The Moxon rectangle
The Moxon is a two-element beam with its element ends folded towards each other, making a rectangle about 70% as wide as a full-size Yagi. The coupling between the folded tips gives it a good front-to-back ratio across a whole band and a feed impedance close to 50 Ω, so it needs no matching network. It's a favourite for portable work, attics and small gardens, and makes an excellent 6m or 2m beam.
The dimensions here use the design equations published by L. B. Cebik, W4RNL, which adjust every dimension for the diameter of the wire or tube. That's why the calculator asks for it. A small change in the gap between the tips (C) moves the front-to-back null, so make the gap adjustable on your first build.
The delta loop
A delta loop is a single full-wavelength loop of wire in a triangle. Compared with a dipole it has a little gain, and many operators find it quieter on receive. It only needs one high support at the top corner, or two at the top for an inverted triangle.
Loop length (ft) = 1005 ÷ frequency (MHz)
Where you feed it decides the polarisation. Fed in the middle of the bottom side, it's horizontally polarised. Fed a quarter of the way round from the top, part way down one side, it's vertically polarised with a lower angle of radiation, which is often better for DX. The feed impedance is roughly 100 to 120 Ω, so a quarter-wave section of 75 Ω coax between the loop and your 50 Ω feedline makes a good match. Its physical length is a quarter wavelength multiplied by the coax's velocity factor.
The cubical quad
The two-element quad uses two square full-wave loops: a driven element and a slightly larger reflector behind it. It gives similar gain to a three-element Yagi in a smaller boom length, and the loops tend to be quieter than straight elements. The classic formulas are:
Reflector loop (ft) = 1030 ÷ frequency (MHz)
Element spacing = 0.15 to 0.2 wavelength
Each side of a square loop is a quarter of the loop length. Fed in the centre of the bottom side the quad is horizontally polarised, with a feed impedance near 50 to 75 Ω at typical spacings. Many builders add a short stub to the reflector so it can be tuned for the best front-to-back ratio.
Tuning tips
- Build slightly long and trim. The formulas assume the antenna is in the clear, and nearby objects lower the resonant frequency.
- Tune the driven element for the lowest SWR, then adjust the reflector or the Moxon gap while watching a signal from behind to find the deepest null.
- Insulated wire makes antennas resonate a few percent lower than bare wire. Allow for it by trimming.
- Use a common-mode choke at the feedpoint. The choke and balun calculator will help.
Questions
How big is a 20m Moxon?
About 7.7 m wide and 2.8 m deep with 2 mm wire, so it fits where a full-size 20m Yagi won't.
Is a delta loop better than a dipole?
A full-wave loop has about 1 dB more gain than a dipole and is often quieter on receive. Fed for vertical polarisation it gives a lower radiation angle, which helps DX. In practice, height and surroundings matter as much as the antenna type.
What's the difference between a quad and a Yagi?
A quad uses full-wave loops instead of half-wave straight elements. A two-element quad gives roughly the gain of a three-element Yagi on a shorter boom, but it's bulkier and harder to make strong enough to survive wind and ice.
Can I use these formulas for VHF?
Yes. The Moxon formulas work well for 6m, 4m and 2m, where tubing is common, so enter the real tube diameter. Delta loops and quads also work on VHF, often built from stiff wire or rod.