Are we all singing from the same hymnal? So that we’re on the same page, let’s first review a little piece of antenna basics in simple terms. If you know this subject inside-out, skip this segment or your eyes may glaze over. Here we go.

Either the dipole or its half brother the monopole over a groundplane is a capacitor-like device we call an antenna to which we connect the output of an alternating current generator we call a radio transmitter. In order for current to flow, any such device must have two terminals connected to the two output terminals of the generator. The radio frequency (RF) alternating current (AC) causes electrostatic and electromagnetic fields to be created between the two elements of the capacitor/device/antenna.

The energy in those fields is proportional to the RF current flowing in the elements. So, more current in the elements means stronger fields. Energy in those fields is “lost” from the system on each phase reversal of the alternating current. That lost energy is what we call our radiated signal.

As in any circuit, maximum current will flow when resistances are reduced to a minimum. The resistances in a monopole/groundplane include losses in conductors and in the plane itself. These are heat losses. Plus, there is “radiation resistance”. This figure is the apparent resistance of the antenna that can be attributed to the radiated energy. Therefore, radiation resistance is the only “acceptable” resistance, if you will, and it is determined by the size and configuration of the antenna. Also, if the antenna isn’t resonant, there will be either capacitive or inductive reactance present that will act as a resistance to AC and will further “impede” RF current flow.

Resonance is the condition that exists when the capacitive and inductive reactances are equal, and cancel each other. Therefore, one of the ways to maximize current and radiation is to “resonate” the antenna by adjusting the length and diameter physically and/or electrically. Another way to improve things is to use lower resistance conductors and in the case of a groundplane, make the “plane” part bigger and/or more solidly conductive. That’s easier said than done in the case of the vehicle we use for our mobile setup and often the backyard we use to erect a vertical for 1.8 or 3.8 MHz, for example. Nevertheless, to achieve maximum radiation the objective is for the RF energy to “see” only the radiation resistance at the feedpoint, or as close as we can come to that condition.

Applying these basics to the case in point, the “full sized” monopole over a groundplane has been “sized” for resonance. As it turns out, at about a quarter wavelength and multiples thereof, depending on such things as cross sectional area, the inherent capacitive and inductive reactances of the monopole element will be equal and will cancel. The monopole is a series-resonant circuit in itself, when fed against an appropriate counterpoise such as a plane or an apposing monopole.

The problem is, full size monopoles for the lower amateur bands are too ungainly for our cars, some of our backyards, and sometimes our pocketbooks, so we often seek to achieve resonance on monopoles much shorter than a quarter wavelength. There are several ways to do this. Since shortening the monopole element reduces both its inherent capacitance and inductance, we can add them back in a more compact form like either “hats” or coils… or maybe both. These may be added anywhere along the monopole, but their positions will determine, to a great extent, the radiation resistance, where in the antenna the current will flow, the size of the fields between elements, and therefore the amount of radiation that occurs. The term “resonator” is often applied to a loading system that has both inductance and capacitance.

The same principles described for monopoles or groundplanes apply to both elements of a dipole. For detailed information about the location of loading elements, read the QEX articles “Actual Measured Performance of Short, Loaded Antennas-Part 1 and Part 2, January and March 2014 by Barry A. Boothe