Antenna current null

When currents flow one way on one part of an antenna and in the opposite direction on another part, they cancel each other with regard to radiation towards the viewer assuming they are in the same plane. This approach works best for planar antennas where the elements are connected together, such as loops or various types of curtain arrays.

The current is maximum in the centers of the two horizontal wires, and minimum in the center of the two vertical wires. That means that there is a major lobe broadside to the loop, that is, in the direction of the reader.

Here are the basic rules that I have used to analyze the current distribution on antennas, even before antenna modeling software was available:. We can then look at the directions and locations of the arrows and make some observations about the resulting antenna behavior. We start by placing a black dot at each end, because the wires are not connected to anything else. Of course, you are welcome to use whatever symbols you prefer. That happens to be the feedpoint, so we go work back from the other end… which comes to the same red arrow at the feedpoint.

Because the phase is the same on each side of the feedpoint, we already know the arrow is pointing in the correct direction. Because there is an arrow at the feedpoint, we know that it is a comparatively low impedance feed. We start the same way, with minimum current at the ends.

The result is that we have two half wave radiators, with the current in phase, so they will add in the direction of the reader. Because the feedpoint is at a current minimum a dot , the feedpoint impedance will be high. Because there are two radiating portions in phase along the length of the wire, we get some colinear gain, making the pattern narrower than a half wave dipole.

Like this:. It is zero at the open end and passes through minima at distances that are a half wavelength away from the end. Fig 2. The arrows represent the direction of current flow i. Equation 3. The current distribution represented by Eq. This condition is never realised perfectly in practice, because obviously an antenna will radiate some energy and therefore will consume some energy.

These are shown in Figure 3 and the calculations are performed in antenna-current. Those familiar with vector network analyser measurements will recognise this as establishing the measurement reference plane at the very base of the antenna.

The Signal Generator is set to a low frequency which is then gradually increased until a null is reached on the DMM at approximately the calculated quarter wave frequency. The electrical length is then calculated from the frequency of the Signal Generator using Equation 1.

The circuit is then re-configured as per figure 12 and the antenna length is trimmed to achieve a current null. The generator frequency can be changed during this process in order to assess how the trimming is progressing but should be returned to the same frequency as found in the cable measurement.

The overall electrical length of the cable and antenna is then calculated using Equation 1. The electrical length of the antenna is then calculated by subtracting the two electrical lengths obtained which in this case gives an antenna electrical length of 1.