25-08-2017, 09:32 PM
Basic Wire Antennas
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Antenna Overview 1
An antenna is a device that:
Converts RF power applied to its feed point into electromagnetic radiation.
Intercepts energy from a passing electromagnetic radiation, which appears as RF voltage across the antenna’s feed point.
The intensity of the radiation launched by the antenna is generally not the same in all directions. This radiation pattern is the same whether the antenna is used to transmit or receive signals
The ratio of the maximum radiation by a given antenna to the radiation of a reference in the same direction is called the directivity:
Antenna Overview 2
Two common directivity measures:
dBi – dB referenced to an isotropic (equal radiation in all directions) radiator.
dBd – dB referenced to a half wavelength dipole (more about dipoles later).
The feed point impedance of an antenna is generally complex. The real component has two components:
Loss resistance due to the conductivity of the antenna itself and losses caused by other objects near the antenna (such as the ground)
Radiation resistance, which represents the transfer of power from the antenna into the radiated field.
In addition to the radiated electromagnetic field, also known as the far field, there is a field that exists only in the immediate vicinity of the antenna known as the near field. Power is stored in the near field, not radiated, although the near field can couple to other objects near the antenna and transfer RF power to them.
Both directivity and impedance are dependent of the frequency of the RF
Dipole Fundamentals
A dipole is antenna composed of a single radiating element split into two sections, not necessarily of equal length.
The RF power is fed into the split.
The radiators do not have to be straight.
Dipole Characteristics
Electrical length - the overall length of the dipole in wavelengths at the frequency of interest.
Directivity - the ratio of the maximum radiation of an antenna to the maximum radiation of a reference antenna. It is often measured in dBi, dB above an isotropic (non-directional) radiator.
Self Impedance - the impedance at the antenna’s feed point (not the feed point in the shack).
Radiation Resistance - a fictitious resistance that represents power flowing out of the antenna
Radiation Pattern - the intensity of the radiated RF as a function of direction.
The Short Dipole
The length is less
The self impedance is generally capacitive.
The radiation resistance is quite small and ohmic losses are high
SWR bandwidth is quite small, ~ 2% of design frequency.
Directivity is ~1.8 dBi. Radiation pattern resembles figure 8
Harmonic Operation of Dipoles
A /2 dipole is also resonant at integral multiples of its resonant frequency.
The self impedance of a /2 dipole at odd multiples of the resonant frequency is 100 - 150 ohms.
The self impedance at even multiples is > 1000 ohms
The directivity is never greater than the extended double Zepp.
The pattern is very complex, with many side lobes.
The G5RV: what is it, really?
The G5RV was originally designed as a 3 /2 antenna for use on 20 meters.
It was used as a multi-band antenna because when fed with ladder line (not coax!) it is easy to match the on any band from 80m to 10m
A G5RV used as a multi-band antenna should be fed with ladder line. Most commercially-made G5RV antennas are lossy because they are fed with coax.
There is no special length for a G5RV; it only needs to be at least /4 long at the lowest operating frequency.
There is nothing magic about a G5RV. It is just a dipole