Full-Wave Loop Antenna Calculator

The horizontal full-wave loop is the antenna experienced operators quietly recommend to anyone with the space for it, and the reason is receive rather than transmit. It hears noticeably less domestic electrical noise than a dipole in the same yard, and on the low bands, where noise rather than signal decides what you can work, that matters more than a decibel of gain would.

Why 1005 rather than 984

A full wavelength in free space is 984 divided by frequency in feet, and an open-ended wire antenna needs shortening from that figure because of end effect. A closed loop has no ends, so the correction goes the other way: the loop behaves as though slightly shorter than it is, and the wire needs to be a little longer than a free-space wavelength to resonate. The commonly used constant is 1005, which lands a typical wire loop close enough to trim.

As always, that is a starting point rather than an answer. Wire diameter, insulation, height above ground and proximity to trees all shift the resonance by a percent or two, so cut long and trim against a measurement at final height.

The practical reading: a 40 metre loop needs a square roughly 35 feet on a side, which fits a typical suburban back garden if there are trees at the corners. An 80 metre loop needs 67 foot sides and is a large-lot antenna. A 20 metre loop at 17.7 foot sides fits almost anywhere and is a genuinely overlooked option for a small garden.

The receive advantage, explained honestly

This is the loop's real selling point and it is worth being precise about why it works, because the explanation usually given is wrong.

Man-made electrical noise close to its source is dominated by the electric field component rather than the magnetic. An open wire antenna such as a dipole responds strongly to the electric field. A closed loop responds primarily to the magnetic component, because current is induced by flux passing through the enclosed area. Close to a noise source, therefore, the loop hears substantially less of it while hearing distant signals, which arrive as fully developed electromagnetic waves, just as well.

The practical result reported consistently by operators who make the change is a noise floor one to two S units lower than a dipole in the same garden, with signal strengths broadly similar. That is a signal-to-noise improvement of 6 to 12 dB, which is worth more than any amplifier, and it costs the price of extra wire.

Two caveats. The advantage is largest on the low bands, where domestic noise dominates and where the loop is physically large. Above about 14 MHz the difference narrows. And the advantage is against local noise specifically; a loop does not reduce atmospheric noise or interference from a distant transmitter, because those arrive as ordinary waves.

Feeding it

A horizontal full-wave loop at normal amateur heights presents roughly 100 to 130 ohms, which is a 2 to 2.6 to 1 SWR into 50 ohm coax. Most modern radios accept that without folding back, so the simplest approach is a 1:1 current balun and 50 ohm cable, and it works.

Better options, in order of effort:

• A quarter-wave section of 75 ohm cable between the loop and the 50 ohm run. Seventy-five ohms is close to the geometric mean of 50 and 112, so this produces a near-perfect match on the design band for the price of a length of television coax. The length is the free-space quarter wave multiplied by the cable velocity factor, which the calculator above gives. Worked through on the impedance matching calculator.
• A 2:1 or 4:1 balun such as a Fumei 1:4 HF balun, 200 W$28.99, which trades an exact match for broadband behaviour. A 4:1 overshoots slightly on a horizontal loop and is the right answer on a vertically oriented delta loop, which runs higher.
• Ladder line into a tuner, which is what people with an 80 metre loop actually do, because it makes the antenna genuinely multiband from 80 through 10 metres with low feedline loss on every band regardless of the mismatch.

Whatever you choose, put a common-mode choke at the feedpoint. Twelve turns of coax on an FT-240-31 ferrite toroid core$11.67 is the standard answer and it protects the loop's main advantage: an unchoked feedline radiates and receives, which reintroduces exactly the electric-field noise pickup the loop was chosen to avoid. Turn counts by band are on the balun and choke selector.

Horizontal or vertical

The delta loop, a triangle fed at a corner or at the bottom centre with a single high apex, is the version that suits a lot with one tall tree. Fed at a bottom corner it produces useful low-angle vertically polarised radiation, which makes it a genuine DX antenna on 40 and 80 metres where a horizontal dipole would need an impossible height. It is a different antenna from a horizontal loop rather than a variant of one.

Multiband behaviour

A loop cut for its lowest band is resonant there and behaves as a longwire array on the harmonics above. Fed with coax it will be usable on some of those and awkward on others. Fed with ladder line into a tuner it is usable on all of them with low loss, because ladder line barely cares about mismatch.

That combination, an 80 metre horizontal loop fed with ladder line into a good tuner, is the antenna a great many long-standing operators end up with as their only HF antenna. It covers 80 through 10 metres, hears quietly, needs no radials, and its only real requirement is a lot large enough to hold 268 feet of wire in a rough square. If you have that space, it is the strongest recommendation on this site.

Building it

• Wire. A Paladin 14 AWG stranded copper wire, 500 ft$74.50 spool covers a 40 metre loop with plenty spare. For an 80 metre loop you will want the whole spool and a little more.
• Corner insulators. Nylon dog bone antenna insulators, 10-pack$29.97 for a ten pack, which covers two loops. Nylon does not shatter in cold the way ceramic can.
• Rope. QNR 3/16 in polyester antenna rope, 500 ft$49.73 in UV-stable polyester, and leave a spring or a weight at one corner so trees can move without breaking the wire. This matters more on a loop than a dipole because a loop is anchored at three or four points and cannot swing freely.
• Getting the rope up. A Forester arborist throw line kit$21.99 puts a line over a branch fifty feet up from the ground, and you will need to do it three or four times.
• A choke. An FT-240-31 ferrite toroid core$11.67 and twelve turns of coax.
• Something to trim against. A NanoVNA H4 vector network analyzer$89.90. A loop's resonance is affected by everything nearby, so measure rather than assume.

One practical note that catches people: a loop must be a closed circuit, and the joint where the two wire ends meet the feedpoint is the only break. Check it with a meter before raising the antenna, because a loop with an unnoticed break is an open wire antenna with a strange feedpoint and it will behave nothing like the calculation.