Quagi Antenna: A Quad and Yagi Hybrid for VHF and UHF Gain

A good beam antenna for 144, 432 or 1296 MHz does not have to be bought. It can be made over a weekend from wire, aluminium rod and a wooden boom, and one of the best homebrew designs for these bands is the Quagi antenna. The name is a blend of quad and Yagi, and it describes the construction exactly: the two rear elements, the reflector and the driven element, are square loops of the kind used in a quad antenna, and the elements in front of them are straight rods, as in a Yagi–Uda antenna. The loops give the antenna extra gain and a wide bandwidth, and the rods make it simple to build for the longer boom lengths. This guide explains how the Quagi works, what to expect from it, what the dimensions look like and how to build and feed one so that it performs.

What a Quagi Is Made Of

A Quagi has three kinds of elements, all mounted on one boom, which is the long bar that holds them in line. At the back there is a reflector, a square loop with a perimeter of slightly more than one wavelength. In front of it there is the driven element, a similar square loop with a perimeter of about one wavelength, which is connected to the feedline. In front of the driven element there are directors, which are straight rods of a length a little below half a wavelength, spaced along the boom. Each loop and each rod works as a parasitic or driven element, in the same way as in a Yagi antenna, and the whole system forms a directional pattern with its main lobe along the boom towards the directors.

The loops are the part that makes the Quagi different. A full-wave loop has more gain than a dipole of the same orientation, roughly one decibel more, and each loop in the antenna adds to the total. A loop also has a lower Q than a thin rod, so the SWR stays low over a wider range of frequencies. The combination is practical on VHF and UHF, where the loops are small enough to make and mount easily. On HF the loops are too large, which is why the full quad described in the article on the cubical quad is a project for the garden and not the shed.

Gain, Pattern and Bandwidth

As with any beam, the gain depends on the number of elements and on the length of the boom. The rule of thumb is that doubling the boom length adds about 3 decibels of gain, up to the limits of a practical design. A Quagi with two loops and a few directors reaches about the same gain as a Yagi that has two or three more elements, which makes it a favourite where the boom length is limited. A longer design with ten or more directors can reach double-digit gain in decibels over a dipole, which at 144 MHz is a very large signal improvement for a modest antenna.

The pattern is a single main lobe with a beamwidth that narrows as gain grows, and small side lobes if the elements are well made. The front-to-back ratio is good, since the loop reflector is an efficient screen. The bandwidth is wider than that of a comparable Yagi, so the SWR stays low across a segment such as the whole 70 cm band, or from the CW and SSB part of 144 MHz up to the FM part. This tolerance is what makes the Quagi forgiving for the home builder, because small errors in length do not turn the antenna into a failure.

Typical Dimensions

The following figures are the starting point for an ordinary design and not a replacement for a published plan. Loop and rod dimensions depend on the diameter of the material, the spacing and the boom. For a good result, build a design that someone has measured and published, and scale only within a small range of frequency.

• Driven loop: a square with a perimeter of about 306 divided by the frequency in MHz, in metres. At 144.2 MHz, this is about 2.12 metres, so each side is about 53 centimetres.
• Reflector loop: about 3 percent larger than the driven loop, with a perimeter of about 314 divided by the frequency in MHz, in metres. At 144.2 MHz, about 2.18 metres.
• Directors: straight rods of about 0.40 to 0.44 wavelength, which is somewhat shorter than a half-wave dipole. The first director is the longest and the later ones are a little shorter.
• Spacing: about 0.2 wavelength between the reflector loop and the driven loop, and about 0.15 to 0.25 wavelength between the following elements.

The formulas come from the usual quad rules, where a loop for a given frequency is one wavelength around, reduced a little by the velocity factor of the wire. At 432 MHz the same Quagi is a third of the size, and at 1296 MHz it is very small and can be made from copper wire or even stiff brass rod. This is why the design is so popular for 70 cm and 23 cm. The article on the 70 cm band explains where such an antenna is most useful.

Building the Boom and the Elements

The boom can be made of wood, fibreglass or plastic, or of metal if the design allows for it. A dry wooden boom, such as a square section of a good hardwood, is easy to work, cheap and non-conductive, and it does not detune the loops. It must be painted or sealed against the weather, with a varnish or a marine paint, because water in the wood changes its properties and lowers the performance. Metal booms need correction factors for the elements that pass through or touch them, and loops mounted on metal behave differently, so for a first project the non-metallic boom is the easy path.

