FM Reception Hints
Introduction
This information leaflet has been prepared to assist listeners to obtain satisfactory reception of Very High Frequency-Frequency Modulation (VHF-FM) radio signals more commonly referred to as FM.
Reception problems for FM listeners include lack of stereo reception (Radio New Zealand Concert), noise, distorted sound, and, for the mobile VHF-FM listener, regular bursts of distorted sound from the receiver when travelling through hilly areas or built-up areas in the central City.
The technical quality of FM transmission is significantly superior to AM transmission as long as reasonable reception is possible or available. Programmes may be transmitted in either mono or stereo. Radio New Zealand’s National Radio Transmissions are in MONO while Radio New Zealand Concert is in STEREO.
FM Broadcasting
Because FM signals operate at very high frequencies the receiver aerial is specially designed to pick up the signals. In many cases in very strong signal areas, a full aerial may not be required. The FM signal travels in a straight line from the transmitting aerial to the receiver. A signal from the same transmitter can also be reflected off nearby buildings and hills and arrive at the receiver from a different direction to that of the main signal.
If the signal path from a transmitter to receiver is clear, i.e. line of sight, the signal strength level reduces at a constant rate with distance. But if the signal is obstructed in any way by hills, tall buildings, forests, etc, then the signal strength reduces very rapidly behind the obstruction. The FM transmitter must be able to see its target area (receiver) to provide the best coverage.
Areas of weak signal strength behind hills, buildings etc, are known as “shadows”. In some of these areas, reception can be difficult to obtain and an outside aerial must be used.
Outside Aerial
Like television, best reception of FM radio is obtained when an outside aerial is used. The most common type of outside aerial used for the reception of FM signals is the Yagi aerial (named after its Japanese inventor). The Yagi aerial has directional properties. It must be positioned the correct way around and normally pointed directly at the transmitter. The polarisation, or sideways tilt, of the aerial may be vertical or horizontal as for television or it may he slant for FM radio. The rule for slant polarisation is that the aerial, when viewed from behind, that is, looking toward the transmitter, should be tilted ‘right-hand-down’ 45o.
A Typical 3 element antenna
Although outside television aerials are not designed to work at the FM frequencies, it is sometimes possible to obtain enough FM signal from an outside mounted television aerial to provide satisfactory reception on receivers fitted with terminals for connecting an external aerial
This aerial helps ensure that the receiver is supplied with a main signal strong enough to overcome any interfering signal. A typical 3-element FM antenna costs about $70 - $ 100.
For listeners who wish to try out their TV aerial as an FM aerial, the Procedure is as follows:
Disconnect the aerial feed from the TV set and connect it to the appropriate aerial input terminal on the FM receiver. If good reception is obtained and 75-ohm coaxial cable is being used then a device known as a "Splitter", can be inserted into the TV antenna feed which will provide two outputs for the one input. One feed is used for the television set, and the other is used for the FM receiver. Your local radio/TV serviceman can help you if necessary.
Receiver Type
In most cases, the city FM listener is provided with a signal that allows satisfactory reception with a cheaper type of receiver. The rural FM listener, however, if living in an area that is not "line of sight" to the transmitter, and that is surrounded by hills and valleys, will require a receiver that has good sensitivity and provision for connecting an outside aerial.
The portable FM receiver, which is probably the most common receiver in use, is fitted with a telescopic whip aerial for receiving the FM signals. To receive a signal, this whip aerial MUST he extended. On some receiver models, the whip aerial can also be tilted and rotated to obtain best reception.
Some portable FM receivers are provided with terminals for connecting an external aerial to improve reception and some stereo systems provide a simple indoor "300-ohm Ribbon Dipole" aerial with the receiver and terminals for that aerial, or for an external aerial connection via 75-ohm Coaxial Cable.
The owner’s manual for the receiver usually shows how to connect the "Dipole" aerial and also how to connect an outside aerial. The "300-ohm Ribbon Dipole" aerial can be bought from Radio and TV appliance shops and costs less than $20.
Multipath Distortion
Some listeners will switch their receiver to FM and find that even after careful tuning of the receiver, the reception is distorted. The most likely reason for the distorted sound is that the aerial is picking up, in addition to the main signal from the transmitter, a reflected signal off a hill or some other nearby reflecting surface. This is referred to as multipath distortion, caused by the signal arriving at the receiver aerial via multiple paths.
For the fixed-location listener, the best method or reducing multipath distortion is to use an outside Yagi antenna designed for the FM band.
Reflected rays interact with direct rays causing distortion.
Technical Notes
Information
Transmission
Scheduled transmission outages
Downloads
Radio New Zealand frequency list (PDF, 32 KB)
RNZ National FM coverage map (PDF, 1MB)
RNZ National AM coverage map (PDF, 1 MB)
RNZ Concert coverage map (PDF, 1 MB)
AM Network coverage map (PDF, 1 MB)
Frequency Finder
Frequency Finder has been developed by a consortium of radio broadcasters comprising members of the Radio Broadcasters Association (mainly commercial operators), Radio New Zealand, Iwi broadcasters and a number of independent AM and FM stations.
The purpose is to help radio listeners find their favourite stations.