Speed of Light with a TV Receiver
Randy Landsberg, instructor
Ashley Garrison, TA

Ghost in the Machine

In the days before cable television and widespread use of satellite dishes, most people used antennas on their roofs to receive television signals. Often these antennas do not produce a clear picture for many reasons, hence their declining popularity. In this lab we are going to look for an unclear signal on purpose. The signal that we are seeking is called a ghost or a reflection. We will use this signal and what we know about how TVs work to measure the fastest thing in the universe!

Spying the Ghost

1

Set up the TV and antenna. Experiment to determine which channels provide the best reception in Williams Bay, WI. Which channels are they? What makes them the "best". Where do they originate?

RANK

CHANNEL

ORIGIN

WHY

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2

Pick a channel with a strong signal, and move the antenna around and to different locations, what do you observe?

 

 

 

 

Move the antenna around until you find what looks like a double image. There will be two copies of the picture slightly offset from each other and one much fainter then the other. Congratulations! You’ve found the GHOST. That is what this second image is called. Use your compass to record the location and how the antenna is aligned to produce the most pronounced ghost.

LOCATION

ANTENNA ALIGNMENT

   

3

What do you think causes the ghost?

 

 

 

 

 

How A TV Works

A TV creates the impression of moving images by slightly changing the picture on the screen continually at a rate faster than the human eye can detect. TVs in the US conform to the National Television System Committee (NSTC) standard. This means that the entire screen is refreshed 30 times a second. Think about it. If each screen has 525 horizontal lines, and a horizontal scan rate of 15,750 lines/second (i.e., 30scans/sec x 525 lines/scan = 15,750 lines/second). Scanning an individual line takes slightly less that 1/15,750 second because there is a small lag time to get from one part of the screen to another, so one scan across the screen takes 1/15,735 of a second, or 0.0000635525897680331 second!

If the refresh rate was much slower we might notice the small changes and see a flicker. The entire image is not changed at once, rather a new image is produced as the TV scans through each line of the image. This similar to a person reading printed text. They read from left to right, from top to bottom, character by character, line by line. The image on a TV screen is produced by an electron beam scanning across it and hitting the phosphor coating on the screen that glows when struck by the electrons. Bright spots in the image correspond to more electrons; darker spots less electrons. Things get a bit more complicated with color TV, but it is essentially the same process. The difference is that instead of just one spot being hit, three different ones are hit - each corresponding to a specific color (cyan, magenta and yellow).

The frequency range assigned to broadcast TV is actually part of the Radio frequency of the electromagnetic spectrum. Radio frequencies are on the order of a million cycles per second or a million hertz (Hz) or 106 Hz or one megahertz (MHz). The specific bands in the radio frequency range are:

CHANNEL

MHz

TYPE

2 — 6

54 — 88

VHF (Very High Frequency)

7 — 13

174 — 216

VHF (Very High Frequency)

14 - 69

470 -806

UHF (Ultra High Frequency)

The TV’s scan rate is fixed and independent of the frequency of the carrier signal. These are very long wavelength waves on the order of centimeters to meters (10-1 to 100 m), compared to visible light that has wavelengths in nanometers (10-9m). The TV set coverts the information contained in these radio waves to voltage changes that correspond to a specific spot or pixel of the screen, every 1/30 of a second.

Antennas

TV antennas are designed to interact with the radio waves that carry the TV signal. Most TV antennas are placed on roofs and orientated in specific directions for the best reception.

Was That Halle Berry Or A Teletubby? — Or Why Is The Reception Terrible

There are a number of reasons that a TV connected to an antenna will display a poor image.

4

Assuming that the TV set is working fine, list some possible reasons for a poor signal.

 

 

 

 

 

5

Test your predictions with the TV/antenna assembly. What did you find out?

 

 

 

 

 

Interference

Waves can add together and create bigger or smaller waves. This is called interference. When the crests and troughs of the two waves line up, we say that the waves are in phase, and they will have constructive interference. When the are out of phase we say that is destructive interfere. Waves of different frequencies can interfere. The waves do not have to align exactly to create 100% constructive or destructive interference.

6

It’s time for a Wave Workout. You will be given a handout and asked to sketch some waves. Enjoy!

Measuring the Ghost (distance = time????)

If a TV signal is reflected and hits an antenna a second time, the TV will just treat both signals as the same and try to display them at once. (Imagine everyone singing "Row, Row, Row Your Boat" slightly out of sync). But just how out of sync are they?

If we know how long it takes to scan across the TV screen (which we do, 1/15,735 second), how long does it take to scan half way across? Right, half the time to scan the whole screen (1/2 x 1/15,735 second). In fact, since the scan rate is fixed in the US, if we know how wide a screen is we can determine how long it takes to scan any fraction of the screen. This can be determined by a simple ratio:

Ghosts As A Timer

7

Now it’s time for us to combine our knowledge of how TVs work with a simple length measurement to determine an incredibly short time.

Measure the width of the TV screen (mm) and record.

Find a good ghost and record the antenna location and direction.

Measure how wide the ghost is (mm). {This part is tricky and may require a creative solution}

Calculate how long the time lag was between the ghost and the main signal. How far behind the main signal is your ghost?

TV Screen Width

 

Ghost Width

 

Lag time

 

Sketch what you measured. (Does this make you a Ghostly Artist?)

 

If the ghost is a result of the radio signal reflecting off of some object and bouncing back to hit the antenna again, what do you think it bounced off? Why?

 

 

 

 

Speed from Distance and Distance from Speed

Since we know the time difference between the ghost signal and the main signal, if we can measure how far away the object it reflected off is, we can then determine how fast the broadcast TV signal travels.

8

Measure the distance from the antenna to the reflector, and describe how you measured the distance.

DISTANCE

METHOD FOR OBTAINING DISTANCE

 

 

 

 

 

9

Now that you have the time (Lag time from # 7) and the distance, calculate the speed of the TV signal. Recall:

Speed = distance/time

CALCULATION AREA:

 
Speed=

Speed of Light

In 1983 the General Conference on Weights and Measures officially redefined the meter as the distance that light travels in a vacuum in 1/299,792,458 of a second. That means that we now have a universal standard for the Speed of Light (C):

C = 299,792,458 meters/second

10

Compare the speed you determined for the TV signal to that of the speed of light in a vacuum. What do you notice?

11

Now let’s review all of the data that you have gathered during this lab. You should be able to complete the data summary section below.

TV Channel

 

City of Channel’s Origin

 

Compass Direction of Antenna

 

Width of Ghost (mm)

 

Calculated Lag Time (seconds)

 

Reflector

 

Distance to the Reflector (m)

 

Calculated Speed of Broadcast TV (m/sec)

 

Some Electromagnetic Radiation (a.k.a. Light) Facts

  1. Light can travel in a vacuum
  2. Light will travel in a straight line unless something interferes with it
  3. Light acts like both a particle and a wave
  4. Light can be reflected, refracted, and diffracted

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