AM Reception Hints
This information sheet has been prepared to assist listeners to obtain satisfactory reception of AM (Amplitude Modulation) radio signals.
Transmission
AM radio signals travel between the transmitter and receiver along the surface of the earth, gradually losing strength as they move away from the transmitting point. This is known as Ground Wave Propagation.
Obstructions to the signal, such as hills and tall buildings will cause the signal to lose strength more quickly, but generally this reduction is only a problem if the listener is outside the transmitter's Primary Coverage Area (the area that the transmitter power allows it to cover satisfactorily).
AM signals travel long distances at night and can cause interference to other stations in neighbouring countries that are operating on the same frequency. This long distance interference is known as “Night Time Skywave Reception”
Reception
An important factor in the reception of AM signals is receiver design. Adequate reception will be obtained by using a radio that is best suited to the level of signal strength available. The typical $9.95 pocket transistor is better suited to the listener living closer to the transmitter where strong signals are available. The more remote listener on the other hand requires a receiver that has the ability to pick up weak signals, i.e. has good sensitivity, and is fitted with terminals which allow the connection to an outside aerial.
Problems
Some AM reception problems experienced by listeners are:
Atmospheric and receiver noise masking the programme.
Interference from other stations at night time.
Fading of the signal at night time.
Interference from electrical devices, e.g. electric fences, TVs and personal computers.
Most AM reception problems can be cured or alleviated by use of an aerial of one type or another.
Fig a. Loop Antenna
Nearly all AM domestic receivers manufactured in recent years have a LOOP type receiving antenna (see above) built into the set. A characteristic of this type of antenna is that it is directional.
Because of the directional property of the AM receiver’s loop antenna, the listener needs to turn the receiver for best reception, particularly when the signal is weak.
Noisy reception caused by electrical devices such as TVs and PCs can be alleviated by turning the receiver for minimum interference.
Frame Aerial
The reception of AM broadcasts on portable transistor radios may be improved by the use of a tuned loop aerial. The aerial (see fig b) consists of two parts.
A number of turns of insulated wire wound around a non- metallic former to make an inductor or coil.
A variable capacitor connected across the inductor to tune the aerial
The value of the tuning capacitor is 350 picofarad (350pf). It is the type that would he used in a crystal set and may be purchased at most shops selling radio components. The number of turns of wire and the diameter of the inductor as shown are matched to this value of capacitor. The loop aerial is very directional. It should be mounted vertically with the edge of the loop pointed towards the station to be received. The transistor radio is placed inside or beside the loop aerial at a right angle to the loop.
Parts for diagram below (figure b)
1. Fifteen turns of single strand bell wire (fine insulated wire) totalling approx. 20 meters.
2. Timber frame with outside measurements on three sides of 380mm and underside 130mm.
3. Position of transistor radio.
4. Base Board.
5. 350 pf capacitor
6. Wire connected to fixed plate connection on capacitor.
7. Wire connected to moving plate connection on capacitor.
Fig b. Frame Aerial
Tune the radio to the desired station and rotate the loop aerial and radio for maximum pickup. Adjust the tuning capacitor on the loop aerial for maximum improvement.
Further improvement can sometimes be obtained by moving the loop aerial from one end of the radio to the other, but always keep the radio at a right angle to the loop aerial. Also, some radios may respond better with the loop aerial beside the radio rather than the radio inside the loop aerial.
In other words, move the radio around and about the loop aerial to obtain the best results.
Outdoor Aerials
Some AM receivers are provided with terminals, which allow an outside aerial to be connected to the receiver. An outside aerial can he very useful when the reception of distant stations is required, especially in rural areas. Figures c and d are examples of outside aerials suitable for AM reception.
Figure c. “Inverted L”
Direction to Transmitter
Inverted L
1. Aerial wire and lead-in in one continuous length (connects to receiver aerial terminal).
2. Earth wire clamped onto earth stake (wire connects to receiver earth terminal).
3. Earth stake (ensures stake is not driven into water/gas pipe or power cables).
4. Wire suspensions.
5. Insulators.
Position the aerial wire (1) so that it is in line with the direction to the transmitter. Also, the end of the wire which is furthest from the house should be closest to the transmitter.
Figure d. “Whip”
Whip Aerial
1. Metal rod aerial mounted on insulating support.
2. Down-lead (connects to receiver aerial terminal).
3. Insulator supporting down-lead clear of gutter.
4. Earth wire clamped onto earth stake (wire connects to receiver earth terminal).
5. Earth stake (ensure stake is not driven. into water/gas pipe or power cables).
If, despite your best efforts you are unable to get decent reception, try a different radio receiver, or try installing an active antenna. Here’s an extract from an advertisement for an active antenna kit…
Active Antenna
Provides improved reception up to 30MHz. Can be used with either a telescopic rod, a few meters of wire draped over a picture rail or dangling from an indoor dipole. Improves the performance of elderly shortwave receivers in selectivity & sensitivity. With tuneable selectivity & a healthy, smoothly–adjustable gain from 500kHz (below bottom of the AM broadcast band) up to 30MHz (the top of the HF shortwave band.) Power source required: 12 volts DC. Supplied with components, hardware, PCB, case, pre-punched screened front panel, pre-punched rear panel & telescopic antenna.
This should be obtainable from your local electronics shop.
Technical Notes
Radio receivers are sensitive to radio waves present all around us. Putting up an aerial of any sort helps collect these waves & feeds them into a coil in your radio receiver.
Depending on the type of radio receiver you have, the earth connection is important too.
The aerial terminal on your receiver is connected to one end of the coil & the earth terminal to the other. Given that radio signals always want to travel to earth as easily as possible, it then follows that if you connect the earth terminal to earth, it helps to “pull down” the signals through the coil. The more signal through the coil, the better your reception & listening pleasure.
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.