Contents
What is the radiation resistance of short dipole antenna?
Radiation resistance of common antennas
| Antenna | Radiation resistance ohms |
|---|---|
| Short dipole of length | |
| Base-fed quarter-wave monopole over perfectly conducting ground | 36.5 |
| Short monopole of length over perfectly conducting ground | |
| Resonant loop antenna, 1 circumference | ~100 |
What is the formula for antenna efficiency?
Radiation efficiency is defined by IEEE Std 145-1993 “Standard Definitions of Terms for Antennas” as “The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.” It is sometimes expressed as a percentage (less than 100), and is frequency dependent.
What is the effective length of a short dipole antenna?
The effective length d eff of a short dipole is approximately half its physical length [see (10.2.25) and Figure 10.2.3]. The reactance X of a short dipole antenna can be found using (10.3.15); it results primarily from the energy stored in the near fields.
What is the relative gain of a dipole antenna?
The dipole antenna has a relative gain of + 2.15 dB compared to an isotropic antenna. This relative gain is not surprising as the dipole has some directivity. Although the half wave dipole exhibits useful power gain over an isotropic antenna, its dimensions can be cumbersome for portable devices.
How big does a half wave dipole need to be?
Although the half wave dipole exhibits useful power gain over an isotropic antenna, its dimensions can be cumbersome for portable devices. As we saw previously a VHF dipole designed for operation at a frequency of 150 MHz would need a total length of one meter.
Which is better a 5 / 4 wave antenna or a half wave antenna?
A 5/4-wave dipole antenna has a much lower but not purely resistive feedpoint impedance, which requires a matching network to the impedance of the transmission line. Its gain is about 3 dB greater than a half-wave dipole, the highest gain of any dipole of any similar length.