Moxon Calculator

For Designing 2-Element Rectangular Beam Antennas

What is a Moxon Calculator?

A moxon calculator is a specialized engineering tool used by amateur radio operators and antenna builders to determine the precise dimensions for constructing a Moxon antenna. The Moxon antenna, named after its inventor Les Moxon (G6XN), is a compact and efficient 2-element directional beam antenna. It is essentially a modified Yagi-Uda antenna where the element ends are folded towards each other, creating a rectangular shape. This design significantly reduces the antenna’s physical size (to about 70% of a full-size dipole) while maintaining good performance characteristics.

This calculator is essential because the performance of a Moxon antenna—specifically its gain, front-to-back ratio, and impedance—is critically dependent on its dimensions. The calculator takes a desired operating frequency and provides the five key measurements needed for construction.

Moxon Antenna Formula and Explanation

The Moxon calculator uses a set of empirical formulas derived from extensive modeling by experts like L. B. Cebik, W4RNL. These formulas calculate the dimensions based on the desired frequency (F) in MHz. The standard diagram identifies five critical dimensions: A, B, C, D, and E.

• Dimension A: The total side-to-side length of the driven and reflector elements.
• Dimension B: The length of the “tail” or folded-back portion of the driven element.
• Dimension C: The gap between the tips of the driven element and the reflector tails. This is a critical dimension for impedance matching and front-to-back ratio.
• Dimension D: The length of the tail of the reflector element.
• Dimension E: The total front-to-back spacing of the antenna (E = B + C + D).

The core formulas used in this calculator (providing results in feet) are:

• A (ft) = 303.3 / F (MHz)
• B (ft) = 53.6 / F (MHz)
• C (ft) = 10.3 / F (MHz)
• D (ft) = 66.8 / F (MHz)
• E (ft) = (53.6 + 10.3 + 66.8) / F (MHz) = 130.7 / F (MHz)

Variables Table

Variable	Meaning	Unit	Typical Range
F	Frequency	MHz	1.8 MHz – 440 MHz
A	Element Width	Meters, Feet, Inches	Varies with frequency
B	Driven Element Tail	Meters, Feet, Inches	Varies with frequency
C	End Gap	Meters, Feet, Inches	Varies with frequency
D	Reflector Tail	Meters, Feet, Inches	Varies with frequency
E	Total Spacing	Meters, Feet, Inches	Varies with frequency

Practical Examples

Example 1: 20-Meter Band (DX)

An operator wants to build a Moxon for the 20-meter amateur band, targeting the DX calling frequency of 14.200 MHz.

• Input Frequency: 14.200 MHz
• Results (Feet):
  • A = 21.36 ft
  • B = 3.77 ft
  • C = 0.73 ft
  • D = 4.70 ft
  • E = 9.20 ft

Example 2: 6-Meter Band (VHF “Magic Band”)

Another operator wants a compact beam for the 6-meter band, targeting 50.125 MHz.

• Input Frequency: 50.125 MHz
• Results (Inches):
  • A = 72.61 in
  • B = 12.83 in
  • C = 2.47 in
  • D = 16.00 in
  • E = 31.29 in

How to Use This Moxon Calculator

Using this calculator is a simple process to get you from frequency to finished design in seconds.

1. Enter Frequency: Input your desired center frequency in the “Frequency” field. The most common unit is MHz.
2. Select Output Units: Choose your preferred unit of measurement (Meters, Feet, or Inches) from the dropdown menu. Many builders find Feet or Inches easier for workshop construction.
3. Review Dimensions: The calculator will instantly update and display the five key dimensions (A, B, C, D, E) in the results section. The dynamic chart will also resize to give you a visual representation.
4. Interpret the Table: The table provides a clear, side-by-side summary of each dimension’s value, which is ideal for printing or referencing during the build. Check out our guide on {related_keywords} for more details.
5. Copy Results: Use the “Copy Results” button to save a text summary of all dimensions and the input frequency to your clipboard for easy note-taking.

Key Factors That Affect Moxon Antenna Performance

While the moxon calculator provides the ideal dimensions, several real-world factors can influence the final performance of your antenna.

• Height Above Ground: The antenna’s height, measured in wavelengths, dramatically affects its radiation angle and gain. A lower height is often suitable for local (NVIS) communication, while higher placements are better for long-distance (DX) contacts.
• Wire Diameter/Element Tubing: The formulas are slightly dependent on the diameter of the wire or tubing used. Thicker elements can slightly increase the antenna’s bandwidth. Our calculator uses an average wire gauge for its formulas.
• Wire Insulation: Using insulated wire instead of bare wire will change the wire’s “velocity factor,” making it electrically longer. This often requires shortening the physical elements by a few percent (typically 2-4%) to achieve resonance at the target frequency.
• Construction Materials: The material used for the supporting structure (the “spreader”) should ideally be non-conductive, such as fiberglass, PVC, or wood, to prevent it from interacting with the antenna’s electromagnetic field.
• Nearby Objects: Proximity to metal objects like gutters, siding, other antennas, or even dense foliage can detune the antenna and skew its radiation pattern. Always try to install it in as clear an area as possible.
• Accuracy of Construction: The gap spacing (Dimension C) is the most critical measurement for achieving a high front-to-back ratio and a 50-ohm impedance match. Precision in this dimension pays the most significant dividends in performance. You might find our article on {related_keywords} useful for comparison.

Frequently Asked Questions (FAQ)

Q: What is the main advantage of a Moxon antenna?
A: The main advantages are its compact size (about 30% smaller than a full-size Yagi) and its excellent front-to-back ratio, which helps reject noise and interference from behind the antenna. It also provides a direct 50-ohm feedpoint impedance, meaning no complex matching networks are required.

Q: Can I use insulated wire for my Moxon?
A: Yes, but you must account for the velocity factor. Insulated wire is electrically longer than bare wire of the same physical length. You will need to shorten the dimensions from the moxon calculator by about 2-4% to compensate.

Q: How critical is the gap (Dimension C)?
A: Extremely critical. This dimension has the largest effect on the antenna’s feedpoint impedance and its front-to-back ratio. Measure this gap as accurately as possible. For more on antenna theory, see our page on {related_keywords}.

Q: Can I build a multi-band Moxon?
A: While possible, designing a multi-band Moxon is notoriously difficult because the elements for different bands interact with each other, making tuning very complex. Single-band Moxons are far more common and reliable.

Q: Does the material of the spreaders matter?
A: Yes. You should use a non-conductive material like fiberglass, PVC pipe, or bamboo to hold the wire in its rectangular shape. Using metal spreaders will interfere with the antenna and detune it.

Q: Should the antenna be mounted horizontally or vertically?
A: It depends on the desired polarization. For HF bands (like 20m or 10m), horizontal mounting is standard for DX. For VHF/UHF FM operating (like on the 2m or 6m bands), vertical mounting is typically used to match the polarization of repeater and mobile stations. You may want to check out some {related_keywords} to visualize this.

Q: How high should I mount my Moxon?
A: A common rule of thumb is to mount the antenna at least one-half wavelength above the ground. For the 20-meter band, this would be around 33 feet (10 meters). Higher is often better for long-distance communication.

Q: My SWR is too high. What should I adjust?
A: First, re-check all your measurements, especially Dimension C (the gap). If the SWR is lowest *below* your target frequency, the elements are too long. If it’s lowest *above* your target, the elements are too short. Small adjustments to the driven element (A and B) length are the best way to tune the SWR. To learn more, read about {related_keywords}.