Space is often a premium for radio amateurs, but you don’t have to sacrifice performance to enjoy HF bands. The capacitor-loaded shortened dipole antenna, popularized by G7UNF, is a compact, multi-band antenna design that uses a center-mounted adjustable capacitor to tune resonance across several amateur radio bands.
This post explains how the antenna works, gives the key dimensions, and shows you the capacitance values for various frequencies.
The Antenna at a Glance
- Two arms of 2.5 m each (total span about 5 m).
- Center-fed with a coaxial cable.
- Adjustable series capacitor (C) at the feed point.
- Works on multiple HF bands by adjusting the capacitance.
This design is essentially a shortened dipole antenna. Because the elements are physically shorter than a half-wave at most HF bands, they are inductive. Adding a series capacitor cancels out this inductance and brings the antenna into resonance.
How It Works
A standard dipole resonates when each arm is about a quarter wavelength at the desired frequency. Shorter elements act inductively. By inserting a capacitor in series, the reactance of the antenna is neutralized, resulting in a near-50 ohm impedance that your transmitter loves.
The capacitor can be adjustable (ideal for experimenting and multi-band use) or fixed (optimized for a single frequency).
| Freq (MHz) | Cap (pF) | Z (Ohms) |
|---|---|---|
| 3.8 | 800 | 52.4 |
| 7.2 | 435 | 49.85 |
| 14.18 | 225 | 49.9 |
| 14.2 | 223 | 50.3 |
| 14.3 | 220 | 50.6 |
| 18.168 | 175 | 50.1 |
| 21.450 | 150 | 49.5 |
| 24.990 | 125 | 50.9 |
| 29.700 | 107 | 50.1 |
Building Tips
- Use high-quality wire for the dipole arms.
- Choose a high-voltage RF-rated capacitor— air-variable or vacuum capacitors are ideal for transmitting.
- Mount the antenna horizontally or as an inverted V for best performance.
- Test with an analyzer at low power and adjust the capacitor for minimum SWR on your band of interest
The capacitor-loaded shortened dipole antenna is a smart way to enjoy multiple HF bands from a small space. With just 5 meters of wire and a variable capacitor, you can cover everything from 80 m up to 10 m with excellent results.
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