A word about impedance: FM receivers
(except most portables) have provisions for connecting an external
antenna. The screw terminals mentioned above are intended for 300-ohm
impedance twinlead and 300-ohm antennas.
Another type of antenna connection might be a 75-ohm F-type
connector jack, which requires that 75-ohm coaxial cable and a 75-ohm
antenna be used. It is
important to remember that maximum
signal strength is passed from the antenna to the receiver when all
impedances are matched. If
the antenna, cable and receiver impedances are not matched, the received
signal will be degraded. If
you must use a 75-ohm antenna with a 300-ohm radio, you must insert a
small matching transformer (called a balun) between the coax cable and the
radio screw terminals. Matching
transformers are readily available at your local Radio Shack, Home Depot,
Circuit City or other electronics stores.
An antenna may be as simple as an
inexpensive "twin-lead dipole" which can be purchased at most
electronic or builders supply stores. This will only be effective in the
primary coverage area of the station, about 20 miles or so from the
transmitter. Ordinary TV "rabbit ears" will work reasonably
well in this area as well, with the rods extended out to 31 inches from
side of center.
outdoor antennas, such as "turnstiles" and "S-shaped" may
be helpful in many instances and usually give better results than any
type of indoor antenna. But if eliminating multipath and/or interfering
stations is the goal, an omni-directional antenna may not help.
Only directional antennas can attenuate signals from unwanted directions,
such as reflections from nearby hills or buildings, or an adjacent
frequency or same-channel interfering station.
For fringe-area reception, an outside
antenna similar to a TV antenna is required. In fact, some TV antennas
can be used for FM reception. If your VHF TV antenna is equipped with
rotator, you may experiment by connecting it to the antenna input of
the FM receiver and aiming it at the WHFC transmitter site located at
Community College about 5 miles east of Bel Air on Route 22.
If this experiment works satisfactorily,
a splitter can be purchased to connect both the TV and FM receivers. If
the trial does not produce improvement, it is possible that the TV antenna
is of the type designed to reject signals in the FM band.
For simultaneous reception of TV and
FM, a good outside FM antenna (called a Yagi, after its inventor) can
purchased at most consumer electronic stores. This should be mounted
on a mast as high as practical. If there is an existing TV antenna, an
antenna may be mounted on the same mast, a few feet away from the TV
In extreme fringe
areas it may be necessary to use more than one Yagi antenna, vertically
or horizontally stacked, for more signal pickup.
One type of FM antenna that is
especially unobtrusive and easy to mount on a house is a ½-wavelength
vertical. The vertical antenna consists of a 56-inch stainless steel
rod with an impedance-matching coil and mounting clamp at the base. A
vertical antenna is non-directional and should be mounted up in the clear,
away from obstructions. With the vertical antenna, reception should be
satisfactory for local and regional stations, but keep in mind that it
cannot attenuate signals from unwanted directions.
Antennas should not be placed near or
behind metal of any type, including gutters, aluminum
siding, metal furniture, electrical wiring, the "guy" wiring
that secures an outdoor antenna mast, etc. Metal objects can reflect
signals and create unwanted multipath distortion problems or detune the
antenna by their proximity, reducing its efficiency.
Apartment dwellers that cannot erect a
mast and antenna may wish to purchase an indoor amplified antenna. Hint: for new home construction in areas with antenna restriction
covenants, you might consider placing a twinlead dipole antenna on the
sheathing of an outside wall before vinyl siding is applied. The lead-in
from the dipole may be passed through a hole in the sheathing and can be
any length. After the vinyl
siding is up, no one will be the wiser.
Before purchasing any antenna or
receiver, it would be a good idea to make sure that it can be returned
to the dealer if it doesn't perform well at your particular location.
If you have any questions, I will be
happy to discuss them with you. Send
an email or snailmail to me at WHFC and I promise to respond.
Host of Desert Island Jazz