Antenna Calculators

Radio frequency energy travels through free space as coupled oscillating electric (E) and magnetic (H) vectors perpendicular to each other and to the direction of propagation. The ratio of the electric field intensity to magnetic field intensity in vacuum equals the intrinsic impedance of free space, exactly 376.73 Ohms (commonly rounded to 120*pi Ohms).

When RF energy is delivered through a transmission line to an antenna, current flows along the conductive elements, accelerating electrons back and forth. This acceleration generates time-varying electromagnetic fields that detach from the physical structure and radiate into the far-field region.

In practice, the boundary conditions at the ends of an open wire force the RF current to zero while voltage reaches a maximum. For a half-wave dipole, the center represents a current antinode (maximum) and a voltage node (minimum), producing a theoretical center-feed resistance of approximately 73 Ohms in free space.

One mistake engineers often make is treating the speed of wave travel on an antenna wire as equal to the speed of light in vacuum. In physical reality, conductor surface resistance, skin depth, and capacitive fringing at wire ends reduce the guided wave velocity by 2% to 5%, shifting the resonant point downward.

Radiation resistance is an equivalent resistance that accounts for the power radiated away by the antenna as electromagnetic waves. A full-size half-wave dipole exhibits a radiation resistance of 73 Ohms, whereas an electrically short dipole (length less than 0.1 wavelength) can have a radiation resistance of less than 1 Ohm.

When radiation resistance drops to very low levels, ohmic resistance from the copper wire, coil windings, and soil connections dominates the circuit. In an electrically small antenna with 1 Ohm of radiation resistance and 2 Ohms of ground resistance, two-thirds of your transmitter power is lost as thermal waste.

A better way to think about antenna matching is conjugate matching. For maximum power transfer from transmitter to antenna, the generator impedance must equal the complex conjugate of the load impedance (Z_generator = Z_load*). When an antenna is operated off-resonance, inductive or capacitive reactance appears, requiring matching networks or tuning stubs to restore 50 Ohm resistive balance.

The performance of any antenna installed within several wavelengths of the Earth is heavily governed by ground reflection. Moist soil, clay, and ocean water act as lossy dielectric reflectors that bounce radiated energy back upward into the sky.

For horizontally polarized antennas such as wire dipoles, reflection at the ground interface produces a 180-degree phase shift. At an antenna height of 0.25 wavelengths, the reflected wave reinforces the direct wave directly overhead at 90 degrees, producing a high-angle cloud-burner pattern suited for Near Vertical Incidence Skywave (NVIS) communication.

When the dipole height is elevated to 0.5 wavelengths, ground reflection reinforces radiation at low take-off angles between 15 and 30 degrees. Low take-off angles are necessary for long-distance skywave communication because the signal enters the ionosphere at a shallow grazing angle, traveling greater distances per hop.

A single resonant dipole radiates uniformly in a figure-eight pattern perpendicular to the wire axis, yielding a peak gain of 2.15 dBi. By placing unpowered parasitic elements in close proximity (typically 0.1 to 0.2 wavelengths), mutual electromagnetic induction transfers energy between elements.

A reflector element cut slightly longer than resonance presents inductive reactance, delaying the induced current phase so that radiated waves cancel backward and reinforce forward. Conversely, director elements cut slightly shorter introduce capacitive phase advance, pulling the beam in the forward direction.

The practical difference is that adding parasitic elements lowers the feedpoint impedance of the driven element, often dropping it from 73 Ohms down to 15 to 25 Ohms. Dedicated matching systems, such as gamma matches, hairpin loops, or folded dipoles, are required to transform this low impedance back to standard 50 Ohm coax.

In workshop practice, wire antennas should never be trimmed by cutting off copper during initial tuning. Instead, strip the wire ends, pass them through the end insulators, and fold the excess wire back along the main element, securing it with electrical tape or cable ties.

Folding the wire back electrically detunes the folded section while preserving the physical copper. If you subsequently relocate the antenna to a new site with different soil conditions or higher tree supports, you can easily unfold the wire to lengthen the element without splicing.

Teams often run into this when tuning vertical antennas over real soil. Adding more radial ground wires does not shift the resonant frequency significantly, but it dramatically lowers ground loss resistance. This increases antenna system efficiency, which can cause measured VSWR to change because the total feedpoint resistance approaches the true radiation resistance.