Delta Loop Antenna: A Full-Wave Loop for HF DX
If you have one tall support and some wire, a delta loop antenna is one of the most rewarding HF antennas you can build. It is a single loop of wire, roughly one wavelength around, shaped as a triangle. It needs no radials, it is often easier to hang than a full-size beam, and depending on where you feed it, it can favour low-angle DX or higher-angle regional contacts. It is also forgiving to build, which makes it a favourite for portable operators and for stations that want a step up from a dipole without a tower.
This guide explains how a full-wave delta loop works, how the shape and feed point change its behaviour, how to match it to coax, and how to tune it. It deliberately avoids quoting exact cut lengths for specific bands, because the right length depends on wire, insulation, height and surroundings. Start from a standard loop formula in a reliable handbook or model it, and then trim on site.
How a Full-Wave Loop Works
A full-wave loop is a closed conductor whose total perimeter is close to one wavelength at the operating frequency. When you feed it at one point, current flows around the loop and forms a standing wave pattern with current maxima at the feed point and at the point opposite it, and current minima a quarter wavelength either side. The direction of the current in different parts of the loop determines how the fields add up.
The result is a radiation pattern that, in the plane perpendicular to the loop, has maximum radiation broadside to the wire, much like two closely spaced dipoles stacked one above the other. That stacking effect is the origin of the small gain a full-wave loop can show over a single dipole at the same average height. The loop is also the basic element of the cubical quad antenna, which uses square loops as driven element and reflector.
Unlike a small magnetic loop, which is a tiny fraction of a wavelength and behaves very differently, a full-wave loop is a resonant antenna with a relatively wide bandwidth and modest feed point impedance. The general family of loop antennas covers both, so it is worth being clear which one you are building. If you are short of space and considering a small loop instead, our guide to magnetic loop antennas for restricted spaces covers that very different design.
Why a Triangle?
A full-wave loop can be a square, a circle, a rectangle or a triangle. Circular loops enclose the most area and are slightly more efficient in theory, but they are awkward to build from wire. A square needs four supports or a spreader frame. A triangle needs only three points, and one of them can be the top of a single mast or a tree branch. That is the practical appeal of the delta shape.
The triangle can hang in two main orientations:
- Apex up: one high point at the top, two lower corners near the ground or on short posts. This is the most common configuration because it needs only one tall support. The lower corners must still be high enough to keep the wire clear of people and objects.
- Apex down: two high supports at the top corners and the third corner below. This puts more of the wire high up, which helps on the higher bands, but it requires two tall supports.
An equilateral triangle encloses the most area for a given perimeter and gives a feed point impedance that is reasonably easy to match. Squashed or uneven triangles still work, but the impedance and pattern shift, so you may need more trimming and a different matching arrangement.
Feed Point Position and Polarisation
The single most important design decision on a delta loop is where you feed it, because the feed point sets the polarisation.
Horizontal polarisation
If you feed an apex-up delta at the centre of the bottom wire, the strongest currents flow in the horizontal bottom section and in the region near the apex. The radiation is predominantly horizontally polarised. Like a horizontal dipole, the elevation pattern then depends heavily on height above ground. At low heights, much of the energy goes upward, which is useful for regional work but less so for DX. Our article on antenna height and takeoff angle explains why height in wavelengths matters so much for horizontal antennas.
Vertical polarisation
If you feed the loop on one of the sloping sides, roughly a quarter wavelength from the apex along the wire, the current maxima move into the sloping sides and the horizontal components largely cancel. The radiation becomes predominantly vertically polarised. A vertically polarised delta loop can give a low takeoff angle without needing great height, which is why many DXers on the lower HF bands like it. Feeding at a bottom corner gives a mixed polarisation that sits between the two cases.
Vertical polarisation brings the usual trade-off: the ground in front of the antenna matters. Over good ground or near salt water, a vertically polarised loop can perform very well at low angles. Over poor ground, ground losses in the reflection zone reduce the benefit, although the loop does not need a radial system the way a ground-mounted vertical does.
Delta Loop Configurations Compared
The table below summarises the most common ways to build and feed a delta loop, and what each is generally good for.
| Configuration | Supports needed | Polarisation | Typical strength |
|---|---|---|---|
| Apex up, fed at bottom centre | One tall, two short | Mainly horizontal | Regional and medium distance; needs height for DX |
| Apex up, fed on a sloping side | One tall, two short | Mainly vertical | Low-angle DX from modest height |
| Apex up, fed at a bottom corner | One tall, two short | Mixed | General purpose compromise |
| Apex down, fed at top centre | Two tall, one short | Mainly horizontal | More wire high up; good on higher bands |
| Delta with reflector loop | Frame or several supports | Either | Directional gain, like a two-element quad |
Matching the Loop to Coax
A resonant full-wave loop has a feed point impedance noticeably higher than the 50 ohms that most transceivers expect. The exact value depends on the shape, the height and the surroundings, so the practical approach is to measure it rather than assume it. Two matching approaches are common.
Quarter-wave matching section
A quarter-wavelength section of coax with a suitable characteristic impedance, inserted between the loop and the main 50-ohm feed line, transforms the loop impedance towards 50 ohms. Many builders use 75-ohm coax for this section. Remember to account for the velocity factor of the coax when cutting it. This method is simple and low-loss, but it only works properly on the band for which the section is cut.
