Loaded and short antennas

What it is

Any antenna shorter than its natural resonant length, made to behave as though it were longer. The shrinking is done by adding inductance (a loading coil), adding capacitance (a top hat), or folding the wire into a helix or meander. It makes antennas fit on a car, a handheld or a small lot. It never makes them as efficient as a full-size antenna.

Why short means inefficient

An antenna turns power into radio waves through its radiation resistance, a fictional resistor that stands in for the power that leaves as waves. Everything else, such as the coil's wire, the ground and the conductors, is loss resistance. They sit in series, so the same current flows through both and power splits in proportion:

A short antenna has little radiation resistance. Losses do not shrink with it, so the wasted share grows. Approximate numbers for a short whip over ground, with 5 Ω of coil and ground loss (illustrative):

Whip height	Radiation resistance	Efficiency
¼ λ (full size)	36 Ω	88%
0.15 λ	about 9 Ω	about 64%
0.10 λ	about 4 Ω	about 44%
0.05 λ	about 1 Ω	about 17%

The slider shows the other half of the trade: the coil that cancels the whip's reactance has its own resistance, equal to its reactance divided by its Q. A better coil, with thick wire and a good form, wastes less. A shorter whip needs a bigger coil and loses more.

Narrow bandwidth

To add reactance without adding radiation, the antenna ends up with large stored energy compared with the power it radiates: a high Q. High Q gives a narrow bandwidth, so a loaded antenna must be retuned when you move across a band. The loss and the narrowness are two sides of the same fact.

Making the best of it

• Spend length where the current is. Radiation comes from current, and current is greatest near the feed point. Top-loading with a hat and center-loading with a coil let more of the radiator carry current than a coil at the base; see Mobile whips and loading coils.
• Build a good ground. On a vertical, ground loss adds to the problem; see Radials and ground planes.
• Know when it matters less. On the low HF bands the noise from the sky is high enough that a weak signal is limited by noise, not by the antenna, so a small loss hurts receiving less than it hurts transmitting.

Related

• Mobile whips and loading coilsShort antennas that work on a vehicle.
• Small magnetic loopsTiny, high-Q antennas with a narrow bandwidth.
• Bandwidth and efficiencyHow wide an antenna works and how much power is actually radiated.
• Antenna impedance and radiation resistanceWhat the feed line sees at the feed point.
• Resonance and QTuned circuits, bandwidth and selectivity.
• Handheld radio antennasRubber ducks, whips and getting more from an HT.
• Portable and field antennasLight, quick designs for SOTA, POTA and camping.