna resistance is too high?

Turn off the multimeter to preserve the batteries. Make sure that there is never any electrical power running to the device you are measuring as this could damage the device, multimeter or cause personal injury if the current is high enough. Laurie Brown has worked as a high school English teacher for the last several years and loves writing.

Is the radiation resistance only part of the feedpoint resistance?

The radiation resistance is only part of the feedpoint resistance at the antenna terminals. An antenna has other energy losses which appear as additional resistance at the antenna terminals; ohmic resistance of the metal antenna elements, ground losses from currents induced in the ground, and dielectric losses in insulating materials.