|OddMix CIRCUITS - CRYSTAL RADIO|
A Good Antenna is Best RF Amplifier for Crystal and AM Radio Sets
Noting is as important to have a success with a crystal radio like a good antenna. Most often radio experiments fail, because there is not enough signal to make stations detectable. Often, local radio stations are very much limited for transmitter power during the daylight hours by the FCC. After sunset many small stations has to stop transmitting altogether to avoid generating interference signals with farther away powerful stations. Thus only powerful far away stations available more often.
|Figure 1. Inverted-L Antenna for crystal detector radio|
In vacuum tube type or solid state radios where there is plenty of circuit gain, smaller input signals may be tolerated, within reason. Even for those radios when the incoming signal into a high gain amplifier is near zero, than the output is not getting that much greater. And even the small signal picked up by the inadequate antenna will be covered up by equipment generated or atmospheric noise.
All common amplifiers have a noise of their own. An important parameter for RF (Radio Frequency) signals is a parameter called a S/N (Signal per Noise) figure. If the small incoming signal is nearly the size of the noise, the reception will be difficult at best. Nothing known for present day technology is as effective in the improvement of reception strength as a good antenna. So build a good antenna!
For crystal detector and other AM radios an almost ideal antenna, that is also easy to construct is the one named Inverted-L. As the name implies the Inverted-L antenna is a longer piece of horizontal wire with a lead wire at one end - Figure 1. For this antenna to be successful, it has to be of good conductor, long and tied as high as possible, preferable higher then the surrounding tall objects. The connection to the two anchor points has to be using glazed porcelain strain type insulators, at least one in each end - Figure 2. Since antenna wire is metal, ideally multi strand copper that has considerable length and significant temperature dilation, it is wise to tie it to its support using the constant force method.
|Figure 2. Airplane strain insulator glazed low absorption porcelain|
The simplest way to achieve a constant force is using a spring or a pulley and an appropriate weight. With the pulley method the antenna can be tied to a large tree in the distance without worry too much about movements caused by the wind - see the antenna Figure 1. In older Radio Shack catalogs the company used to sell AM radio antenna kits complete with the strain type multiple stranded, substantial, 12 or 14 gauge, bronze wire and the insulators for a reasonable prize. Radio Shack or others may still sell similar kits presently. A stranded hard copper antenna wire has lower signal impedance - resistance to RF - therefore it is an ideal material.
The long wire, inverted-L antenna has significant capacitance on its top therefore its effective height is higher than single wire vertical antennas. The higher the effective height the better the antenna will be able to supply the radio with strong signals. Since the end point of the antenna can have very high voltages, it is proper to use two or three insulators in a chain Figure 1. Commercial antenna installations or transmitter antennas always use lots of porcelain insulators of many different types. For simplicity, the longer and higher the antenna the better chance it has to collect the radiated signal from the surrounding space. Use at least 50 to a hundred feet of wire for the length of the top wire between the insulators. This 50-100 ft top length of the antenna is marked L on Figure 1.
To protect against lightning it is very important to connect the free end of the lead in wire to an antenna knife switch. The knife switch that it is made for antenna switching has two positions and a spark gap in addition. During electrical storm the if a voltage induced on the antenna wire gets greater then the breakdown voltage of the gap, the charge can harmlessly jump to ground, instead of ruining the radio or other items connected to it. It is wise to use a neon bulb, or some spark gap or MOV type protectors in addition to the antenna switch. Commercial lightening protectors are available for TV antenna installation can also be used for AM antenna protection.