Wednesday, November 11, 2009

Antenna Analysis

This morning's discussion on 75 m covered the characteristics of a commercially-available doublet antenna of 130 ft total length, center-fed with 75 ft of 450-ohm open line. The following additional parameters were used in analysis performed with the 4NEC2 antenna modeling software:
  • 12-gauge copper conductors
  • height above ground 30 ft
  • average ground characteristics
Below is a sequence of horizontal radiation patterns, computed at one frequency for each of the 9 HF bands. The patterns for 80-40 m are very similar in shape. Starting at 30 m, additional lobes appear. You can click on the figure to select and examine any slide more closely.



Using the program's 3D features, the following plot shows the complexity of the antenna's radiation pattern at 28.5 MHz. This is an overhead view, with antenna oriented from left to right. Click on the figure to enlarge.


A frequency sweep from 1.8 to 29.7 MHz was made to investigate the behavior of the antenna's impedance at the transmitter end of the feed line. The following figure shows the calculated values of resistance, reactance, impedance, and phase at increments of 0.2 MHz. Numerical values are available in the program's output file, and one set of numbers (at 7.2 MHz) are shown on the figure. Click on the figure to enlage.


It can be seen that the antenna's impedance (green curve) is fairly high (several hundred ohms) at many of the ham band frequencies, confirming the need for a tuner to achieve matching to a 50-ohm transmitter. The program includes a feature for calculating the needed matching components. This could be used to predict the ability of a given tuner to achieve a match at any given frequency.

John WA5MLF

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