A Portable Magnetic Loop Antenna

To start with, I claim no originality for this antenna. It is based on an antenna described in RADCOM, June 2005, by G3HBN.

My variation on the theme was built using what was to hand – no point in paying good money for something that may not work.

Portability is achieved by making the loop in the form of a wire cage (6 wires, cage diameter about 35mm. G3HBN used 12 wires, which he claims outperform 6 and 8 wire versions). The loop itself is hexagonal, about 1m diameter, and the 6 wires are soldered into a spade terminals. The spokes are softwood dowel, each with a plastic cage former and are mounted in holes (lined with copper tube to prevent splitting) in a piece of 22mm water-resistant chipboard, which is fixed to a 20mm (approx) dowel. This in turn can be fixed to the base unit by an adjustable clamp, used to tension the cage after assembling

 The base unit consists of a piece of push-fit waste pipe which very conveniently can be attached to the tripod support of a portable TV satellite dish. Also attached to the base unit is a box (I never got round to making a lid) which contains the tuning capacitor and the loop terminals.

The loop was measured to have an inductance of about 2.6microhenries. The tuning capacitor was a twin-gang unit of unknown law that I had in the spares box. To avoid contact losses, it is connected stator to stator, halving its capacity but doubling it’s working voltage. The tuning range is about 6MHz to 31MHz.

The coupling loop is made of RG213 with a braid break. The shape of this affects the VSWR. Mine has been squashed to an oval and, over the whole tuning range, VSWR is better than 1.6. Other references suggest going as far as a cardioid shape, with part of the coupling loop outside the antenna loop.

This is a narrow-bandwidth antenna. As a receiving antenna it can null out some noise. When used as a transmitting antenna, I have found that when tuned to maximum noise it is just about the minimum VWSR at that frequency, so no ATU is required.

It was apparent from the start that this antenna was too lossy for SSB. As my CW leaves a lot to be desired, it has seen very little use, However, I did get a 599 report from LA8BCA/P on a Norwegian summit (20M) in Sept 2012, and 449 from RD9CX (Ekaterinberg - about 2600 miles) on 30M from the 2013 garden party using just 20 watts erp..

Further reading:

1)      Extensive discussion in International Antenna Collection, pub 2003

2)      Subsequent articles and arguments in RADCOM. These add little info, seemingly based on computer simulations rather than practical measurements

Magnetic Loop Antenna

Coupling Loop

Support Bracket

Cage Support

Tuning Capacitor