Antenna tuner approach

Pre-selector approach

What's the difference?

In many shops, dealers will tell you there is no difference betweenan antenna tuner and a pre-selector. But there most certainlyis. Most communications receivers have an antenna input with animpedance of 50 ohms. In order to make the best use of 50 ohmcoaxial cable and get the maximum signal transfer between antennaand receiver, it is important that the antenna should have a fairlyconstant impedance over the frequency range for which it was designed.But many simple shortwave antennas (such as dipoles or trap dipoles(W3DZZ) only have a 50 ohm impedance when the receiver is tunedto one or just a few frequencies. Only on these channels (oftenin the amateur radio bands) are the antenna, coax and receiverinput perfectly matched.

Antenna tuner approach

An antenna tuner (sometimes called an antenna matcher) can beplaced between the antenna and receiver to "match" theimpedance of the antenna to the 50 ohm impedance the receiverwants. Proper matching is important. It is particularly noticeableon weak signals which can be boosted to levels significantly higherthan the background noise. If you have such a device rememberthat adjusting your tuner for maximum signal often will not improvereception. It is better to look for a weaker signal and adjustfor maximum readability.

Antenna tuners usually consist of a tapped coil of wire, tunedby two variable capacitors. They come in different versions. The"Pi" configuration is one, a "T" configurationis another. The units only make a better match possible... nothingis done to improve the receiver's selectivity.

The antenna tuners which use a "Pi" configuration havea low-pass function. Frequencies higher than the one being "matched"are attenuated a bit. An antenna unit with a "T" configuration(not seen very often) attenuates frequencies lower than the onebeing matched. The "T" type of tuner is better for shortwavelisteners because of the high power transmitters using lower frequencies...longwave, medium wave, plus 6, 7 and 9 MHz shortwave.

Pre-selector approach

A pre-selector is a different device. The aim is to filter outall the signals operating on the radio dial except the one selectedon the receiver dial. That is needed because even fairly shortSW antennas receive very strong signals from all parts of thespectrum and these are all presented to the front end of the receiver.Too much signal is often as bad as too little signal.

Most communications receivers have some sort of input filteringto suppress signals on unwanted frequencies. But this is ofteninadequate because the input filters are too wide, usually anoctave, e.g. a filter many let though signals between 7 and 14MHz and reject the rest. This still causes problems in high signalstrength areas like Europe. Broadcast stations use high power(in excess of 250 kW) and even simple antennas can deliver signalstrengths in the order of 100 milliVolts or higher. Even thoughthe receiver may be tuned to a different part of the dial, strongsignals from the 41 meter band overload the sensitive input circuitryand "ghost" signals appear on parts of the dial wherethey shouldn't be. All receivers in a price bracket of US$3000and lower are prone to overload. The simple solution is to switchin the attenuator. Intermodulation products can be pushed downin the background noise much easier than desired signals. Forinstance, if you switch in 10 dB of attenuation the desired signalwill become 3 times weaker. On many signals, that is hardly noticeable.Third order intermodulation products will be 31 times (30 dB)weaker. If the level of interfering signals drops, weaker broadcastsignals become intelligible above the noise floor of the receiver.

But what happens if you are listening to a very weak signal, and10 dB of attenuation make it so weak that it disappears into thenoise? This is where the pre-selector comes into its own. In fact,the pre-selector is not a new concept. Thirty years agomost communications receivers had some form of pre-selector. Thedisadvantage was that there were two knobs to turn instead ofone. The arrival of frequency synthesizers made receivers farmore frequency agile. Memories, remote control by computer andscanning capabilities made it desirable to simplify the tuning.Manufacturers decided to take out the pre-selector. Today, onlythe top end receivers like the Japan Radio Company NRD-535 havea pre-selector. The NRD-535 pre-selector is tuned automaticallybeing linked to the main tuning, but its performance is not nearlyas good as an external pre-selector such as the Lowe PR-150.