Broadband transformer
A ferrite-core transformer with a suitable ratio can match the loop on its fundamental band and keep the system reasonably usable elsewhere. It must be rated for your power level and mounted in a weatherproof enclosure at the feed point. Our guide to baluns and ununs explains the difference between current and voltage devices and the ratios you are likely to meet.
Choke the feed line
Whichever matching method you use, a loop is a balanced antenna fed with unbalanced coax. A common mode choke at the feed point keeps current off the outside of the shield. Without it, the coax becomes part of the antenna, which distorts the pattern, brings RF into the shack and raises your received noise. This matters even more on a vertically fed loop, where the feed line often drops away from the loop at an angle and can couple strongly to it.
Multiband Use
A loop that is one wavelength around on its lowest band shows further resonances near its harmonics. In practice the impedance on those bands differs from the fundamental and the pattern breaks into more lobes, so you will usually need an antenna tuner. Many operators feed a large horizontal loop with ladder line and a tuner and use it on several bands, accepting that the pattern changes from band to band.
If you want efficient multiband operation with open-wire feed, the principles are similar to those in our guide to the doublet with ladder line. If you want the low-angle advantage of a vertically fed delta, it is usually best treated as a single-band antenna, with its matching section and feed point chosen for that band.
Building and Tuning a Delta Loop
Construction is straightforward, but a few practical points make the difference between a loop that works first time and one that needs repeated trips up the mast.
- Start long. Cut the wire longer than your calculated perimeter. Insulated wire, nearby objects and height all lower the resonant frequency, and it is much easier to shorten a loop than to lengthen it.
- Use proper insulators at the corners. Corners carry mechanical load and, at some points, high RF voltage. Use insulators or low-friction pulleys so the wire can adjust without chafing.
- Keep the lower corners safe. With an apex-up loop, the bottom corners can be within reach. Keep them high enough that nobody can touch them while you transmit, and remember that loops can have high voltages at the current minima.
- Measure at the feed point. An antenna analyser lets you see the resonant frequency and impedance directly. Our NanoVNA guide shows how to calibrate and read a sweep.
- Trim symmetrically if you can. On a horizontally fed loop, shortening both sides equally keeps the current distribution symmetrical. On a side-fed loop, trimming near the far corner changes resonance with less effect on the feed point.
- Retune after installation. Raising the loop to its final position and installing the matching section and choke will shift resonance slightly. Make the final adjustment with everything in place.
Is a Delta Loop Quieter?
Operators often report that loops receive with less noise than dipoles or verticals. There are plausible reasons: a closed loop is less sensitive to some forms of local electric-field noise, and a well-choked balanced loop picks up less common-mode noise from the feed line. But much of the improvement people notice comes from installing a new antenna in a better location, with a properly choked feed line, rather than from the loop shape itself.
If noise is your main problem, start with the source. Tracking down and fixing local noise sources is a better first step than changing antenna type. A delta loop is a good antenna, but it is not a cure for a noisy switch-mode power supply in the next room.
Modelling Before You Build
Because the feed point, shape and height interact, modelling a delta loop before you build it saves time. A simple model tells you whether your chosen feed point produces the polarisation you want, roughly what impedance to expect, and how the elevation pattern changes with height. You can also compare the loop against a dipole at the same height and see whether it suits the paths you care about.
Free and low-cost modelling tools handle a single wire loop easily. A single loop is a good second modelling project after a dipole. Model with realistic ground, because ground quality matters a great deal for the vertically polarised versions.
The Bottom Line
A delta loop antenna is a full-wave loop shaped as a triangle, which lets you hang a resonant HF antenna from a single tall support. Feeding it at the bottom centre gives mainly horizontal polarisation, while feeding it on a sloping side gives mainly vertical polarisation and low takeoff angles from a modest height. Match the higher feed point impedance with a quarter-wave section or a suitable transformer, choke the feed line, start with the wire long and trim with an analyser. Treated that way, the delta loop is one of the best value DX antennas a wire builder can put up.
Frequently Asked Questions
What is a delta loop antenna?
A delta loop antenna is a closed loop of wire about one wavelength in circumference, arranged as a triangle. It is a resonant full-wave loop that can be hung from a single tall support with the apex at the top.
How do I make a delta loop vertically polarised?
Feed the loop on one of the sloping sides, roughly a quarter wavelength along the wire from the apex. The strongest currents then flow in the sloping sides and the radiation becomes predominantly vertically polarised, favouring low takeoff angles.
What feed point impedance does a delta loop have?
It is higher than 50 ohms and depends on the shape, height and surroundings. Measure it with an analyser after installation, then match it with a quarter-wave coax section or a suitable broadband transformer.
Does a delta loop need radials?
No. A full-wave loop is a complete antenna in itself and does not need a radial system. Ground quality still affects the low-angle performance of the vertically polarised versions, but there is no counterpoise to install.
Can a delta loop work on several bands?
Yes, with an antenna tuner. A loop resonant on its lowest band shows further resonances on harmonically related bands, but the impedance and pattern change. Fed with ladder line and a tuner, it can be a useful multiband antenna.
Is a delta loop better than a dipole?
It can show a small advantage over a dipole at the same average height, and the vertically fed version gives low angles without great height. Whether it is better for you depends on your supports, ground and the paths you want to work.