Re: How to display radar (including diffraction/reflection) experimentally

Question ID Number 1206956750.Eg

References:

1. Robert Buderi, The Invention that Changed the World (RADAR), Simon and Schuster, 1996

2. Mad Science Archives: How does a Doppler Radar use the Doppler Effect to Find Information?

http://www.madsci.org/posts/archives/1997- 05/862442804.Ph.r.html

Greetings Peter:

It is possible to actually make radar measurements using simple, inexpensive equipment. I often use signals
from local FM radio stations as transmitters to have students perform radar related experiments. In the USA
our FM stations are centered around 100 Megahertz (100 MHz) which is 100 million cycles per second with a
wavelength of 3 meters. I am not familiar with frequencies of the FM stations in South Africa; however,
they probably operate in the Very High Frequency (VHF) band between 30 MHz to 300 MHz. You can calculate the
wavelength in meters of a transmitter by dividing the speed of light, 300 million meters per second, by the
frequency in cycles per second.

For example the wavelength at 100 MHz = 300,000,000/100,000,000 = 3 meters.

1. For radar experiments we use battery powered FM transistor radios with small whip antennas. The FM radio
I use for these experiments costs about 20 dollars USA. It is important that no power wires be connected
to the radio that can distort the received signals.

2. You must determine the general direction toward a local FM station's transmitter antenna. They usually are
located on a high tower, on top of a high building or on a mountain. As long as you can receive a strong
signal, I have used signals from FM transmitters located on a mountain as far as 100 km away.

3. You want to perform the experiments in an open field, such as a football field or a park away from
buildings and electrical wires and electrical lighting systems.

Experiment 1. Locating the direction of the FM stations transmitter antenna.

In an open field tune your FM radio to the desired station. It is best to listen to talking or classical music
programs for the experiments. Rock type music with drum beats make it difficult, but not impossible, to
discern FM radio signals reflected from objects. Place the whip antenna parallel to the ground and hold the
radio far away from your body. Point the end of the antenna toward the horizon in the general direction of
the transmitter and rotate around until the radio signal decreases. This happens when the antenna is pointed
directly toward the transmitter. You may also move the antenna tip slightly up and down and right and left
to find the best cancellation of the FM signal. This is technically called a null. It should be possible to
completely reduce the FM signal close to the noise level of the receiver. FM radios have automatic gain control
(AGC) circuits to decrease the receiver sensitivity for signals from strong stations. By reducing the level
of the transmitter signals detected by the antenna the receiver will become more sensitive to detect weak
signals reflected from moving targets. Thus tilting the antenna to reduce a strong FM signal greatly increases
the sensitivity of the receiver and we use this technique in our experiments.

Experiment 2. Detecting Doppler shifted radio wave reflections from moving vehicles.

I discuss the Doppler Effect in radar in Reference 2.
This experiment simulates the classic experiment performed near Daventry, England on February 26, 1935 when
radio engineers used the signals from a BBC short wave transmitter to detect the Doppler shifted radio signals
reflected from a moving RAF aircraft. This demonstration proved that radio signals reflected from aircraft can
be detected with suitable receivers. The result of the Daventry experiment led to the development of the
"Chain Home" radar defense system used during World War II to defend the British Isles from attack by German
bombers.

Tune your FM receiver to the frequency of the transmitting station that you selected and set the radio on a
wooden or plastic table. Next tilt the whip antenna toward the transmitter until the signal strength starts
to fade into static but does not completely null the signal out.
Now listen closely and if there are any large moving aircraft between the transmitter and your receiver you
should be able to detect a fluttering in the amplitude of the station's programming caused by the Doppler
shifted reflections from aircraft. These aircraft can be many kilometers away and out of sight. I have
detected Boeing 747 and 767 aircraft flying at 10,000 feet, more than 30 kilometers from my receiver. In
fact, depending on the air traffic in your area, you may hear signals from several aircraft at the same time.
If smaller airplanes pass close to you, you will receive a strong fluttering signal. When this fluttering
signal decreases in frequency and passes through a zero beat frequency, the aircraft is directly between
you and the transmitter. After the aircraft passes between the transmitter and your receiver, the frequency
of the fluttering will increase. This fluttering in amplitude of the transmitter programming material is
caused by the Doppler shifted signals reflected from the moving target beating with the steady signal coming
directly from the transmitter to your receiver. Once you have detected aircraft you can adjust the receiver
antenna height and rotate it to maximize the fluttering signal.
If you have to wait for aircraft to be in your area you can have someone drive an automobile at different
speeds coming toward your receiver in the direction of the transmitter antenna and your receiver.

Experiment 3. Measuring the transmitter's wavelength and the speed of light using radio mirrors and the radar
characteristics of various materials.

Obtain a large metal lid, similar to those used on trash cans or steel drums, for a mirror reflector. Note the
metal does not have to be polished at radio frequency wavelengths. You can also make a mirror from aluminum
foil attached on a flat piece of wood or use a wire mesh screen on a wooden frame. A mirror between one and
two meters wide works well.

Set up the radio in an open field as described in Experiment 2.

Face the mirror toward the transmitter and move back away from the FM radio receiver. You should hear the
radio signal fade (null) every one half wavelength as you move backward. This is caused by the signal
received directly from the transmitter being canceled by transmitter power reflected by the mirror
to your receiver. Measure the distance between a number of nulls and average them to find the wavelength.

The speed of light is equal to the transmitter frequency times the wavelength. Calculate the value that you
measure and compare it to 300 million meters per second. Your number for the velocity of light will not be
very accurate but it demonstrates the techniques that scientists use to make these measurements.

Next, cover your metal mirror with various materials of different thickness such as wood, cardboard etc
and repeat the wavelength measurement technique. Materials that absorb radio frequencies will not produce
deep nulls when compared to metal mirrors. A completely absorbing material will produce no reflections and
no nulls in the received signal. Of course using meters would make these experiments more accurate; however,
this demonstrates techniques that scientists actually use to measure materials at radio and microwave
requencies.

Experiment 4. Diffraction

Diffraction is very difficult to measure without very sophisticated equipment. If you can, find a metal
building or large metal fence that completely blocks the signal from an FM transmitter from which you can detect
a strong signal when you are in the clear. By measuring the station's signal strength as you pass into the
object's shadow will demonstrate that the edge of the shadow is not sharp. This blur of the edge is caused
by diffraction.

Best regards, Your Mad Scientist
Adrian Popa