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Forming Reasonable Expectations
of Modern Tri-Band Beam Designs
By L. B. Cebik, W4RNL
(A Posthumous Publication)

The most modern tri-band beams for 20, 15, and 10 meters have given up traps in favor of using individual elements for each band. Optibeam, Force 12, and Bencher all have beams based on this premise. Even for relatively close total boom lengths, the element positions vary considerably. The feed systems use either open-sleeve coupling or direct-drive systems—or a combination of both. Compared to older trap and hybrid designs, the newer beams present a somewhat bewildering appearance, as if the boom were a porcupine on a bad-hair day.

The somewhat confusing appearance presents many would-be users with more than one quandary. Perhaps the most central is this question: what should I reasonably expect by way of performance from these beams? We are not wholly at the mercy of the manufacturer’s specification sheets in forming our expectations. Instead, if we can understand what goes into the design of such beams and can develop an understanding of the performance of appropriately comparable monoband beams, we can develop expectations that fit virtually any of the newer designs. The goal of this set of notes is to go some distance in this direction.

Electromagnetic Fields of a
Small Helical Toroidal Antenna
By Kirk T. McDonald, PhD
Joseph Henry Laboratories, Princeton University

In these notes, we shall discuss the electromagnetic fields of a helical toroidal winding with major radius a and minor radius b, as shown below, that carries is an oscillatory surface current that has no azimuthal component. There is no accumulation of charge anywhere. It suffices to consider the limit that we shall calculate where a component is the wave number and c is the speed of light.

A physical realization of this winding is shown as perhaps first proposed by Corum where two counter windings of N/2 turns are each driven with current such that the magnetic dipole moment is zero. A complication in this case is that the N close crossings of the two windings, there is localized, oscillating charge accumulation due to the capacitance between the nearby wires, which leads to azimuthal variation of the currents in the windings.

A solution follows which assumed sheet currents as stated above. We first consider the static limit of sheet currents represented by N terms of steady current with no azimuthal component. Then, the electric field is zero everywhere, the magnetic field is zero outside, the torus, while inside the torus we use Ampere's law (in Gaussian units) to find the answer.

The fact that both static electric and magnetic vanish outside the torus implies that all multiple moments vanished for a conventional multipole expansion in this region. However, the vector potential is certainly nonzero inside the torus, so that continuity of the vector potential at its surface implies that the vector potential is nonzero outside the torus as well.

To find the vector potential and the electric and magnetic fields in the case of oscillating currents in the windings, we follow a method due to Hertz. The electric and magnetic fields, E and B can be derived from the scalar and vector potentials V and A according to an equation in Gaussian units.

Measurement of a High-Frequency Current
By Igor Grigorov

Transmission Lines
By Ted Hart

Every antenna book usually has a long detailed section on transmission lines. You should read all that on a cold winter evening when you have nothing better to do, or if you want to design a special application antenna configuration. For simplicity, and adequate information for the average amateur, all you really need to know is a little bit about coax.

Sure, there are special applications for parallel conductor open wire transmission lines, but they don't hook up directly to the coax connector on the back of your rig. You need matching networks on both ends and, if you don't really know all about those things, you can't build a system that will use those techniques to advantage.

Home-Brew Design and Construction
By L.B. Cebik, W4RNL (SK)

Not every amateur wishes to purchase equipment. Some amateurs prefer to build rather than buy the basic equipment which goes into the shack. On the chance that thoughts of this nature have entered your immediate or future plans, we should look at some of the design and construction features of home-brew equipment in at least enough detail to permit some decisions.

Transmission Line Theory
A Contribution From Academia
National Institute of Technology, India