July, 2008

– getting it right!

The Smith Chart was invented by Phillip H. Smith and first published in January 1939. It is a sophisticated tool designed primarily for solving transmission line problems, in which its inventor was very much involved. With all the advances in computer technology, the Smith Chart still has great relevance for radio engineers today. It is still the basic tool for determining transmission line impedances. At first glance its complexity may be somewhat of a challenge and somewhat off-putting, but if you take the trouble to find out how it works, you will find that it can be a very useful tool indeed – and one that you can use remotely - after an antenna has been installed...

Complex as this may sound, the Smith Chart is really just a specialised graph with curved coordinating lines, in the form of two sets of circles. The outer circle forms the Reactance axis. The Resistance Axis is its diameter. Values along the Resistance Axis range from 0 ( left) to infinity (right) with the Prime Centre in the middle. In fact 0, zero resistance, zero reactance is short circuit (0 +j0). Infinity, infinite resistance, is open circuit.

Touching the outer circle at infinity and centred on the Resistance Axis are a series of concentric circles representing Resistance. Where it cuts the axis, each circle is given a value. All of these values relate to the assigned value of the Prime Centre.

The use of coefficients or ratios, makes it possible to chart any impedance values for any type of uniform transmission line. Impedances (or admittances) go through a process of normalisation before being plotted, based on the value assigned to the Prime Centre. All you have to do to read a value is multiply the value on the Resistance Axis by the Prime Centre value. So, if the Prime Centre, 1.0, stands for 100 ohms, then the circle at 2.0 will have a value of 200W and the circle at 0.5 a value of 50 ohms. Similarly to plot a value you will need to divide by the Prime Centre value.

If you are just working with 50 ohm coaxial cable, it is possible to obtain charts with a 50 ohm prime centre value to make it easier.

The Reactance series of circles (actually only segments are visible) centre on infinity and fan out to the Reactance Axis. Values relating to the Prime Centre are shown where the segments cut the axis. Values above the resistance axis are positive (inductive), while those below are negative (capacitive).

When the resistance and reactance circles are superimposed, you get the Smith Chart. Using this chart you can plot complex impedances (R + jX). For clarity, the diagrams shown above only give a few values of resistance and reactance. Obviously normal Smith Charts are more detailed. (to be continued)

Directional Wideband Surveillance

- transmit and receive

The Moonraker BRX1500D system is designed for transmission and reception across the bands from 50 to 1500 MHz. The 25 element log periodic array can be swivelled through 90 degrees permitting horizontal, vertical or slant polarisation to be selected prior to erection. The complete mast and antenna can be rotated manually through 360 degrees when erected.

The antenna/mast can be erected by two men and may be either ground or roof mounted. The log periodic array has foldable elements (max length 1.5m) with a boom length of 2m. Maximum antenna length is 10.5m (34.5ft) in the fully vertical configuration. There are 3 sets of 4 RF transparent guys. Guy radius is 5m (20ft).

- BRX1500D

• optional erection kit (gin pole, guy stakes)

VHF Broadband Helical System - high performance, high durability The MHB3090 provides broadband performance, typically at <2.5:1 from 30 to 90 MHz. Antenna height is only 400mm (15.7 in). The heavy gauge copper wire helical is protected by black thermoplastic pressure tubing. This makes the antenna rugged, lightweight and unobtrusive, being stiff enough to stand upright yet flexible enough to bend into a U shape. The connector fitted into the antenna base connects direct to the connector on the transceiver. - type MBH30/90 various connector types available

Compact Guyed MF Whip

- keeps losses to a minimum

The Moonraker 15 metre (50ft) 150MF guyed whip offers compact performance in the 250 kHz to 3 MHz range for low power AM broadcast stations, non directional beacons and differential GPS base stations. Systems are tailored to suit individual frequency and ground conditions. A suitable ATU is required (not supplied). Antenna wind survival is to 216 km/h (134 mph). Supplied complete with guys and swing up base mount system. Earth mats are available to suit.

- type 150MF

• custom designed to your frequency and ground conditions