As noted in the page for demonstration 76.09 -- Radio transmitter lights antenna bulb, the light blue box on the cart in the photograph above (with an upright lucite strut attached, which has a thin rod screwed into it on either side at the top) is a radiofrequency transmitter. It oscillates at 147.496 MHz, which corresponds to a wavelength of (2.998 × 108 m/s)/(1.47496 × 108 cps) = 2.033 meters. (The output power is about 20 mW.) The two rods at the top form a radiating dipole, and their ends are one meter apart. This corresponds, at least fairly closely, to one half wavelength.
The darker blue box mounted to a lucite strut is a receiver. Mounted to either side of the top of the strut is a telescoping antenna, and at the top (in the middle) is a light bulb. For normal operation of the unit, each telescoping antenna half should be adjusted so that it is the same length as one of the antenna halves on the transmitter. When the (receiving) antenna is oriented so that an incoming radio wave can induce an EMF in it, the receiver turns on the light bulb. The brightness of the light bulb corresponds to the amplitude of the radio wave, i.e., the greater the amplitude, the brighter the bulb. A sensitivity setting allows you to adjust how much the brightness of the lamp changes with the amplitude of the incoming radio wave. IMPORTANT: If this control is changed from its proper setting, some aspects of this demonstration may not work.
The antenna of the transmitter is a dipole antenna, to which the transmitter applies a sinusoidal voltage at 147.5 MHz. This sets up an oscillating electric field across the antenna, with a corresponding oscillating magnetic field perpendicular to it. These propagate together from the antenna as an electromagnetic wave – a radio wave. This wave propagates in all directions along a plane that cuts through the center of the transmitter’s antenna, perpendicular to its axis, with the oscillating electric field parallel to the antenna axis, and the oscillating magnetic field perpendicular to it. It travels with the speed of light, and its wavelength and oscillatory frequency are related by λ = c/ν, as noted above. When you orient the receiver’s antenna parallel to the transmitter’s antenna (but not along its axis), the transmitted radio wave induces an oscillating EMF in the receiver’s antenna, which sets up an oscillating current in the receiver. The circuit in the receiver then lights the bulb with a brightness that is, as noted above, proportional to the amplitude of the received radio signal.
Polarization:
By orienting the receiver’s antenna in different ways, you can show two things. First, when you orient the antenna along the axis of propagation of the radio wave, that is, perpendicular to the transmitter’s antenna but in the same plane with it, the bulb does not glow. If we think of the emitted radio wave as a plane wave, this illustrates that the receiving antenna must be parallel to the wavefronts for the receiver to detect it. Second, if you have the receiving antenna perpendicular to both the axis of propagation and the transmitting antenna, the bulb does not glow. This shows that the radio wave is horizontally polarized. That is, the electric field oscillates in the horizontal direction, while the magnetic field oscillates in the vertical direction. The receiving antenna must be parallel to the electric field (and thus perpendicular to the magnetic field) for the receiver to detect the radio wave.
Also, if you move the receiving antenna to either end of the transmitter antenna, either in line with it or not, the light will not light, because along the axis of the transmitting antenna there is no oscillating field.
In the photograph above, next to the receiver is a 1-meter-long aluminum rod.
If you hold the receiver so that its antenna is horizontal, i.e., parallel to the transmitter antenna, the bulb, of course, lights. If you now interpose the aluminum rod between the two antennas, parallel to them, the light goes out. When you do this, it is important that you grasp the rod in the middle (or close to it).
If you hold the receiver so that its antenna is vertical (so the bulb does not light), then interpose this aluminum rod between the receiving antenna and the transmitter antenna, perpendicular to the line between them (so in the plane of the wave fronts) at an angle of 45 degrees, the bulb lights. As in the previous activity, when you do this, it is important that you grasp the rod in the middle (or close to it).
This is similar to the phenomenon that occurs when you place two crossed polarizers (i.e., at 90 degrees) in front of a light source, and then place between the crossed polarizers one whose easy axis is at 45 degrees to theirs. (See section 1) Polaroid sheets (polarizers) in the page for demonstration 84.36 -- Polarizers).
Standing waves:
By placing a sheet at an appropriate distance from the transmitter, with the transmitter antenna parallel to the plane of the sheet, you can set up a standing wave between the transmitter and the sheet, whose nodes and antinodes you can detect with the receiver. The transmitter antenna is an antinode. The sheet acts like the fixed end of a mechanical system, or the closed end of an organ pipe, and reflects the radio wave out of phase with itself. It thus creates a node. An oscillating system with a node at one end and an antinode at the other allows only those oscillations that correspond to odd integral numbers of quarter-wavelengths to survive. Since the wavelength of the radio wave is approximately two meters, this means that the distance between the sheet and the transmitter antenna must be some odd multiple of 0.5 meters. Of course, if we place the transmitter and sheet 50 cm from each other, all we see is a node at the sheet, and an antinode at the transmitter. We could place the sheet at any odd multiple of 50 cm from the transmitter, but it is best to place it at least 4.5 meters from the transmitter. The radio wave diverges as it travels from the transmitter, and the reflected wave continues to diverge, so that by the time the reflected wave returns to the transmitter, it is much smaller in amplitude than the wave being emitted by the transmitter. Because of this, for any distance between the sheet and the transmitter, it is impossible to detect the node that should appear at 50 cm from the transmitter. With a separation of 2.5 meters, you can observe the node at the sheet, an antinode about 50 cm from the sheet, a node about one meter from the sheet, then an antinode that appears at about 1.5 meters from the sheet, but continues up to the transmitter. A separation of 3.55 meters gives you three nodes and two clear antinodes. Longer distances, up to 6.59 m in Broida 1610, give several clear nodes and antinodes in the middle of the space between the transmitter and the sheet, but the first node after the sheet becomes difficult to observe.
Another method of performing this demonstration is to set the transmitter up in the middle of the room, place the receiver at a distance of 4.06 meters from it (so two wavelengths; one wavelength, 2.03 meters, also works), and then take the sheet and hold it up behind the transmitter. As you walk away from the transmitter, you will observe the light bulb dimming at roughly one-meter intervals. In room 1610, you should be able to get at least four nodes and three antinodes. (With the transmitter and receiver about two meters apart, the receiver being between the exit door and the end of the main demonstration table, and the transmitter about 3-1/2 feet away from the end of the demonstration table (towards the wall), you can easily observe six nodes.)
With the second method, the nodes are clearer, and the behavior is consistent throughout the travel of the sheet. Which method you choose depends on how you would like to present the standing wave – whether you would like to have it exist over a fixed distance, along which you probe with the receiver to find the nodes and antinodes, or you would like to change the distance and show that constructive (or destructive) interference occurs only for certain distances between the sheet and the transmitter. Also, it depends on whether you prefer to carry the receiver or the 40-inch-square sheet.
References:
1) Halliday, David and Resnick, Robert. Physics, Part Two, Third Edition (New York: John Wiley and Sons, 1977), pp. 904-905.