EC312 Security Exercise 16
Drivers start your engines.
Today we looked at various ways radio waves propagated through space, air. For this lab, we will be using
radio control (RC) cars, our communication system, to evaluate the propagation of electromagnetic waves as
they traverse through space.
Now we have gone out of the way to purchase the best radio control cars in the world. That’s right! Only the
best for you guys. We acquired Ferraris, Audi R8, Lamborghinis, Camaros, etc. Don’t they look so pretty?
The cars that you have available to you today operate at a couple of different frequencies.
Question1: Examine the cars and write down the frequencies at which the cars operate.
a. First car (lower frequency). _______________________________________
b. Second car (higher frequency): ____________________________________
Question 2: Based on the frequencies you just determined for the cars in you, what are the possible
propagation paths (based only on frequency) which are used? (i.e. ground wave, sky wave, space wave)
_______________________________________________
Question 3: Based on the lab environment you’re working in, which of the propagation paths determined in
Question 2 is actually used in the RC cars? __________________________
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Even though one of the propagation paths was sky wave, the signal still travels in a straight line. In fact, do you
think the signal from the RC car transmitter is strong enough, or powerful enough, to take advantage of sky
wave propagation? It’s not. When you think of sky wave propagation you should be thinking about more
powerful transmissions such as AM radio.
You’ve determined some good information. Now let’s use the information that information to evaluate the
abilities of the RC cars.
Question 4: What are the wavelengths of the frequencies associated with the RC cars?
a. Lower frequency wavelength. _____________________
b. Higher frequency wavelength. _____________________
Now that you know the wavelengths associated with the frequencies, how far do you expect the cars to travel?
You need some information to calculate the distance. The gain for the transmitter is -8dB. The gain for the
receiver is also -8dB. The power of the transmitter (PT) is 10dBm. The power necessary at the receiver (PR) to
control the car is -50dBm. Rearrange and use Friis Free Space equation to determine the distance.
Question 5: Expected distance:
a. First car (lower frequency). _______________________________________
b. Second car (higher frequency): ____________________________________
Alright. You got your calculation. Now, it’s time to take measurements. Measure how far the lower
frequency car will go. Make sure your measurement is in meters. Drive from the front of the classroom to the
back and around back benches, not out of the classroom. Remember the distance be should a straight line to the
car, not the path is takes. So how far did it travel?
Question 6: Empirical distance:
Lower frequency car. _______________________________________
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What happened?!?! The car didn’t go nearly as far as you calculated. Why?
Think back to the equation you used to calculate the distance. What did we say about the equation? It needs to
be used in free space. That means no terrain, mountains, buildings, ground, or atmosphere. Do you have
anything like that in the classroom? Look around. Any desks, lab equipment, people? Oh yeah there is! So
Friis Free Space equation isn’t going to give you an accurate distance.
When we have all this lab equipment that can interfere with the signal, the equipment will (1) reflect the signal,
(2) diffract the signal, or (3) scatter the signal. Remember that:
(1) Reflection occurs when energy (or the signal) reflects off a large (relative to the λ) conductive surface.
(2) Diffraction occurs when energy bends around objects
(3) Scattering occurs when energy diffuse re-radiation off rough (smaller than λ) objects.
As the signal is manipulated by all the lab equipment, people, etc., the signal at the receiver is a combination of
many variations of the original signal. This variation leads to a reduced signal. So how are you going to
determine how far the higher frequency car should go?
Let’s use a new equation known as the Log-Normal or Log-Distance model. This model is widely used to not
only predict coverage for a particular mobile user (i.e. the RC car), but also for predicting the interfering signal
power that the mobile user will experience from other Radio Frequency sources (i.e. the cell phones in your
pocket).
The Log-Normal Path Loss (PL) equation: In short, this equation is used to predict the path loss (i.e. power
lost during signal propagation from transmitter to receiver, in dB) when the distance between the transmitter
and receiver is d meters. Since different environments will affect a signal in different ways, we characterize
an environment by a path loss exponent n, which would be determined experimentally. Based on this, we can
����(𝑑𝑑), by
estimate the path loss over a distance d, i.e. 𝑃𝑃𝐿𝐿
𝑑𝑑
����
𝑃𝑃𝐿𝐿(𝑑𝑑) = 𝑃𝑃𝐿𝐿(𝑑𝑑0 ) + 10𝑛𝑛 log10 �𝑑𝑑 � [dB]
0
Here, 𝒅𝒅𝟎𝟎 is some close-in reference distance (much smaller than the expected d), and 𝑷𝑷𝑷𝑷(𝒅𝒅𝟎𝟎 ) is the path loss
at that reference distance. (Note that this would be either measured or calculated with the Friis’ Free Space
equation.)
Note: Antenna gains, wavelength, etc. are embedded in the model (PL(d0), and n) parameters.
Now, we want to use the Log-Normal Path Loss equation to predict the max distance at which we should be ablt
to control the high frequency car. Here’s some info you’ll need.
- Inside a building with obstructions (i.e. your classroom), you would expect a path loss exponent of 4-6.
For your classroom, use 4 as the path loss exponent.
- Based on information provided on the previous page (about power of the transmitter and required power
at the receiver), you can calculate the maximum acceptable path loss (i.e. the path loss when d is at its
max).
- You are told that at d0 = 1 meter, the path loss is 10dB.
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Question 7: Using the Log-Normal Path Loss equation, calculate the new expected max distance (d) for your
higher frequency car.
Question 8: Now go drive the higher frequency car. Drive from the front of the classroom to the back and
around back benches, not out of the classroom. How far did it go in meters? Remember the distance should
be a straight line to the car, not the path is takes. ___________________________
Aw yeah. Smooth as butter. You did it. So now you can calculate, at least for a RC car, the distance a radio
wave will travel. Just understand that if you change the configuration, (i.e. you go into the hall) you will have
different model parameter and therefore different results.
One last test of your mathematical skills. Using the Log-Normal Path Loss (PL) equation, calculate the distance
the lower frequency car will travel if you were outside. The path loss exponent (n) is 2.6. All other
parameters are the same.
Question 9: What is the expected distance (d) for your lower frequency car? __________________________
Watch the Youtube video, RC Car Outside Distance (http://youtu.be/YzbxHcp3UEk).
Question10: Were you correct (roughly)? ____________________________
Your final test. Using either car, place the car against the wall inside the classroom next to the door. Go
outside the door where you can no longer see the car. Just on the other side of the wall should be fine. Try to
move the car using the radio controller.
Question11: Did it move? Why or why not?
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EC312 Security Exercise 16
Name:
__________________________________________________________________________________________
Question 1:
a.
b.
__________________________________________________________________________________________
Question 2:
__________________________________________________________________________________________
Question 3:
__________________________________________________________________________________________
Question 4 (SHOW WORK):
a.
b.
__________________________________________________________________________________________
Question 5 (SHOW WORK):
a.
b.
__________________________________________________________________________________________
Question 6:
5
__________________________________________________________________________________________
Question 7 (SHOW WORK):
__________________________________________________________________________________________
Question 8:
__________________________________________________________________________________________
Question 9 (SHOW WORK):
__________________________________________________________________________________________
Question 10:
__________________________________________________________________________________________
Question 11:
__________________________________________________________________________________________
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