Matching problems

Polarization

Active antennas solve a number of problems -- space and governmentor deed restrictions -- immediately.

Few people have the space to hang a dipole of some 30 meters tomatch their interest in the 60 meter tropical broadcast band.Additionally, it seems that many local government authoritiesaround the world are becoming more restrictive in their attitudetowards outdoor antennas. Pick up almost any electronics magazineand you'll find someone advertising a solution. The "active"antenna is small, easy to install, and its box of electronicsmeans that it performs as well as any "passive" outdoorantenna. That's the claim at any rate. In practice, tests at RadioNetherlands together with the World Radio TV Handbook,edited in Amsterdam, show that passive antennas always outperformactive antennas, providing you have the space to mount a passiveantenna outside away from sources of interference.

Active antennas often take the form of a telescopic whip whichcan be unobtrusively mounted on a outside wall or balcony. Suchan small antenna has a high internal resistance. In fact it consistsof a real resistor (usually a few ohms, and therefore rather insignificant)and a capacitor. For a whip antenna, this usually has a valuearound 12 pF. The capacitor offers an apparent resistance, whosevalue is frequency dependent. At a frequency of 200 kHz this isof the order of 55,000 ohms, and at 30 MHz around 230 ohms. Inthe longwave part of the spectrum, the impedance is quite high.It is useless trying to hang a 50 ohm coaxial cable to it, forin effect you're short-circuiting it. The trick in an active antennais to connect the whip to an amplifier with a high-impedance input.The output impedance of the amplifier is designed to be 50 ohms,and a coaxial cable matches perfectly.

But won't a small antenna receive much less signal than a largeone? You'd be surprised. Theoretically, the smallest antenna youcould design is a point, i.e. an isotropic antenna. Say this receivesan energy from the electromagnetic spectrum equivalent to 0 dB.An ordinary half-wave dipole antenna receives 2.5 dB (or 1.6 times)more energy than an isotropic antenna. If you were to use a smallactive antenna, such as the Dressler ARA60 or RF Systems DX-7,this gives 1.76 dB more than an isotropic antenna, or 1.5 timesthe energy. The difference between the standard and small activeantenna elements is therefore just 0.39 dB, insignificant in fact.Unlike a small antenna, a large aerial can be directly matchedto a 50 ohm cable, which is why an active antenna uses an electricalsolution to change the impedance. Most active antenna manufacturersadd some form of amplification to their impedance matching unit.The active antenna itself may take many forms such as a miniaturedipole, a loop, or some type of whip. But there are some disadvantagesin using an active antenna!

Matching problems

The weak point of most active antennas is the impedance matchingunit and/or the amplifier. Many factors affect performance. Wenoted earlier that a whip antenna has an apparent capacity ofaround 12 pF. It is therefore up to the manufacturer to designa circuit which has a very low input capacity. Easier said thandone. Budget antennas which don't heed to this point have a noticeablylower sensitivity, i.e. efficiency, above about 10 MHz. Higherpriced antennas have usually solved this problem. A far more seriousdrawback in most active antennas is ability to handle strong signalsthat can produce intermodulation products. If there are a lotof strong medium wave signals available, then check the range100 to 400 kHz. If longwave reception on the receiver is maskedby noise and whistles, the active antenna may be producing a numberof intermodulation products. This overloading problem also appliesto the shortwave broadcast bands, especially the overcrowded 6,7, 9, and 11 MHz bands. For example, mixing from stations on 7,200and 11,800 kHz give products at 19,000 and 4,500 kHz. The lattermay well mask reception of weak 60 meter tropical band broadcasterswith an unacceptable noise level.

Polarization

Whereas polarization is important and VHF/UHF frequencies, itbecomes less critical below 30 MHz. Even if a signal starts outas a horizontally polarized waveform, the effects of the ionosphereand the curvature of the earth cannot guarantee the polarizationat the receiving end. In fact it is constantly changing, as isthe ionosphere. The antenna therefore receives both horizontallyand vertically polarized signals, plus those some way in between.This effect is also noticeable with active antennas. On long andmedium wave, where the wavelength is huge in comparison with thesize of the antenna element, this effect is difficult to determine.But above 5 MHz, our tests showed that polarization made a signalstrength difference of up to 3 "S" points (18 dB). Asignal was just audible on one antenna, and unreadable on theother, depending again what polarization was arriving. The seriouslistener should ideally have two active antennas, one vertically,the other horizontally polarized. A coax switch can then be usedto select between the two. Another possibility is the multi-polarizationfunction of an antenna such as the RF Systems DX-7 or DX-1 Pro.

Active antennas exhibit a specific interference problem. Becausethey can receive signals at very low frequencies, they are particularlysensitive to the interference signals produced by domestic TVsets. At night, when TV sets are on, the noise level ( as a resultof the 15.625 kHz TV line frequency, and harmonics) rises by about6 dB or 1 "S" point on the average European housingestate. Personal computers and their associated monitors becomea serious source of interference, even if they are some 10's ofmeters away. Especially in areas where the household power supplycomes via overhead cables, the power socket brings not only power,but also all types of low-frequency interference. Unless you liveon a secluded farm away from interference sources (in which caseyou'd probably have space for a passive outdoor antenna), an indooractive antenna will probably disappoint you. If man-made interferenceproves to be troublesome, it is a good idea to try different placesin the house (e.g. on a balcony), even some 1.5 meters above theroof if this is permitted.