The Loose Coupler, by Eric P. Nichols, KL7AJ

Small loop antennas are very inefficient, either for transmitting or receiving. You can read the fine writings by W8JI on his website, www.w8ji.com and learn the truth about antenna efficiency. This does not mean the small receiving loop is ineffective. In many instances we willingly sacrifice efficiency for effectiveness. A multi-element, close-spaced Yagi antenna, for example, has much higher losses than a simple dipole, primarily because the radiation resistance is lower. But it is generally considered a fair price to pay for gain in the desired direction.

Antennas don't necessarily have to be SMALL to be inefficient, either. A terminated rhombic dissipates half the transmitted power in the termination resistor. Incidentally, my partner-in-crime, Joel Ballek, WL7AI, and I learned the real story about rhombics back in 1988. A few years before, Joel and I had put up a 160 meter rhombic on his farm. It was about two wavelengths per leg, aimed toward the lower 48. It worked okay, but nothing to write home about. In the intervening years, the ground-mounted termination resistor had been snagged by a moose and dragged off somewhere into the wilderness.

1988 came around with a HUGE opening on 10 meters. The rhombic was still standing, minus its termination resistor, so Joel had the bright idea of using this on 10 meters, which was somewhere around 30 wavelengths long. We decided to use a few 2 watt resistors to replace the moose-trampled terminator of the previous incarnation. We had both bought the conventional wisdom that, in a very long rhombic, all the energy is "radiated before it gets to the terminator anyway." Joel had a "full gallon" on ten meters, so we decided to impress some lower 48 "DX" stations with our death ray. Our termination resistors, nearly a third of a mile away, instantaneously evaporated. So much for "all the power being radiated." Lesson learned. We replaced our evaporated resistors with a genuine high-power terminator (a Heathkit Cantenna and 16:1 balun), and THEN enjoyed our death ray.

Now, when it comes to receiving antennas, efficiency has a slightly different connotation. We aren't too worried about things burning up, because the power levels are minuscule. But in a very small loop, the radiation resistance is still minuscule, and the resistive component is primarily wire loss. This would normally increase the thermal agitation noise as well, except that small antennas also have a very high Q, which limits the noise bandwidth considerably. So we can STILL come out ahead. Additionally, at the frequencies where small tuned loops are used, mainly the lower frequency ham bands, atmospheric noise is the limiting factor. Having either a very low noise, or very sensitive antenna benefits us little, if anything.

When sensitivity is at a premium, one wants to carefully match the antenna impedance, or antenna SYSTEM impedance to the receiver front end. This assures maximum power transfer from the antenna to the receiver. However, this situation doesn't necessarily give you the best advantage for a small loop...extremely high Q and a very usable sharp null. Back in the dark ages, many crystal radio operators intuitively understood this, though perhaps not knowing why it worked. The "loose coupler" was a prized possession of early radio pioneers.

The fact of the matter is, the small, tuned receiver loop exhibits its BEST performance when nothing is attached to it...when it's floating out there in space. To take full advantage of the small loop, we need to allow it to, as closely as possible, exist in that ideal state. We can approach that state by extremely LOOSE coupling to our receiver. For example, if we build a one-foot-square, 100 turn frame loop, we might use a single turn 6' loop concentrically located to transfer the energy to our receiver. This is NOTHING RESEMBLING an impedance match. But it's more than GOOD ENOUGH, because we gain the advantages of the small loop....lots of selectivity, nice sharp nulls (excellent for getting rid of B.C. band overload) and immunity to many types of local man-made noise. Yes, it's a serious compromise, but it's the CORRECT compromise.

If there is a "moral" to the story, it's that EVERYTHING we do in amateur radio involves tradeoffs. The effective radio amateur knows which tradeoffs work and which ones don't. We might be aghast at the thought of operating ANYTHING with a 10:1 or even a 100:1 mismatch. But if it works, it works!

73,

Eric P. Nichols, KL7AJ