Abstract: Typically UHF frequencies perform poorly when trees, mountains and / or man-made objects form significant RF obstructions, making them less suitable for voice and digital emergency communications. This article discusses a UHF RF diffraction experiment that provided communications adequate for simulated Search and Recovery (SAR) operations by using a UHF repeater to transmit a strong, directed signal over an intervening mountain to a mountaintop ‘incident site.’
Key Words: UHF, Mountains, RF Obstructions, ‘Knife Edge’ RF Diffraction, Emergency Communications, SAR Operations
Background
The ability to reliably send and receive voice and data over radio waves is essential when establishing radio communications whether over a local, amateur repeater or through a statewide digital packet network. The presence of RF obstructions such as dense stands of trees, mountain peaks, and man-made objects can limit UHF radio communications particularly in the western, mountainous areas of Virginia when a signal must propagate along a phalanx of tree-lined mountain ridges. The situation is worsened if one of the radio stations is located on a valley floor or in a RF shadow next to an adjacent mountain.
Radio
Wave and Laser Experiments
Experiments have shown that when radio waves are transmitted over a ‘knife-edge’ mountaintop projection, that diffraction will cause some of the signal to enter the valley or RF ‘geometric shadow region.’ [1] The same principle generally applies to laser beam diffraction when transmitted over the edge of a sharp object, such as a razor blade. (This photograph courtesy of ITS.)
Experiment Parameters and Equipment
In this experiment, the authors used a mountain ridge as a topographic ‘knife edge’ to determine if UHF RF diffraction could be reliably used for emergency communications. A 45-watt UHF repeater and radio operators were located at an actual emergency helicopter landing zone known as LZ-7. Radio operators were also located at the ROCKD mountaintop ‘incident site,’ as might be done during SAR operations for an air crash. Medevac LZ-7 was established as an Eagle Scout project and is near the crossing of the Appalachian Trail and a U.S. Forest maintained road in Rockbridge County.
Radio
operators were equipped with commercial 4-watt UHF portable radios. The experiment
was to determine if the radio operators at the LZ could remain in contact: 1) while
at a LZ, 2) while driving along mountain roads approaching ‘incident site,’ and
3) while hiking up to the mountaintop. The complicating factor was that a
mountain – or the topographic ‘knife edge’ for this experiment – was located
between the repeater and the ‘incident site.’ In this photo of LZ-7, the mountain
on the left, under a clear, azure sky, presented a sizeable RF obstruction in
height, length, and in distance, making an ideal test range for RF diffraction
experimentation.
A
commercial 6-element, rear-mounted Maxrad® UHF yagi (10.2 dB gain) was placed ~8 feet above the valley floor and
was pointed slightly upward and directly at the distant mountain ridge. A short
run (~20 feet) of LMR-400UF coaxial cable connected the repeater to the
antenna. The UHF yagi produced an ERP of ~360 watts with a 42 degree forward
lobe. Depending on the height of the ridge or mountain, the RF diffracted
signal can be 15 to 20 dB below the primary light-of-sight signal. A strong
signal, nevertheless, was needed for the experiment to work. To limit potential
RF exposure hazards once the repeater was turned on, no one was permitted to
stand directly in front of the antenna. (The RF field was strong enough to lock the
doors on my 4x4 Ford pickup truck with the keys inside.)
Experimental Test Results
Tests results showed that when going through the 45-watt repeater at LZ-7, 4-watt portable radios were adequate to reliably communicate from the LZ to the mountaintop, as might be required during SAR operations.
On the back slope of the mountaintop ‘incident site,’ the 4-watt portable radios did not have a strong enough signal to accomplish reverse RF diffraction to reach the repeater at the LZ valley. However, it demonstrated that the principal thrust of this experiment worked. When on the back slope, a 35-watt UHF mobile with a roof-top gain antenna was needed to reach the repeater. Even then, the signal was about half-scale with some ‘white noise.’ This indicates communications quality RF diffraction is dependent upon: 1) distance, 2) antenna height, 3) RF power output, and 4) antenna gain, preferably using a yagi or beam antenna.
As the team progressed up the mountain, they briefly encountered several areas of marked fading, presumably from secondary obstructing ridges and rock outcroppings that blocked the signal. Once they were in UHF simplex range, the repeater and the use of RF diffraction were longer necessary. Overall, this experiment demonstrated that an innovative approach to UHF communications could be used with good success on amateur and public safety frequencies.
Photo Credit
[1] Telecommunications: Glossary of Telecommunication Terms, Institute for Telecommunication Services, Boulder Colorado, 1996. www.its.bldrdoc.gov
About the Authors
Tom Goyne, N4NSP, is the newly appointed DEC7 and previously served in this position. He is a medicolegal death investigator, an EMT with Gloucester Volunteer Fire and Rescue, and a member of York County CERT. Tom is the control operator of ROCKND on 145.730 MHz.
Earl Evans, KE4NBX, is the EC for Gloucester County and Mathews County. A master electrician of many years, Earl has installed and repaired mountaintop packet nodes in the VDEN system and helped to conduct this RF diffraction experiment. He served with the Appalachian Search and Rescue Council for 10 years, where he recovered remains from mountaintop air crashes. He has extensive experience semi-technical and technical (vertical) rope rescue and in SAR tactical public safety communications.