|
FM Antennas
Selecting the proper antenna is crucial
to achieving optimum performance from your tuner. What follows
is an excellent treatment of the subject that appeared in
the users manual for the superb Day Sequerra Reference Tuner.
***
The selection and installation of an appropriate FM antenna
is arguably the most important aspect of receiving high
quality FM broadcasts. No single antenna type, whether indoor
or outdoor, is perfect for all receiving situations.
The intent of this overview is to aid in the selection
of an FM antenna that best suits your receiving location.
The next few paragraphs describe the types of readily available
FM receiving antennas.
Even though a common outdoor television antenna may, in
a few instances, provide acceptable results, antennas specifically
designed to receive the FM broadcast band, generally 88
MHz to 108 MHz, are the most effective for FM reception.
The FM broadcast signal is VHF (very high frequency) in
nature and generally can only be received within a distance
known as "line of sight"... about 40 miles.
Without hills, mountains, or buildings to impede its progress,
an FM phenomenon commonly known as "diffraction"
causes the FM broadcast signal to conform to the curvature
of the earth for a distance of approximately 30 miles beyond
normal line of sight. It is this phenomenon that provides
for long distance or "fringe reception" of the
FM broadcast signal. Fringe reception can usually provide
a listenable FM signal, although it is often noisy and of
low audio quality.
FM Interference
FM interference problems will occur more often as the
distance between the transmitting and receiving antennas
increases. Tropospheric scatter causes part of the FM transmissions
to be refracted toward the earth at some distance within
the normal line of sight. This transient phenomenon may
occur, for example, when a temperature inversion is located
in the FM broadcast signal path. Such signal refraction
can create FM reception where there was none before, but
it can also disrupt reception of a high quality local FM
broadcast transmission.
Obstructions to an FM broadcast signal can range from
small hills and buildings to office towers and mountains.
The resultant effects could be partial absorption, which
lowers received signal strength, or reflection. This causes
the desired line of sight FM signal to be received along
with reflected out-of-phase, time distorted signals. This
"multipath interference" distorts the FM signal
resulting very often in low fidelity reception.
Other sources of interference affecting audio quality
of the FM signal include co-channel and adjacent channel
interference as well as "noise flutter" (e.g.
modulation interference caused by aircraft).
Outdoor FM Antennas
As a general rule, an outdoor FM receiving antenna that
has many elements can be assumed to be highly directional.
That is, it will be more sensitive to FM signals received
perpendicular to its active elements than to signals arriving
from other angles or even in the opposite direction. Such
directionality is desirable in an outdoor FM antenna for
mainly two reasons: Antennas that are highly directional
usually have higher gain. Antenna gain will provide increased
signal strength when receiving a transmitted FM signal.
When referring to an FM antenna, gain is most often referenced
to a standard half-wave dipole, such as a simple flat twin-lead
antenna. For example, an FM antenna with an advertised gain
of 3 dB will deliver to your FM Reference approximately
twice the power compared to a standard half-wave dipole.
A directional antenna will receive only those stations
within the defined antenna beamwidth. Antenna beamwidth
is the general measure of the amount of directionality and
is defined by the half power points (-3 dB) of the angle
of the front radiation hemisphere. If the angle is too great
(wide antenna beamwidth), the antenna will receive the desired
FM broadcast as well as reflected signals (multipath) from
different horizontal directions that may cause FM broadcast
interference. Conversely, it may prove difficult to precisely
position an antenna with a very narrow beamwidth for proper
reception of the desired FM broadcast.
An omnidirectional antenna, although much lower in gain
and lacking any significant directionality, may prove to
be adequate in a few applications. For example, if you live
in an area which is relatively free of multipath problems
and receives strong signals from many directions, a low-cost
omnidirectional outdoor antenna may suffice.
Of the outdoor FM broadcast receiving antennas available,
the following are most common:
Unidirectional Dipole - This is a large dipole having
two elements and slightly more gain than the common indoor
twin lead dipole. It concentrates its signal reception mostly
in one direction. This type of antenna could be used where
the desired stations are in one general direction and within
about 20 miles.
Turnstile (crossed dipole) - This antenna attempts
to provide an omnidirectional receive pattern, without the
need for rotating the antenna. Unfortunately, this type
of antenna has relatively low gain (about -3 dB) in all
planes.
Multi-element Array - This is a unidirectional antenna
capable of receiving very distant stations due to its high
gain. However, this gain is directional and may in some
cases require that the antenna be rotated each time you
tune to a station (usually using an antenna rotator system).
A good quality antenna of this type, mounted high on a mast
in most suburban and rural installations, should provide
you with the best FM reception possible. Yagis, collinears,
and log periodics are the most common of these high-efficiency,
multi-element array types.
Indoor FM Antennas
For urban, near-suburban or in other situations where
an outdoor antenna installation is not possible or desirable,
a high-quality indoor FM antenna should provide excellent
reception.
It should be noted that the urban FM receiving location
poses a totally different set of problems than that of a
suburban or fringe reception installation.
The radio frequency environment in and around modern metropolitan
areas is jam-packed with voice and data transmissions, over
modulated broadcasts, FM multipath, and out-of-band spurious
interference, all competing for valuable airspace...spilling
over and interfering with your favorite FM radio station.
Because the L-R portion of the FM stereo broadcast signal
is more easily affected by this RFI and multipath than the
L+R portion, stereo reception of even a very strong local
FM station might be noisy and distorted, while monophonic
reception of the same station is clean and clear.
To complicate matters, multipath signals in an FM urban
environment are generally received from almost the same
direction (in the horizontal plane); even a directional
outdoor antenna with its horizontal angular discrimination
is often incapable of separating the desired direct path
signal from this vertically oriented multipath interference.
Amplified Indoor Antennas
Most amplified antenna designs have inherent tradeoffs
in signal-to-noise as well as other negative RF attributes
that tend to exacerbate the difficult receiving conditions
in the urban RF environment. The greatly increased FM noise
floor caused by the amplified antenna degrades the real
world sensitivity of the tuner.
Unamplified Indoor Antennas
There are two basic antenna configurations generally used
in unamplified indoor antennas, both are dipoles.
Bidirectional Dipole - This is perhaps the most
commonly-used indoor FM antenna. The single, 1/2 wave, dipole
design exists as an industry gain reference (O dB) for most
FM broadcast band antennas. The crudest form of a dipole
is the "ribbon dipole" found packed with most
FM tuners and receivers.
1/2 Wave Vertical - A 1/2 wave vertical design features
relatively high gain for an indoor antenna, approximately
twice the gain of the bidirectional dipole. The 1/2 wave
vertical dipole is probably the antenna of choice for high
quality FM reception in an urban environment.
Cable FM Service
In most cases, cable FM (provided with some cable television
services) is distinctly inferior in the areas of signal-to-noise,
cross-talk and RF intermodulation distortion when compared
to a properly installed FM antenna, whether indoor or outdoor.
***
In general, we have found that good performance in most
areas can be achieved with the use of the simple folded
dipole antenna that is included with most tuners. The specific
characteristics associated with your area will determine
whether you need to take more drastic measures. A multi-element
outdoor antenna on a rotor is usually the best answer, but
in many cases not a viable option.
Magnum
Dynalab offers a range of excellent indoor and outdoor
antennas Here is a
link
to get details
and coverage maps for any station in the US
Some interesting technical
information and measurements of various antennae
|
