This article investigates the near field radiation characteristics of a quarter wave monopole antenna with a proven current, phase and voltage distribution in order to understand the effect of current direction on radiation. It has been proven that the vector difference current observed in the first article of this series Antenna Current Distribution in a grounded monopole antenna is sinusoidal. This current is referred to in the text books simply as 'antenna current distribution' and it is not indicated that this is a vector difference of the INCIDENT current going into the antenna and the REFLECTED current from the end of the antenna. The concept of reflected currents should be easily understood by those that are familiar with transmission lines but some readers may also wish to study Understanding Coaxial RF Transmission Lines by Measurement and Calculation in order to be convinced of reflected currents and their true behaviour.
1.2) Measurement Principle
In order to measure the radiation in one direction a bi-plane probe is required that has one measurement plane and one screening plane. The basic principle is shown in figure 1, although the actual probe used is a little more sophisticated with some extra screening and antistatic protection. The probe is positioned very close to the antenna but not touching it and is orientated to receive radiation in either the upwards or downwards directions.
1.3) Measurement Equipment
The probe used for the original measurements was made out of brass but copper would work equally as well. It can be seen in figure 2 that the rear and sides of the probe have been screened as the probe is prone to picking up external interference. The measurement plane is basically one plate of a capacitor and is very badly matched to the 50 Ohm coaxial cable, which causes in excess of 20 dB mismatch loss. However, with good quality coaxial cables and sound measurement techniques it is possible to obtain valid readings of radiation. Two (18K) resistors have been used to hold the measurement plane in position and also to provide a dc path to ground in order to prevent static build up which may damage the analyser input. The lower screen has an extension fitted in the photograph and this extension provides better isolation when the bi-plane probe is used at a small distance from the antenna.
1.4) Measurement Test Circuit
The test equipment is connected as per figure 3 using good quality cables with double screening to give high isolation.
The amplifier is used to bring the transmit level above any external interference and to provide adequate input level to the bi-plane probe assembly. The bridge enables the antenna input return loss to be constantly monitored and the readings to be taken at the optimum antenna matching frequency. Due to the fact that there are many reflections and subsequent phase additions and cancellations, the time domain facility of the vector network analyser is used to gate out all unwanted signals. Judicious use of the gate start and stop times is required to ensure that only wanted signals are measured and care has to be taken to calculate delay times both within and external to the antenna at their relative speeds (approximately c x 0.82 depending on the antenna velocity factor and c, the speed of light).
The original electrical measurements were made with a vector network analyser (VNA) in a quiet radio area (no screened room was available).
1.5) Sample Antenna
A simple monopole antenna is made using copper wire and should ideally be the actual antenna that was used for the previous antenna current and voltage distribution measurements. The antenna is mounted in a vertical orientation in order to ensure that any ground effects are appropriate to a real life antenna. The ground should be sufficient to ensure that the antenna is well matched to the 50 Ohm coaxial cable.
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