Notes on Reactive Antenna Loads and Their NEC Models

L. B. Cebik, W4RNL

Although the basics of changing the electrical length of an antenna wire by adding a reactive load to it are seemingly well-known, the precise relationship between real loads provided to real antennas and modeled loads provided to modeled antennas is not so certain as one might think. Therefore, this series of notes will try to look at some of the ins and outs of the matter--all without pretending either to authoritativeness or completeness. It will be enough if we can get a better handhold on the subject.

In the first note, I focus on the means for implementing loads in modes as complex impedances, that is, as resistance and reactances, and as series R-L-C loads. The effort will be to translate a reactance into an inductance as might be found in a solenoid inductor of standard single layer winding. Moreover, we shall also focus only on center or source- point loading, whether the math of the effects of loading is simple addition. Eventually, however, we shall have to return to R+/-jX and translate this into the other major means of implementation: the shorted transmission line stub of under 1/4 wl. That means of load implementation has another name: the linear load.

Center or source-point loads are not the only reactive loading we can use to electrically lengthen an antenna. With dipoles and similarly balanced antennas, we can place loading reactances almost anywhere along the element, with the proviso that they be symmetrical in placement and value with respect to the element center. Actually, such loads do not have to be symmetrical, but symmetry does simplify most design and analysis problems. So we shall remain symmetrical to a fault.

Next, we shall address both solenoid inductors and transmission line stubs placed well away from center on antenna elements. They will show some interesting patterns of which we should be aware before taking the next more difficult modeling step.

NEC offers us short-cut methods of modeling both the solenoid and the stub reactive load via purely mathematical models. Before we conclude this sequence of notes, we shall investigate modeling both types of reactive loads--linear or transmission line loads and solenoid loads--as physical wire assemblies. One reason for taking pains to develop the models used in the preliminary investigation of using the LD and TL functions of NEC for antenna loads is that physically modeling both transmission line loads and single layer solenoid (or helical) loads requires care lest one exceed one or another limitation of the NEC modeling calculation system. The early models are developed with the later physical models in mind.

The results of physically modeling linear or transmission line loads and of physically modeling coils (helices) may hold a few surprises relative to some commonly held beliefs about load models. For one example, not all apparent mid-element linear loads are in fact mid-element loads. For another example, loading coils turn out not to be simple inductances (or inductive reactances), but are instead a complex combination of inductance and radiating antenna wire.

Although these notes may be useful in demonstrating a few principles, their numerical results cannot be automatically transferred to some existent antenna making use of loads. The final note will suggest some of the possible transfer errors that might be made, as well as some of the modeling challenges yet to be met when working with existing antennas.

Updated 2-6-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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