Surfin’: Dust Particles Reflecting RF in the Atmosphere
By Stan Horzepa, WA1LOU
This week’s installment of Surfin’ examines the radio propagation mode known as “dust scatter.”
Tampa Floridian Mike Schaffer, KA3JAW, at EL87su, sent the following intriguing report in response to last week’s installment of Surfin’ “Falling Meteors and Acorns”: With all the propagation modes available to the modern Amateur Radio operator today, how many have tried dust scatter?
That is a misleading term just to grab your attention. But it is not far from the truth, as the majority of Amateur Radio operators have had their signals reflect off this medium everyday and land hundreds and even thousands of miles away without their knowledge!
If you have not yet figured out where I am going with this, perhaps I will mention one of the major annual meteor showers called the Perseids to help clue you in.
The Perseids is a prolific meteor shower associated with comet Swift-Tuttle. It is called “Perseids” because the point in the sky from which the meteors appear to come from -- the “radiant” -- lies in the constellation Perseus.
Meteor forecasters predicted that the peak of the shower would take place on August 11-12 with 100 meteors per hour zipping across the sky.
As a scientific experiment, I wondered if I could detect a “passive” meteor back-scatter from the Perseid meteor shower. First, let me define “passive” for this study -- a radio transmitter used to generate the required RF power to a target (meteors, in this case) that is not located at the receiving site and is not assigned within the Amateur Radio bands.
Early in the night (2200-2300 UTC) on August 11, the radiant appeared near +58 degrees inclination above the horizon in the sky towards the northeast. At this point in time, longer shooting stars (fireballs) were zooming across the sky at velocity speeds of 58 km/s.
According to the International Meteor Organization (IMO), the typical maximum of the Perseids is in the period between 1200–1430 UTC on August 12 for observers in the Northern Hemisphere. Mid-northern latitudes are favored compared to observers from the Southern Hemisphere, such as Australia and New Zealand.
For those who spotted the shower visually, the moon was in the waning crescent phase, which was a nuisance due to its relative proximity to the radiant. Other factors that impaired visibility were local weather conditions, such as overcast clouds, rain and fog.
During the predawn hours, the radiant decreases towards the horizon, which is the perfect time to point your high-gain horizontal outdoor antenna at the horizon, because the ping (meteor detection) rates will increase substantially.
At this juncture, you should turn on your recording devices.
At 0704 UTC (3:04 AM EDT), I parked my Visteon HD Jump RDS FM radio (analog mode) on a vacant FM channel, 107.5 MHz. Within minutes, the baseline background noise increased to a moderate signal strength with a Spanish announcer abruptly being heard (ping) for about four seconds in duration. Since the time was excessively short, no possible station identification could be determined. Possibilities could be WAMR-FM in Miami “Amor 107.5 Radio la Nueva,” 193 miles to my southeast, which is the only Spanish broadcaster in Florida on that assigned frequency. But then again, it could have originated outside the USA, such as Cuba, Mexico or even Central America.
Four minutes lapses, then another ping occurred at moderate signal strength for 10 seconds. This time, there was a male English announcer with a fair amount modulation distortion due to Doppler shifting of the main carrier.
I have noticed the above event occurring eight times between the hours of 0707–0811 UTC with duration times ranging from 3 to 15 seconds due to meteor dust particles the size of grains of sand with their associated ionized trials flashing overhead from the debris field stream of comet Swift-Tuttle.
The Doppler source originates from a local St Petersburg broadcaster, WXGL (slogan “Tampa Bay’s Classic Hits Station, 107.3, The Eagle”) with a RF power output of 100 kW.
Their transmitter site is 7.5 miles west of my location across Old Tampa Bay. WXGL is assigned a frequency 200 kHz lower than my parked frequency at 107.5, therefore, when a meteor fireball torches through the atmosphere heading towards my location, it shifts the stations RF carrier up in frequency. When it pass overhead and flies farther away, the signal shifts back down rapidly and produces the distorted out-of-phase modulated audio.
Normally, meteor scatter is detected in the forward scatter mode, that is, the transmitter is located well below the horizon. In this case, the transmitter site is not below my horizon. It is 7.5 miles away, thus we can call it co-located.
Their 100 kW signal is radiated out above the horizon at angles greater that 35 degrees inclination into the sky, which then strikes the ionized plasma trail and causes the signal to reflect back to its emitter source within a co-located area. My cross-phased antenna is roughly aimed north-northwest (338 degrees). Co-located WXGL transmitter tower site is roughly west by south (259 degrees) from my station. The perpendicular angle is 79 degrees.
This form of propagation is called “passive meteor back scatter” and essentially it is radar.
According to the IMO, at 0119 UTC, there were 86 meteors observed per hour (averaged) during the mid-evening of August 12. In the predawn local hours, it rose to 122.
I used a roof-mounted Antennacraft CCS-1843, 180-inch boom length, extreme deep fringe cross-phase multi-element U/V/FM antenna that is 25 feet above the ground aimed north-northwest, with 50 feet of RG-6 quad coax cable terminated to a Visteon HD Jump FM RDS radio set in analog mode. I didn’t use an antenna preamplifier; however, if you wish to achieve more efficiency, thereby increasing meteor detection rates, obtain a low-noise, medium-to-high gain (10-20 dB) antenna preamplifier and mount it as close to the antenna feedpoint as possible.
Hopefully this report will provide some enlightenment and be an encouragement to experiment using alternative techniques, even if the probability of success is low to non-existent. If you do not attempt to make an effort, you might miss out on the next discovery.
Until next time, keep on surfin’!