Effective Diffraction of Radio Waves
By: Edward Czajka
Radio waves act differently when their frequencies (wavelengths) are changed. UHF frequencies, between 300 MHz and 3 Ghz, have a tendency to only operate by near line of sight, while HF frequencies, between 300 KHz and 30 MHz, tend to bend and hug the surface of the earth or bounce off the atmosphere. VHF is between the aforementioned frequency regions, between 30 MHz and 300 MHz, and it will act like both HF and UHF, as it will sometimes bend around solid objects. This is a graphic demonstrating the different radio bands.
AM Broadcasts are between 520 KHz - 1710 KHz, Citizen Band (CB) is around 27 MHz, Family Radio Service (FRS) is around 462 MHz, Cell phones operate around 800 MHz, and WiFi networks operate on 2.4 GHz. My intent is to test the ability for a radio wave to bend around objects, and compare that to other frequencies, to measure performance.
Experimental design
This experiment will be conducted with two radio sites, one radio station with a long wire antenna for receiving, and the other station will transmit with a handheld radio through a multiple band antenna. I am a licensed Amateur Radio Operator (KI6PSP), and I will be transmitting on Amateur Radio Frequencies to my receiving station (KC6UWM).
This experiment will involve transmitting on frequencies with wavelengths of 2m (144-148 MHz), and 70cm (420-450 MHz). Multiple locations will be used while transmitting, and the positions will be near a mountain, so I can measure how the signal will bend around a solid object. My transmitting station will be a Yeasu VX-7R with a SRH940 Antenna. The receiving station will be a Kenwood TS-2000 utilizing a 6m dipole antenna. Nearly every Amateur radio has what is called an "S Meter", for Signal Strength meter, that will show us how strong the signal being received is. We will use the S Meter to determine the signal strength of the transmitting station, and compare the signal strength values when we change location of the transmitter. I will begin my tests with a clear line of site to the receiving radio station, and establish a baseline to plot my data from. I will transmit on a given frequency from my control point with 2.5 watts, and the receiving station will log the signal strength he receives, then without changing any settings on the radio, I will walk 25 feet away from my control point, into the canyon, and transmit again for 10 seconds. The receiving station will log the result, and I will continue this process until the signal is unreadable due to the frequencies inability to diffract around the mountain. As I move away from my line of sight control point, there will be more mountain mass between the two radio sites, thus we can measure the diffraction of the radio waves around the mountain. Once my signal is unreadable, I will return to my control point, and then change frequencies to test the next frequency band. I will plot my data, and base the loss of signal on the line of sight control. I chose this design because I could easily obtain the required equipment, I could easily perform the experiment, the results can be easily duplicated, and the transmitting station would be easy to operate due to it's simple setup. To reduce threats to internal validity, I will utilize the same equipment with the same settings (Antenna, Transmitter Power, Height above the ground) for each transmission, and perform my tests on the same day concurrently, so environmental variables (changes in temperature) will not affect the results.
Because I will not be able to correct for Free Space Loss, I will baseline my data on a line of sight data set, and plot the reduction of signal as I move behind the mountain.
The Red Pin is the receiving station, while the Purple Pin is the Transmitting Station. Notice the small mountain near the transmitting station.
Google Map (2011)
Literature review
A RF diffraction experiment was performed by 2 amateur radio operators in Virginia, and they demonstrated that radio waves can bend around an object like a mountain, even on Ultra High Frequencies (between 300 MHz and 3Ghz) (ARES/RACES of VA, 2007). This is a good example for my experiment because it shows that signals will diffract (bend) around solid objects, such as mountains.
A similar radio wave experiment was conducted by students at Kansas State University, they tested the effective range of radio frequencies with limited power levels (Kansas State University, 2009). In their experiment, they measured the effective amount of signal they could receive at a distance.
Dependent, independent, and controlled variables
Independent: Frequency (wavelength), Location relational to a solid object
Because I am comparing the performance of different frequencies, I must have two independent variables, otherwise this would simply be a demonstration of how one frequency diffracts when I change position, verses comparing the results of several frequencies.
Dependent: Measured signal levels of transmitted signals on other side of object, relative to line of sight control.
Controlled Variable: Height of Transmitting station above the ground, Transmitter power, Antenna used to radiate the signals, Length of transmission.
Hypothesis
The ability for a radio wave to bend (diffract) around a solid object is inversely proportional to it's frequency (wavelength).
I developed this Hypothesis based on some personal experience and the desire to quantify radio frequency performance. Upon research, I discovered that there are various mathematical equations that Radio Frequency Engineers use to build RF links when objects are blocking line of sight (Afar Communications, 2011). They use a Fresnel Zone to calculates the area of the object that is blocking the path, factoring the wavelength of the radio frequency used, and use the resulting data to plan their links. They will usually plan on having no more than 40% of their Fresnel Zone obstructed, to have reliable communications.
Experiment Data
Conclusion
The data collected clearly shows that the different frequencies diffracted around the mountain in the experiment. When you compare the results from 2m and 70cm, they confirm my hypothesis. The amount of diffraction of the 2m signal is clearly more than the 70cm signal. The 70cm signal dropped to 50% of it's signal strength at 50 away from the control point, while 2m reached 125 feet before it yielded the same results, clearly an improvement of diffraction.
Experimental design is essential to a reliable experiment, as lack of design and planning can influence the experiment results. I had to redesign this experiment because of lack of equipment, and technical difficulties. I originally started with a Service Monitor, capable of giving data by the dBm, but it wasn't sensitive enough to receive the signal at the distances we were using. During the redesign process, I was able to figure out, that I didn't need to have the expensive equipment, and I could achieve the same results by using a control point, and keep everything relative to the control point. This would eliminate various discrepancies with the equipment used, and how they operate with different frequencies. Proper planning was still a big factor to producing accurate results.
I performed this test with two basic amateur radio stations, so it can be easily replicated by another group of people with minimal equipment. This would simply involve using two licensed operators, and two multi band radios, with a solid object to test with.
References:
Google (2011) Terrain Map. Retrieved on July 22, 2011 from:
http://maps.google.com
ARES/RACES of VA (2007) Knife Edge RF Diffraction. Retrieved on July 20, 2011 from:
http://aresracesofva.org/index.php?option=com_content&view=article&id=71&Itemid=116
Kansas State University (2009) Propagation Comparisons at VHF and UHF frequencies. Retrieved July 20, 2011, from:
http://www.ece.ksu.edu/~wkuhn/pubs/Propagation_Comparisons_at_VHF_and_UHF_frequencies.pdf
Afar Communications (2011) Fresnel Zone Calculator. Retrieved July 20, 2011, from: http://www.afar.net/fresnel-zone-calculator/
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