The loops are made from copper wire, brass or aluminium rod, or stiff tubing, bent into a square or a diamond. For 144 MHz, a rigid rod of 4 to 6 millimetres holds its shape without supports, while a thinner wire needs spreaders made of plastic or fibreglass, in the form of a cross in the plane of the loop. At the feed point the loop has a small gap, which is the place where the coax connects. The directors can be aluminium rod or brazing rod, mounted through holes in the boom or on plastic clips. Use a stainless or plated fixing material, because dissimilar metals corrode, and seal the ends of tubing against water.

Feeding the Quagi

The impedance at the feed point of a loop in free space is about 100 ohms. The reflector and directors lower it, and a typical Quagi ends up in the range of 50 to 75 ohms, which can be fed directly by coax with an acceptable match. The loop is a balanced element and the coax is unbalanced, so a balun is advisable. Without it, current flows on the outside of the shield, the pattern is distorted and the SWR changes as the cable is moved. For VHF and UHF, a simple choke balun is effective: a few turns of the coax in a coil of about 10 centimetres in diameter, or ferrite beads placed over the cable near the feed. The article on baluns and ununs describes the principle.

The place where the coax attaches defines the polarization. If the feed is at the middle of a horizontal side of the loop, the polarization is horizontal, which is the standard for weak-signal work on SSB, CW and digital modes. If the feed is at the middle of a vertical side, the polarization is vertical, which is the choice for FM and repeaters. Pick the polarization before you build, and use the same orientation as the stations you want to work, because a mismatch between horizontal and vertical costs many decibels.

Tuning and Testing

A homebuilt antenna needs checking, but the Quagi is tolerant. With an antenna analyzer or a NanoVNA, measure the SWR across the band, with the antenna on its mast and some distance above the ground, because nearby metal changes the result. If the SWR is lowest below the band, the elements are too long, and a small trim of each loop or rod moves the curve up. If the SWR is high everywhere, check the feed point, the balun and the connector. Remember that the ground, a metal roof and the mast all change the match, so test in the place where the antenna will work. The guide on the NanoVNA shows how to calibrate and sweep.

What the Quagi Is Good For

For VHF and UHF weak-signal work, the Quagi is a natural choice. It works well in tropospheric openings, which are the subject of the article on tropospheric ducting, and it is a good antenna for meteor scatter, for contests such as the VHF and UHF events and for general SSB and CW use on 144 and 432 MHz. Two Quagis on one mast, stacked at a proper spacing, provide a further gain of up to 3 decibels, as the guide to stacked antennas explains. For satellite work with linear polarization the Quagi does not follow a satellite as well as a circularly polarized antenna, and for FM satellites a handheld Yagi or a turnstile antenna is usually better. For fixed point-to-point links, such as a link to a repeater or a local net, a Quagi with horizontal or vertical polarization gives a strong, clean signal.

Practical Tips From Builders

• Build a published design and follow its dimensions precisely, including the diameter of the material.
• Seal the boom and the cable entry, and drill a small drain hole in any tubular element.
• Keep the coax at a right angle to the boom for as long as possible behind the reflector.
• Choose low-loss coax. On 432 MHz, a poor cable can waste more than the gain of the antenna, as discussed in the article on coax cable choices.
• Do not use paint with a metallic pigment on the elements, and avoid insulating coatings that change the electrical length.
• Mount the antenna away from metal masts: a few metres of non-metallic mast section is ideal.

The Bottom Line

The Quagi antenna combines two square loops, the reflector and the driven element, with straight Yagi directors. It gives slightly more gain than a Yagi of the same boom length, has a wider SWR bandwidth and can be matched to 50 ohms with a balun. A loop perimeter of about 306 divided by the frequency in MHz, in metres, is the starting point for the driven loop, with a reflector about 3 percent larger. It is a good homebuilt beam for 144, 432 and 1296 MHz, especially for weak-signal work and contests. Build a published design on a dry, sealed boom, choose the polarization first and check the SWR on the mast.

Frequently Asked Questions

What is a Quagi antenna?

A hybrid beam with a square loop reflector and driven element and straight Yagi directors, used mainly on VHF and UHF.

Is a Quagi better than a Yagi?

For the same boom length it often gives a little more gain and a wider bandwidth, but it is harder to build because of the loops.

Do I need a balun on a Quagi?

Yes, it is advisable. The loop is balanced and the coax is not, so a choke balun keeps current off the shield and keeps the pattern clean.

What boom material is best?

A dry, sealed wooden or fibreglass boom is easiest, since it does not disturb the loops. A metal boom needs a design that allows for it.

Can I build a Quagi for 432 MHz?

Yes. It is a third of the size of a 144 MHz version, and popular among builders for 70 cm and 23 cm.

How do I choose horizontal or vertical polarization?

Match the stations you want to work: horizontal for SSB, CW and weak-signal work, vertical for FM and repeaters. Place the feed point on a horizontal or a vertical side accordingly.