Half-wave dipole antenna radiation pattern

[zak8000]

Homework Statement

i just have a few questions about the radiation pattern of a half wave dipole antenna.

firstly i know the for the half wave dipole we can dervie and expression for the electric field in the zenith angle and the magnetic field in the azimutal angle by using the far field approximation. so if i was asked to draw the radiaton pattern in the horizontal and vertical plane would the following images suffice i have provided a link: http://www.astrosurf.com/luxorion/Radio/dipole-radiation-pattern-low.gif

also if i was to compare a one wave dipole to a half wave dipoles radiation pattern would i expect a similair shape but double the magnitude?

finally if i had a choice to design between a half wave dipole and a 3/2 wave dipole antenna, which would result in a better antenna?. Is a half wave dipole a more practical antenna than the 3/2 wave dipole or vice versa or they are both practical

Homework Equations

The Attempt at a Solution

 

[vk6kro]

Your radiation patterns are for a vertical dipole.

I used the free program Eznec Demo to produce the following radiation patterns for horizontal antennas. Anyone interested in antennas should probably get this program.

These are azimuth diagrams, so they are like looking down from above.
In each case, the antenna is on the vertical line down the center of the circle.
[PLAIN]http://dl.dropbox.com/u/4222062/dipole%201%20and%202%20half%20waves.PNG
[PLAIN]http://dl.dropbox.com/u/4222062/dipole%203%20half%20waves.PNG

Notice that the pattern becomes more complex as the antenna gets longer in terms of wavelength and also the gain increases slightly.

The 2 half wave antenna has to be fed at the point 25% of the total length from one end. The center point is high impedance and so the antenna can't easily be driven there..
The other antennas are fed in the center.

These are free space diagrams and proximity to ground has a profound effect on the pattern of these antennas.

 

[zak8000]

thanks for the clear demonstration. so i was wondering if i had a choice between a half wave dipole and a 3 half wave dipole to design. then i should design a 3 half wave dipole antenna because it will have a higher gain ?

 

[vk6kro]

A 1 dB gain would not be worth putting up an antenna that was 3 times as long, if that was the only reason you were doing it.
You could get that much increase in signal by raising the antenna a few feet higher.

The advantage, if you had room for the bigger antenna, would be that the extra gain applied in more directions. A half wave dipole has a fairly broad radiation pattern but it is mostly at right angles to the dipole.
Three half waves have radiation as shown in the diagram above. Some lobes are at 45 degrees to the dipole and this may give better coverage in those directions.

Such antennas are mostly the sort of thing you can achieve if you own a 100 acre farm, or work for someone who does, though.
A half wave dipole on 3.5 MHz (a popular Radio Amateur band) is 132 ft long. Even this barely fits into a large suburban block. Three half waves would be 396 ft long which is out of the question for most of us.
At higher frequencies, where such an antenna might be reasonable, the Yagi antenna is a better choice. It gives good gain and it can be rotated.