Whilst the antenna is probably the single most important aspect for a SWL, it’s frequently not made the most of, perhaps because it is made an unduly complex undertaking. Simply put, without spending heaps of money, the better we feed our radio, the better it will perform.
Crud, otherwise known as interference, may make the S meter read high, but the info we want can be overwhelmed. Our guide here is not the strength reading as shown by the meter, but how much desired signal competes with unwanted signals, termed the (S/N ratio), or signal to noise ratio.
Loop antennas have the effect of minimizing the noise picked up, by automatically feeding the interference to ground in the radio. Loops are somewhat inefficient, in that a loop picks up less signal than many types of antenna, but because the S/N ratio is so much greater, the signal it picks up is more usable – and the volume control is a great equalizer.
Efficiency of a loop is determined by the area enclosed – greater area = greater efficiency. Small loops will need a signal amplifier, but a giant loop will actually provide gain merely by it’s size. Make the loop big enough, it is effectively self-tuning, meaning that for reception purposes, no matching unit (ATU) is needed.
Height is a common myth for a receive antenna. So long as there isn’t a metal obstacle blocking the loop, anything over head-height is great, and with a loop, even laying it in the grass of your garden will work adequately. Why cause yourself headaches?
Finally, on the theory side: Loops do not need be fed by coax. The inherent match of a large aperture loop is around 75 ohms. Simple lamp wire (fig 8 wire) has a match of about 72 ohms, ideal for your loop, and dirt cheap. Ensure a mechanically strong join (knot the loop and the feedline together). Solder your joins.
Now to the practice – Simplicity rules
Make a loop that will reach around your roof, at a height of about 6 inches (15cm) above your gutter, in a horizontal form. One wire is sufficient, so you can split the same lamp type wire in half. Try for a square or rectangular over-all shape, it’s the easiest way to maximize the area of capture, but whatever is most convenient.
Tie each corner to your gutter. In practice, if you leave the insulation intact, you can even use wire as your tie-piece. This keeps the antenna in position, allowing use of thinner wire than if you needed the antenna wire to have structural strength.
Put a suitable (what does your radio use?) plug on the far end of the feedwire, which is connected to the two ends of the loop, one side of the fig 8 wire to the left side, other to the right side. Feed it into the house through a window … which happily closes over the wire- unlike with coax. Plug it in, and away you go.
The Loop In Use
How does it actually work? I have such a construction on a single story building. My home is at the bottom of the South Island of New Zealand, effectively Antarctica. Using an ATU lowers my signals, so the naked antenna is used. Last night was typical. I was listening to Hams using between 100 – 400 watts situated in Norway, UK, and USA at a minimum of strength 7, usually a 9+.
For the cost of a reel of cheap wire, I’m very satisfied. Play, have fun! Just remember, if it’s a receive only antenna, you can get away with breaking rules. My transmit antenna is much more complex.
Why Take The Stealth Antenna Approach?
Wives, landlords and neighbors can make life hard for SWLs, as they don’t see the beauty in an antenna that we do, and so we have to manage life making due allowance. Mounted as described, this loop is invisible, and what they cannot see they cannot complain about.
This aspect is vital in such as retirement villages and other places with covenants against antennas. With some small changes, this invisible loop can also be a transmit antenna, enabling continuance of the radio hobby.
This is the first guest post from one of this websites readers, Barry. A radio enthusiast of many years Barry is willing to field any reasonable questions about this antenna design and can be reached via email : firstname.lastname@example.org