  Will I always get better reception with an external antenna? Theanswer really depends on what type of receiver you're using. Isit a "portable" or a "communications" type? Portable typesPortable radios with a built-in telescopic whip have sensitiveinput circuits that are carefully matched to that antenna. Ifyou lengthen the whip (e.g. by clipping on 12 meters of wire witha crocodile clip) you may find that the radio is bursting withsignals (especially after dark on the bands of 31, 41 and 49 meters).If you listen carefully, you may discover that the majority ofthe "extra" signals you hear when the external antennais connected are not really on the air. They are generated frominside the receiver as a result of overloading. Communications typesWith some radios you do not have a choice. They do not have abuilt-in antenna and so you have to connect an external one. Butthat is no guarantee that your radio can handle everything thatis being pumped in to it. Remember that any antenna you put upis not only receiving the signal you are trying to hear. Especiallywith antennas that receive a wide range of frequencies, the entireshortwave spectrum is presented to the receiver's terminals. Thestrength of the total energy can vary from around 1 microVoltto between 200-500 microVolts. The automatic gain control (AGC) inside most receivers is designedto ensure that the fluctuations in received signal strength (dueto fading) are compensated for by raising or lowering the gain.Because you've chosen a weak signal, the radio frequency (RF)gain is turned up to maximum. That means it is also amplifyingthe nearby strong broadcast signals. The first mixing stage inthe radio therefore receives those broadcast signals a factorof 3 to 10 stronger than they are being delivered to the inputterminals by the antenna. In Europe a simple dipole antenna candeliver signals with a power of 50-100 milliVolts from the 7 MHzbroadcast band without much problem. This level drops a littleat higher frequencies, but it is still in the order of 30-50 milliVolts.If an active antenna is being used, then the signals may be inthe order of several hundred milliVolts! None of the signals areattenuated by the automatic gain control. Mixers and amplifiersin even the better shortwave receivers can only handle a certainamount of signal before things go wrong. Weak signals must beabove the background noise level to be readable. But at the sametime, strong local signals must not be made too strong so thatcircuits are overloaded. Spotting overloading problemsOverloading problems are often audible in two ways. First, a strongtransmitter is heard all over the spectrum and adjusting the tuningknob does nothing. Or secondly, the signal strength of the desiredsignal drops, until it disappears in the background noise. Thereceiver has become insensitive, and this problem is termed "blocking".A receiver's "dynamic range" is therefore a measurementof how strong an unwanted station has to be tocause receiving problems to the wanted signal.It doesn't have to be that strong! Remember that the antenna is delivering hundreds or may be thousandsof signals to the receiver's input terminals. All these intermodulationproducts mix together to produce a background noise, preventingreception of weaker genuine signals. In our opinion, this is one of the major problems of consumergrade and semi-professional receivers today. Not only is therean increasing number of high-power stations on the air, but modernreceivers have simplified their tuning by not including a receiver"pre-selector". Such a device is still available asan optional extra (e.g. Lowe PR-150), and helps to reduce theamount of unwanted signals at the first mixer stage of the receiver.But the pre-selector is not cheap, sometimes being around thesame cost as the original receiver. Europe has many high-power transmitters packed into a relativelysmall area. Radio Netherlands' independent tests in the Pacificand North America show a cheaper communications receiver to bemuch "quieter" there because of lower signal levels.But even so, the re-launching of good pre-selectors for the consumermarket would solve a lot of listeners complaints. If the interference to the receiver is caused by a nearbymedium wave broadcast station, a bandstop filter will help toalleviate the receiver overload and reduce spurious signals. TheIndustrial Communciations Engineers, Ltd., Model 401 filter insertsup to 60 dB attenuation in the range of 500-1800 kHz, yet allowsreception above and below the MW spectrum.. The Model 402X isa highpass filter that allows radio amateurs to use transceiversup to 300 watts on any frequency above 1.8 MHz. The Model 400(receive only) and 400X (transceive) high-pass filters provide60 dB attenuation at 1.6 MHz and are designed to operate above3.5 MHz. Price range for all filters is US$30. Comments? letters@rnw.nl |