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Why active antennas aren't perfect

From: (Brian Frost)
Date: Oct. 20, 1994
Original source: Usenet's

There have been various comments here regarding the effectiveness (or otherwise) of active antennas. This is not a definitive comment, but I though I would add some reasonably objective comments. Since you dont get something for nothing, you ought to have some idea of expected benefits per buck.
Active antennas work by matching the impedance of a short wire (or rod) to the lower impedance of the radio input. For lengths of wire under about 10m, the impedance of the wire falls as its length increases, and when this is combined with increasing length increasing signal level, 'long wire' antennas are simple and effective.
Since an active antenna attempts to work with a short wire, it can only do this by dropping the impedance (so that the radio does not 'short-out' the small signal from the wire) and it usually amplifies the smaller signal to a level where the output is comparable to the output from the long wire.
So far so good, so why doesn't everybody use active aerials? There are two problems with these.
1. Since the wire is short and inherently localised to the set, pickup of unwanted interference (bedside clock/radios, TV's etc) is enhanced over a long wire. When amplifed, these interferences can, at best, require the aerial to be moved around for best effect, or at worst make it unusuable.
2. And the most serious problem....intermodulation and distortion. A good radio has quality filtering as early in the signal chain as possible to ensure that weak signals are not clouded by unwanted strong ones. An active aerial has an amplifier which is not perfect. If two signals f1 and f2 are injected into this (any) amplifer, the output will consist of sum, difference and harmonic products. The radio is then unable to distinguish these from real world signals. For example, in the evenings, 7Mhz is very strong, 14Mhz not so strong. Due to harmonic distortion, with an active antenna, some of the 7Mhz stuff will 'appear' around 14Mhz with obvious problems. Intermodulation will also cause spurious signal to appear at wierd frequencies too.
In conclusion:
Problem 1 cannot be improved by an technique other than siting.
Problem 2 can be improved by purchasing an active antenna that offers good quality filtering. This filter MUST be tunable and is designed to be selective only in the band that you are currenly listening to. Usually, this filtering is offered in association with a gain control, where, due to these distortion and IM problems, reducing the gain can actually improve the quality of the signal.

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