Once upon a time, listening to long-distant radio signals (whether on the broadcast or shortwave bands or even the amateur bands) was a popular hobby.

Hours upon hours were spent, listening for that elusive station . . . the ability to bring very weak stations "out of the mud" was the ultimate thrill.

In the 21^st century (and at the risk of earning the ire of die-hard shortwave listeners!) those days have all but gone.

Today, there is little interest in the big, high-performance communications receivers of last century. A lot of gear came out of WWII, perhaps modified but there was an enormous amount of commercial receiver equipment on the market as well, reflecting the popularity of "listening".

Who can forget (if you were around 50 years ago!), the Marconis, Hallicrafters or Gelosos, the Nationals, Eddystones, Collinses or the build-it-yourself Heathkits?

Speaking of build-it-yourself, the late Ian Pogson described two mighty "Deltahet" Wadley-Loop communications receivers in (the also late!) "Electronics Australia" during the 1960s and 70s.

Later on came the Frogs (Yaesu’s famous FRG-7) and many more. Have I missed your favourite? Sorry!

Those who still indulge in the art of "listening" are these days just as likely to use WinRadio in/on their PCs – which in many ways outperforms even the best of the communications receivers of yore.

In fact, the vast majority of receivers today have little more than the AM and FM broadcast bands.

There are three things which make a receiver "good". Two are fixed (at the whim of the designer or manufacturer).

First is the receiver’s sensitivity, which is its ability to resolve very weak stations.

Second is the receiver’s selectivity, which is its ability to separate stations whose transmit frequencies are very close. Note that NO receiver, on its own, can separate stations which are on the same frequency.

Here's the top PC board mounted inside one of the low-cost Oatley weatherproof cases. The two cables entering left are for the loop antenna above (you can just see the loops behind the case lid). All four wires in this cable are soldered to the underside of the board together. The cable entering in the middle is the downlink - this cable has all four wires individually soldered to the underside of the PC board .

However, the third factor, which can often help a receiver distinguish between stations on adjacent frequencies (and even sometimes the same frequency) is the aerial or antenna.

Even though the terms are virtually interchangeable these days, no self-respecting old-timer would ever call his aerial an antenna. Antennas were those small flimsy things designed to pick up TV!

Unless you have made an in-depth study of aerial/antenna theory, to most people, many (most?) electronics hobbyists included, it’s a black art.

Sure, everyone knows aerials/antennas are the "inductor" part of a tuned circuit which, depending on the antenna length, resonates at a particular frequency, according to the formula

1

2pÖLC

where L is the inductance in Henries and C is the capacitance in Farads.

Wanted frequencies (ie, the station you want to listen to) can pass virtually unhindered but (at least theoretically) all other frequencies are rejected.

If you make the frequency of the tuned circuit variable, then you have a means of tuning over a specific band of frequencies.

Well, at least that’s the way it’s supposed to work.