IAN LANG ELECTRONICS
No 1.The Aerial.
The job of the aerial in a receiver is to have a voltage inducted upon it by an electro-magnetic wave broadcast from a transmitter.
Aerials take many forms but as we are making a small, portable receiver for AM the one we will most likely be using is a ferrite rod. This consists of several hundred turns of Litz wire (very thin enamelled copper wire) wound around an iron core. If you have ever taken the back of a small radio you may well have found one of these; they are very noticeable. Below is a drawing of one which does long wave and medium wave, the data is supplied by Rapid Electronics:
We need certain standards here, and if you place the coils as shown the MW coil will have an inductance of 370 microHenries (or thereabouts) and the LW coil 4.1mH
Unlike other aerials, the ferrite rod works as an inductor directly in the tank circuit (as we shall see on the next page) and it has certain drawbacks. It is highly directional, and if the ends point in the direction of the incoming signal it cannot receive it. Maximum pickup comes when the ferrite rod lies parallel to the direction of the signal; however under these circumstances it does have a very good response; no external aerial is needed. Like all aerials, it responds to all signals and not just one, and so we need to filter out for the one we want. We do this by creating a tank circuit, and we discuss this further on the next page.