Antenna Theory - What's Happening

 Antenna Theory – A small bit, anyway

It is an early Sunday evening, and you have been in the shack for over an hour. You have been tuning the HF bands on your new Icom 7300 transceiver. Linked to the 40-meter half-wave dipole that stretches from the far yard tree to the house eave, you are experiencing noiseless, 1.2 SWR transmissions and picking up DX stations in South America and the Pacific rim.

Your wife calling you to dinner quickly snaps you out of your Tuesday, after work nap. It's time to get up from the Lazy-Boy lounge chair. While at the dinner table, you reflect on the dream that you just had. It was all a fantasy; you get no such performance from your ham shack setup. You know it is not good, but you don’t know why.  It's time to dig into what may be happening.

Radiation occurs when electric charges accelerate. Constant-velocity current does not radiate; time-varying current does. In a half-wave dipole, the RF source forces electrons to speed up, slow down, stop and reverse direction every half cycle. That acceleration is the root cause of radiation. It is all taking place at near the speed of light.

A center-fed half-wave dipole supports a standing wave which consists of: Current that is maximum at the feed point and which falls sinusoidally to zero at the ends. At the same time Voltage (charge) is changing sinusoidally from minimum at the feed point to maximum at the ends. So at any instant: charges are moving fastest near the center, and they are piling up and dispersing near the ends. Both effects matter.

Any current in a wire produces a magnetic (H) field. The alternating current creates a time-varying magnetic field and field lines form loops around the wire. At the same moment, charge buildup at the ends creates an electric (E) field. As charge increases/decreases, the E-field changes in time. A changing E creates an H, and a changing H creates an E.

Near the antenna: the E and H fields are mostly reactive, i.e., energy is stored and returned to the antenna each cycle. Farther out: the E and H fields become perpendicular to each other, in phase, and locked in a fixed ratio. When that happens: the fields no longer return energy, and they propagate outward at the speed of light. That outward flow is electromagnetic radiation.

A ½ wavelength antenna is special. With this length: current distribution is smooth and sinusoidal, fields from different parts of the wire add constructively, destructive cancellation is minimized, and the radiation resistance ≈ 73 Ω. Whereas, for a shorter antenna: they will store more energy than they radiate, have lower radiation resistance, and they demonstrate poor efficiency. An antenna that is longer than a ½ wavelength antenna: develop phase reversals and create multiple lobes and nulls.

The antenna’s radiation pattern, or space behavior is important to note. For directionality it has maximum radiation broadside to the wire and there is near-zero radiation off the ends. The radiation around the antenna has a torus (donut), 3-D shape. The polarization or orientation of the E-field is horizontal polarization for a horizontal dipole, and it is vertically polarized for a vertical dipole.

If you picture a one-cycle mental movie it may look something like this: current increases → magnetic field expands, charges accumulate → electric field strengthens, current reverses → fields reverse while old fields keep moving outward while new one's form. Each cycle launches another “shell” of EM energy.

How about a one-sentence summary: A half-wave dipole radiates because alternating current accelerates charges, creating time-varying electric and magnetic fields that detach from the antenna and propagate outward as electromagnetic waves, strongest broadside to the wire.