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.