Linear Loaded Dipole Antenna: An Efficient Shortened Antenna

A linear loaded dipole is a type of shortened antenna where a portion of the radiating element is folded back on itself to create a shorted transmission line stub. This fold adds the necessary inductive reactance to electrically lengthen the antenna without requiring the massive physical footprint of a full-size half-wave dipole. If you are trying to make an existing 80m dipole resonate on the 160m band without increasing its total physical length or shifting the feedpoint, linear loading is arguably the most efficient way to achieve it.

How It Works

When you fold the wire back parallel to the main element, the parallel section acts like a shorted transmission line stub rather than a pure radiator. This stub introduces inductance to the antenna system. Unlike a traditional loading coil which concentrates the inductance in one localized spot, linear loading distributes this inductance along a section of the antenna.

Why Choose Linear Loading Over Coils?

For homebrewing antennas—especially when running external power like linear amplifier—linear loading offers distinct advantages over traditional base or center-loading coils:

Feature	Linear Loading	Coil (Inductive) Loading
Efficiency (Q factor)	High. Lower resistive losses because the inductance is created by continuous, thick antenna wire.	Lower. Coils introduce resistive losses, which turn your RF output into heat.
Bandwidth	Broader. Maintains a slightly wider usable SWR curve across the band.	Narrow. High-Q coils create a sharp resonance, often requiring a tuner for small frequency shifts.
Power Handling	Excellent. Handled entirely by the wire gauge; no risk of arcing between tight coil windings.	Limited. Coils can overheat, melt, or arc if not heavily built for the amplifier’s output.
Construction	Mechanically complex. Requires rigid spreaders (like PVC conduit) to keep the wires parallel and evenly spaced in the wind.	Mechanically simple. Just insert a wound inductor in line with the wire.

Construction Basics

To build or modify one, the parallel wires must be kept perfectly separated to function correctly.

1. Spreaders: Use lightweight, non-conductive materials like CPVC pipe, fiberglass rods, or even electric fence insulators. Drill holes to route the wire through.
2. Spacing: The exact spacing between the parallel wires isn’t hyper-critical for resonance, but consistency is vital. A gap of 2 to 4 inches is standard for HF bands.
3. Wire Routing: The wire goes out from the feedpoint, passes through the spreaders, turns back at a specific length, routes back toward the center, and then turns again to extend outward for the rest of the antenna leg.
4. The Ladder Line Hack: Instead of dealing with individual wires and custom spreaders, many builders use a length of 300-ohm or 450-ohm twin-lead/ladder line for the folded section, shorting the ends as required to create the stub.

How do I calculate the specific wire lengths needed to build a 160m linear loaded dipole?

Calculating the exact wire lengths for a linear loaded dipole is more complex than a standard dipole because the folded parallel wires act as a shorted transmission line, introducing distributed inductance.

If your goal is to build this 160m antenna without increasing the total physical span beyond the footprint of a standard 80m dipole (approximately 130 feet total, or 65 feet per leg), you will need to rely on a three-wire fold to achieve resonance within that restricted space.

Here is how to calculate the dimensions.

1. The Target Electrical Length

First, calculate the full size of a standard half-wave dipole for your target frequency (e.g., 1.85 MHz). A common formula for the length of a dipole in feet is 468 divided by the frequency in MHz.

L_total = 468/f_MHz

L_total = 468/1.85 ≈ 253 feet

This means a full-size 160m dipole requires roughly 126.5 feet of physical space per leg.

2. The Physical Footprint Constraint

By keeping the antenna confined to roughly 65 feet per leg, you are physically missing about 61.5 feet of wire reach on each side. You cannot simply fold 61.5 feet of wire back into the available space to make up the difference.

When you fold the wire back parallel to itself, it forms a shorted transmission line stub. This stub generates distributed inductive reactance (X_L), which electrically lengthens the antenna. The inductive reactance provided by a shorted stub of length ℓ is calculated as:

X_L = Z₀ tan (2πℓ/λ)

Where:

• Z₀ is the characteristic impedance of the parallel wires (determined by wire thickness and the spacing of your spreaders).
• ℓ is the physical length of the folded section.
• λ is the wavelength.

Because this shorted stub acts as an inductor, the actual length of wire needed in the folded section is slightly less than the straight mathematical missing length.

3. Practical Reduction Ratios

Because calculating exact distributed capacitance and Z₀ in an open-air wire array is highly complex and sensitive to environmental factors, it is best to use established reduction ratios. The linear loading method effectively folds the dipole elements back on themselves.

• Two-Wire Fold: With a two-wire loaded dipole, the resonant frequency is reduced to about 0.67 to 0.70 of a standard dipole of the exact same physical length. This would reduce a 160m antenna to a physical footprint of roughly 85 feet per leg, which is still too long to fit an 80m footprint.
• Three-Wire Fold: With a three-wire loaded dipole, the resonant frequency is reduced to about 0.55 to 0.60 of a reference dipole of the same length. A reduction to 55% brings the required physical reach of our 160m dipole down to roughly 69 feet per leg, which closely matches your spatial limit.

4. Starting Dimensions for a 3-Wire Fold (Per Leg)

To achieve resonance within a 65-foot physical leg, use the following starting measurements.

1. Total Wire: Start with approximately 105 feet of continuous wire per leg.
2. The First Run: Run the wire from the center feedpoint outward for 25 feet.
3. The Fold-Back: Fold the wire back toward the feedpoint for 20 feet, threading it through non-conductive spreaders to maintain a consistent 2-to-4-inch spacing.
4. The Extension: Fold the wire outward one final time and run the remaining 60 feet out to your end insulator.
5. The Geometry Check: This puts your final end insulator exactly 65 feet from the feedpoint (25 ft out – 20 ft back + 60 ft extension = 65 ft physical reach).

Factors like the proximity of the ground and other nearby objects have a significant impact on the actual tuned length, so you should always make the prototype antenna slightly longer than calculated. Leave an extra 2 to 3 feet of wire at the far end of the extension, folding it back on itself, so you can trim the antenna while measuring its performance with an analyzer to find perfect resonance.