Does a resonant antenna perform better than a non-resonant antenna? The answer is no. As long as both antennas have similar impedances to the transmitter, they will perform equally well.
Let us start by defining impedance. Impedance is the resistive element of an AC circuit. It is described as a complex number with a real part and an imaginary part. A typical impedance may look like this: 50+j50 ohms. The real part is the first number 50 ohms which is the resistive component and what most people use when referring to impedance. The second number +j50 (could be -j50) is the imaginary part. The letter j stands for the square root of a negative 1. This number represents the reactive component of impedance and can be inductive (+j50) or capacitive (-j50). It is important when speaking about antenna impedance that the resistive and reactive components are referenced. One use of the imaginary part is to determine the phase relationship between voltage and current. When the imaginary part is +j0, both are in phase and maximum power is transmitted to the antenna system. Looking at our sample impedance (50+j50 ohms)we can deduce the phase using the following formula:
Because the imaginary part is positive (+) we know that the current lags the voltage.
It is important where the measurement is taken. Taking the measurement at the antenna will give a different impedance than one taken at the feed point. For this discussion, we will use the feed point where the coax from the antenna (antenna system) is hooked up to the radio or the antenna tuner. I want to take a moment here and discuss antenna tuners. Antenna tuners do not “tune” an antenna. All they do is match the antenna system’s impedance to the transceiver’s. Most if not all modern transceivers have a 50+j0 ohm impedance. A better term for an antenna tuner would be transmatch, which is an older name for a tuner.
A resonant antenna will have a purely resistive load which will allow the transceiver to transmit at full power. The reactive portion of impedance will be +j0. In most cases that would be 50+j0 ohms. But wait! 30+j0 ohms is also a resonant antenna and so is 80+j0 ohms. While the latter two antennas are resonant, they may not work very well with a transceiver that requires 50+j0 ohms. An antenna is only resonant on one frequency. Once you move off of that frequency, the antenna is no longer resonant. In fact, non-resonant or random length wire antennas often perform better on multiple bands because they are easier to match. I have made 1,000’s of contacts with non-resonant antennas.
Plotting the impedance on a Smith Chart gives us a VSWR of approximately 2.6:1. This shows about 20% of the total power reflected back. Because of a fairly low VSWR, most transceiver tuners can make an acceptable impedance match. The transmatch would apply a capacitance in the amount of -j50 ohms to the feed point which would cancel out the +j50 of the antenna system changing the impedance to 50+j0 ohms allowing full power to the antenna system.
This is a nontechnical view of antenna impedance based on a conversation I had over a year ago. I was musing about the subject and before I knew it, I had 3 technical books and about half a dozen websites open. There is a lot of cool math for antennas and if I can break it up into usable pieces I may try to post more here. In the meantime I’ll close with a quotation from the ARRL Antenna Book for Radio Communications 24th Edition, page 1.6.
An antenna need not be resonant in order to be an effective radiator, There is in fact nothing magic about having a resonant antenna, provided of course that you can devise some efficient means to feed the antenna…It is important to consider an antenna and its feed line as a system in which all losses should be kept to a minimum.
KK4Z 