Many people know that a half-wavelength, center-fed dipole has radiation resistance of about 70 ohms at its resonant frequency. In a good antenna, most of the resistive part is the radiation resistance in free space and depends on the type of antenna. This occurs at the resonant frequency, and the inductive reactance and capacitive reactance are then called conjugates.Īll radio communication is based on sending AC energy through a transmission line and into a circuit that has the special property of radiating some of that energy into free space, which we know as the antenna. If X L and X C are equal, they cancel out, and there is zero phase shift. We denote reactance as “imaginary” numbers, +jX or – jX, so the impedance formula is often written Z=R+jX (“j” is the same mysterious i= √-1 that you met in junior high school). You can see that if the net reactance is zero, there is no phase shift, while if there is zero resistance, a pure inductance causes a +90 degrees phase shift, or -90 degrees for a pure capacitance.įigure 1. Resistance and net reactance form a right triangle the impedance Z is the length of the hypotenuse, and the angle it makes with the horizontal axis is the phase shift in the circuit. Unless the two are equal, there will be some net reactance on the vertical axis. ![]() On the graph, R is on the horizontal axis and X L and X C are on the vertical axis. Inductive reactance increases with frequency, while capacitive reactance decreases with frequency, and the phase shift also depends on the frequency because the inductance and capacitance shift it in the opposite directions. The resistance does not vary with the frequency of the AC source, but the reactance depends entirely on frequency. I will get around to ATUs and antennas soon but let us look at a circuit that you might build on your workbench (Figure 1). The resistor dissipates power just as for DC, but reactance only shifts the phase of the AC current relative to the voltage and does not consume any power. Resistors behave just as they do in a DC circuit, but the inductors and capacitors have the property called reactance. Some antenna matching units in my shack AC CircuitsĪll electrical circuits are a combination of resistance (R), inductance (L), and capacitance (C), that resists (impedes) the flow of current when alternating (AC) voltage is applied. A review of impedance matching will help explain why ATUs are used. Though these all look different, they have the same function they match the impedance of a “load”, such as an antenna, to the impedance of a “source”, such as a transmitter. On the left side of the photo below, an inexpensive automatic mini-ATU rests on a vintage manual tuner, while to the right is a much-modified $15 “eBay” portable tuner and an automatic MFJ-993B ATU. ![]() Why do I make this clarification, and why are they so popular regardless of what they are called? These are more correctly called Antenna Matching Units. If you operate on the high frequency (HF) amateur bands, you might have an antenna tuner built into your transceiver, or an external unit on your desk connected to it. In Part 2 I will cover the design decisions I made in building my own remote ATU and the details of the construction. In Part 1 of this article, I review some theory about antenna matching and the advantage of using a remote ATU. I started working on this project in midsummer, and now as Autumn is fading away, it is almost finished.
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