The small loop, HF from inside a room.

A small transmitting loop is a meter or so of copper tubing bent into a circle and resonated with a capacitor. It will work HF from an attic, a balcony or a basement — anywhere you cannot hang a wire. The physics is unforgiving, and the point of this tool is to show you what it costs before you build rather than after.

How to use it

• Loop diameter and conductor diameter are the two things that decide everything. Bigger and fatter is always better.
• There is no design frequency to set. A loop is not cut for one — you build a size and retune the capacitor — so the tool reports what that size does on every HF band instead of asking you to pick one.
• Transmit power drives the capacitor voltage, which is usually the thing that decides what you can actually build.

What you get

• The loop as built: circumference, inductance (which is geometry only, the same on every band), and the coupling loop.
• Then, band by band: efficiency, the capacitance a variable has to cover edge to edge on that band, the bandwidth, and the volts and amps at your power.

The capacitor range is the one to read first. A single figure at a band center does not tell you whether one capacitor covers the band; the range does, and on the low bands it is both large and high-voltage — usually what really decides how low this antenna can go.

The coupling loop

This is the part people leave out, and it is how the loop gets matched at all.

It is a separate loop, about one fifth the diameter of the main one, mounted inside it and centered directly opposite the capacitor. It is not connected to the main loop anywhere — the coax center goes to one end of it, the braid to the other, and that gap is the only feedpoint on the antenna. The main loop has no feedpoint; it is coupled to magnetically.

It carries almost no current compared to the main loop, so it does not need the same heavy copper — coax braid or thin tube is plenty.

You trim the match by moving it: slide it nearer or further from the main conductor, or squash it slightly out of round, until the SWR dips. Mount it so that is still possible after everything else is built.

A Faraday version — made from coax with the braid opened at the point furthest from the feed — does the same job and picks up noticeably less electrical noise.

Tips

• The capacitor is the hard part. At kilovolts, anything with wiping contacts will arc. Use a vacuum variable, or a butterfly or split-stator type where no current crosses a rubbing joint.
• Joints matter more than metal. Tens of amps circulate in a loop at moderate power. Solder or braze; a bolted joint with a little oxide on it is a resistor in series with your antenna.
• Aim for a circumference between 0.10 and 0.25 wavelengths. Below that, losses dominate; above it, the small-loop assumptions stop holding.
• Keep it clear of walls and wiring — anything close to it becomes part of it.

Good to know

Radiation resistance goes as the fourth power of frequency. Halve the frequency and the useful part of the loop drops sixteen-fold while the losses barely move — which is why the same loop that is respectable on 20m is close to a dummy load on 80m. The tool shows this directly in the band-by-band table.

The pattern is a figure-eight broadside to the loop, with deep nulls off the flat faces. Being able to rotate those nulls onto an interference source is often the real reason to own one.