FM Antennas

by Bruce Carter

This article relies heavily on the four part article:

"What Kind of FM Antenna is Best for You?", by Michael J. Salvatti, appearing in Audio Magazine January through April 1978

Although that article series is excellent, and contains a lot of very detailed information, it is now becoming somewhat dated, with many manufacturers mentioned going out of business, and others changing their product lines extensively and discontinuing specific models mentioned. Those who are curious can find the original article in libraries.

This article is meant for FM listeners who are dissatisfied with the performance of the folded dipole antenna that came packaged with their receiver. Almost without exception, you will need to install an outdoor antenna. This article describes your options.

Terminology

Omnidirectional Antennas

Those listeners that find themselves in the middle of a metropolitan area, or very near to several nearby cities up to 30 or 40 miles away can benefit mostly from an omnidirectional antenna. This type of antenna will be of no help, however, if you are trying to receive a more distant, low power, or translator station. For these type of stations, a good directional antenna will be needed.

Omnidirectional antennas come in two types: The "S" curve type and the "turnstile" type, shown below, along with their polar patterns. In the case of the omnidirectional antennas, the very name implies "from all directions eqaully". The two antenna types have roughly circular polar patterns. Neither type has gain (over a standard dipole), in fact, both types have a slight loss. This can be an advantage in areas where a lot of strong stations are available. If the antenna had too much gain, the received signals might overload your receiver, especially if it is older. The effect of placing the antenna higher than you could place a folded dipole will give you an effective gain, and is the main benefit from this type of antenna.

folded dipole antennacircular polar pattern

"S" Curve (folded dipole) Antenna and its Polar Pattern

turnstile antennarounded rectangular polar pattern

Turnstile Antenna and its Polar Pattern

High Gain Directional Antennas

A high gain directional antenna is needed for weak distant stations. Not only does it add gain, but its beamwidth can serve to reject other stations on the same or adjacent frequencies - coming from directions outside the beamwidth. It can also reject sources of noise the same way. Noise can include reflections from nearby buildings or mountains, automobile ignition, large electrical installations or motors, or harmonics of CB and other types of communications.

Directional antennas come in two basic types - Yagi and Log Periodic, and hybrids are possible. Yagi's are recognized by elements that extend the entire width of the antenna, while log periodics are recognized by elements that alternate between sides of the antenna. Yagi's tend to have higher gain and narrower beamwidths, while Log Periodics tend to have more uniform gain over the FM band and wider beamwidths. A generalization that applies to both types of antennas is that the larger they are (and the more elements they), the more gain and narrower beamwidth they will have. The reference above tends to favor log periodics, but I have found that large Yagi's are an absolute necessity for really weak or distant stations (300 miles is possible but rare).

yagi antenna log periodic antenna
YAGI Antenna Log Periodic Antenna
polar pattern for yagi and log periodic antennas
Polar Pattern for Yagi and Log Periodic Antennas

Using TV Antennas

Extreme caution is advised here, as FM reception from TV antennas is unpredictable at best. It requires the antenna manufacturer to extend the gain of the low VHF band (channels 2-6) from 88 to 108 MHz, which almost doubles the range. This is no small feat, and definitely involves compromising the performance for the low VHF TV channels - something that the antenna manufacturer may be unwilling to do. The majority of TV antennas simply roll off in gain through the FM band. Some have traps to eliminate FM reception, because FM can interfere with TV channel 6. These antennas may even have "break-off" elements to enable FM reception, so the user can make a choice.

Mounting

MANDATORY SAFETY RULES!

  1. Mount it so there is no possibility it could fall on power lines, or power lines on it.
  2. Always use lightning arrestors. Even if you only get 2 inches of rainfall a year, it only takes one strike to destroy your equipment and/or kill you. Lightning arrestors will actually help you by bleeding off static electricity charges.
  3. Strength and stability - mount it no higher than you can mount it securely! If not, the first high winds it encounters will turn it or blow it down.

If you have carefully observed those safety rules - the rule for performance is: "The higher the better, within your budget and zoning limits." In practice, though, a ten foot mast on top of your house will give satisfactory results. FM is line of sight, but there is usually enough reflection in the atmosphere to help you. Mid-afternoon is usually the worst time for distant FM, and that is the time where raw height will help the most. Every mile added to the line of sight comes at the expense of several feet of height, however. It may not be necessary to have line of sight, just line of sight to a region where the signal is more receivable.

In cases where you are trying to receive a very weak station in a metropolitan environment, you may be able to see the tower. Although it is usually best to aim straight at the tower, there may be cases where it is necessary to aim the antenna to reject interfering signals. both Yagis and Log Periodics are least sensitive to signals broadside to them. This is intuitive, because the receiving elements present the smallest surface area on the sides of the antenna. It may be necessary to broadside your antenna to an interfering station, even if you are aiming way off of the desired signal. In rare cases, it is better to aim the antenna at a building or mountain to pick up the reflection of a station instead of the station itself. This happens when the building is relatively near and the station is several miles away. If you aim at the station, the reflection from the building causes multipath. If you aim at the building, you get the reflection, and the station - miles away - is rejected, eliminating the multipath.