In general, an antenna is "cut" or "built" for one frequency, or
for a band of frequencies centering on one particular frequency.
This antenna will respond to all frequencies, but will only exhibit
"maximum" gain at one frequency. Frequencies higher or lower will
not be received quite as well, and the farther from the "tuned" or
"center" frequency the signal is, the less gain the antenna will
provide on that frequency. As we work with an omnidirectional
antenna, we can "tinker" with it a bit in an attempt to get it to
work with a broader range of frequencies, but as we broaden the
usable spectrum over which it will work, we will see less gain on
the "center" frequency. If you're getting the idea that tuning an
antenna involves trade-offs, you're getting the right picture. No
improvements are possible without "cost" in another part of the
operating spectrum. If we move to a directional TV antenna like,
say, one of those antennas (not the dish) that still can be seen on
some rooftops, we see what is basically a modified log periodic
antenna. This antenna will generally be "pointed" or "aimed" in the
general direction of the transmission antenna of the station(s) for
which reception is desired. Stations with broadcast antennas to the
side or the rear of this antenna won't be "seen" as the antenna is
highly directional. Having said all that to get you up to speed on
some fundamental concepts of the antenna, as we move up the
frequency spectrum (starting "low"), the gain of the antenna will
increase until we reach the "center" or the "tuned" frequency for
that antenna. After that, increasing the frequency will result in
signals with increasingly lower gain.
