Reception and Antennae

Posted in Technical Information

 

Quality of radio reception is dependent upon several factors. The most important one for you to consider is the quality of the antenna or aerial you are using.

What is a good Antenna?

A good antenna is the key to successful short wave reception. Time and effort spent making an antenna will result in improved reception. The most common form of antenna is a piece of wire strung between two supports with one end connected directly to the receiver.

Where to Connect?

Most portable radios have no provision for connecting an antenna, but you can attach a wire to its telescopic antenna. Cabinet mounted radios often have provision for both an external antenna and an earth. The earth terminal is marked "E" or by its electrical symbol  or . The antenna terminal is marked "A" or by its electrical symbol  or .

Inside Antenna

Unfortunately, an inside antenna that works well is difficult to achieve. Adding a length of wire to the radio can help reception. Under certain conditions an inside antenna performs very well. Living in a steel reinforced concrete building with a tin roof will give poor results. A brick or cement block house is better. A roof that is thatch or tile is better still. Ordinarily, the radio's own telescopic or built-in antenna is not enough to give suitable reception inside a building. Try moving the radio to different positions in a particular room or even to another room.

A portable radio, for example, can be placed on a window ledge. Try different positions to determine the best place. The simplest inside antenna is a wire running up the wall to a picture rail and then along two walls. Don't make a complete circle around the room. If there is no picture rail, try along the skirting board. This is not as good because it locates your antenna very low. If your roof is not made of tin, another method is to take the wire straight up the wall into the loft. Anchor the wire to a rafter and run it to the farthest end. Take special care not to let the antenna wire be in contact with other wires, metal objects or plumbing.

Various other layouts may be tried. Remember radio waves easily pass through wood, glass, asbestos and curtaining. They are hindered by metal objects, concrete and the like. If you still experience difficulties, you may find that you are located behind a hill, large building, or some other local feature obstructing the radio signal's path. You should certainly try using a "long wire" antenna.

Antenna Earth Switch

 

   A   To Receiver

   B   To Antenna

   C   Receiver Earth (Optional)

   D   To Earth

 Outside Antenna

There are four simple types of antennae that work quite well and are not difficult to erect. The "clothes line" antenna slopes upward in a straight line from the house to its highest point. The "inverted L", the "T", and the "windom" antennae are very similar. Each has a straight horizontal wire supported as high as possible by poles or trees. The down-lead wire from the "inverted L" is at one end. The "T" antenna has its down lead in the centre and the windom has its connection one-third the distance from one end. These antennas can be as long as 30 meters; something much shorter, 10 - 15 meters performs well.

The higher the antenna is from the ground, the more effective it will be. Each of these antennae will work well for any short or medium wave frequency. They will receive equally from any direction. A short antenna tends to favour signals arriving broadside while a long antenna becomes somewhat more sensitive to signals in the direction the wire points.

A   Roof

B   Insulator

C   Rope

D   Copper or galvanised steel plate. (50cm underground)

 

A   Roof

B   10-30 meters

C   Wooden Pole

D   Earth Rod - 1m minimum

 

A   Roof

B   Down Lead

C   Antenna Earth Switch

D   At Least 5m

E   Steel Water Pipe

 

A Roof

B Down Lead

C Weight & Pully

D Earth Rods (where 1m depth is unobtainable)

Noise

There is always some noise present in reception and there are several causes for it. Man-made noise is the only type we have control over. This noise is produced whenever electrical power is switched on or off. The earliest wireless transmitters used this same principle to produce their radio waves. Some sources of electrical noise which should be avoided when placing your radio or its antenna are fluorescent light, petrol engine, television, electrical motors which have 'brushes' (e.g. sewing machine) and high voltage sources (e.g. neon signs and electrical wires). In each case the item in question should have minimal effect if it is installed properly. Often these can be the source of strong interference.

Earthing

Noise may be reduced by 'earthing' the radio. If your radio is a cabinet model, most often there is an earth terminal. Portable transistor radios can be earthed by connecting to the metal framework of the radio. If a good earth is not readily available, it is better not to use any because a poor earth can introduce noise of its own.

Good earth (or ground):  Galvanized steel water pipe; ground rod driven into the ground; copper or galvanized

Poor Earth (or ground):  Tin roofing; earth pin of a power outlet; the screw on a switch cover; stove or other electrical appliance.

The wire from the radio to the earth should take the shortest possible route.

Lightning Protection

With all outdoor installations care should be taken to ensure the radio is safely guarded against lightning. The best step to take is to install an earthing (ground) switch adjacent to the point where the antenna enters the house. When a lightning storm is approaching, or when the radio is not being used, the switch should be put in a position that connects the antenna to earth (ground) while disconnecting it from the radio. A lightning arrester can be purchased and connected between the antenna and earth wires. This gives added protection if lightning were to strike while the radio was operating

Hardware

Wire:  The earth should be connected with stranded bare or insulated copper wire. This is standard 1.5mm electrical wire. The same type of wire can be used for the antenna. A suitable lighter type of antenna wire is made by splitting rip cord in two, lengthwise.

Insulators:  Porcelain or glass insulators should be used at each end of your antenna. These can be bought at an electrical store. A good substitute is to use old porcelain cleats which are used for open electrical wiring in houses.

Points to observe with Outside Antennae
  1. The antenna should be well clear of obstructions such as windmills, tin roofs and wire fences.
  2. Telephone lines and power lines exhibit a shielding effect of a nearby antenna. If possible, place your antenna so that its length is at right angles to other wires.
  3. Never allow the antenna wire, whether insulated or not, to come in contact with any metal surface or any other wires. Precautions need to be taken to ensure even if the antenna were to break, it would fall safely to the ground.
  4. In houses with thatched roofs, particular care should be taken to ensure that neither the antenna nor the down lead go anywhere near the thatch.
  5. The antenna should be erected as high as possible. Five meters above the ground should be considered a minimum height.
  6. The antenna installation should be sturdy and well made. When the antenna is attached to a tree, some allowance needs to be made for the tree swaying in the wind. A rope, pulley and weight can be used to keep the tension consistent.
  7. The best antenna or earth connection has no joints in it at all, but where splices are necessary, they should be well soldered.
  8. An Antenna-earth switch is essential for the safety of the receiver as well as for personal safety. A lightening arrester is an added safeguard.