If you happen to live in a flat with steel central heater pipes and you don’t have the option to make yourself an external antenna, due to e.g. thermal insulation, this might probably be the best indoor shortwave antenna for portable world band radios; when it comes to signal (or noise) strength. The latter happens in unfortunate cases, when there are just many switched mode-based modern electronics around. However, along with a simple attenuating circuit, it beats any indoor long- or random-wire setup. I use this indoor gizmo on my Degen DE1103 for casual shortwave number or oddity stations hunting; and the results were spectacular: positioned in central Europe in a block of flats, I have managed to catch Cuba, Libya, Vietnam or even Bangkok with this setup, besides other stations. So, here’s how!
As you can see, this circuit is the simplest example of an RX antenna attenuator. For the input jack, use as short connections as possible. Use the potentiometer in combination with the internal attenuation circuitry of your world band radio to prevent overloading. Thus, if there’s too much noise in your reception, or stations “bleeding through”, increase resistance between the antenna jack and coaxial.
Then, connect the center conductor of your coaxial directly to any uncovered part of your pipes (find one with no paint/lacquer), and it’s done. Shielding is connected only to the negative pole of your receiver, please see the note below.
Do not try to use this antenna on any receiver which is connected to ground. Or, in other words, this antenna can be used for portable, battery-powered world band radios, as mentioned above.
Why this limitation? Technically, steel central heating pipes are directly connected to ground, both literally and electrically. Thus, if you try to connect this setup to a desktop receiver, or to an SDR powered by a computer running on mains, it might not work properly, or it will introduce a lot of QRM (interference). It would be like shorting your antenna jack directly to mains ground.
On the other hand, this is actually a big advantage for battery powered radios; since ground acts as a zero volt potential like a lightning conductor, this setup will also protect your input stage from any static or electrical charges.
If you own a desktop receiver (or any other connected to mains ground in general) and you are still interested in this setup, you will need to make a ferrite antenna out of some suitable ferrite cores, with the central heating pipes in center, like this. Or, try directly winding some 10-15 turns on the pipes. However, you might introduce interference from anything which is near this “coil”, unless you shield it someway.
Summed up, good luck with shortwave listening! 🙂