Radio Detection Basics

Forward scatter



Since the transmitting stations run high power, 10 to 100 kW, the echoes are quite strong so the antenna does not have to be too complex or expensive.

For TV video carriers forward scatter an old log periodic TV antenna can be used with success. Now that most towns have gone to cable or satellite people often give away their old TV antennas to any one that is interested in hauling it off. Or you can buy one at Radio Shack or the Source.When using a “TV antenna” it’s good practice to use a 300 ohm to 50 ohm transformer at the antenna. This matches the antenna to the impedance of the coax cable that will carry the signals down to the receiver. Matching the impedance this was and insures the signal loss is kept at a minimum.

Log periodic and yagi “TV or “FM antennas” have some directionality to them. This means they exhibit gain in signal strength when pointed at the transmitter while it hears much less well off the sides, or off the back of the antenna. In fact in some cases the front to back ratio can be more useful than the front gain is. Let’s say you have a frequency which would work well for echo counting but here is a weak, semi local station, that is causing interference . By turning a log periodic or yagi it is possible that the antenna’s front to back back or the side lobes will notch out the interference thus rendering an unusable frequency usable.

I have also used a simple half wave dipole which is just two pieces of wire connected to the inner conductor of the coax and to the coax’s shielding. Each piece is a quarter wavelength at the frequency of interest. I often tack together a dipole to test a new frequency before deciding on a better antenna. At VHF frequencies it’s easy to tape the temporary dipole onto a meter stick or other scrap 1×1 wood so you can rotate the antenna during the test. Dipoles exhibit notches off each end of the wire so it is possible to notch out a weak carrier that might prevent meteor counting.

If I have three receivers recording I will user a discone antenna that is seated on my upper deck. The number of echoes are not as good as a normal resonate antenna (log periodic, dipole, yagi, or loop, but during a major shower the discone can preform well enough to capture research quality data. The discone antenna is a omni directional antenna so it theoretically hears in all directions. It has vertical polarization so there is usually some loss of signal from TV video carriers which are usually horizontally polarized.  Discone have large bandwidth with surprising low SWR or mismatching. The one I use for example will work between 28 MHz to 1.3 GHz. I don’t recommend using one but if you have one from a police scanner hopbby give it a try before you try the larger antennas.

My favorite antenna is a full wave length loop antenna operating in a horizontal polarization mode. It consists of a one full wave (at the listening frequency) piece of copper wire. It is formed into a square, with one quarter wave length per side. The bottom 1/4 wave length of the square is cut in the middle and the centre of the coax cable is soldiered to one side and the other side to the copper shield braid.  It is very cheap to make and it is a very quiet, resonate, antenna. It does have some directionality to it so it can be place to maximize the signal or to notch out a weak carrier that is interfering with your count. Four insulators at each corner of the square is all that is needed. I usually have several full wave loops cut for different frequencies in the attic. They have provided over a decade of good echo captures.

In a pinch you can try anything. My HF vertical (3.8-28 MHz) works very well when it comes to hearing SNOTEL’s 40.5 MHz echoes.

I prefer my inside the attic antennas over my out door meteor antennas. The indoor antennas do not shake in strong winds, the coax connections do not weather as badly as outdoor cables do, there is no impedance change when covered in deep snow or rain.

Snowy ants


Above: Snow covered antennas. The discone antenna can be seen just left of the BBQ grill on deck floor. Looking west down the lake.

And most of all attic installed antennas don’t blow down.

Wind damage

Photo of the 5 element log periodic antenna (top), a horizontal yagi (middle), and a vertical yagi (below the arm of the BBQ grill) dangles at an angle on the second story deck after winds with prolong gusts of 90+ km/h. The winds were unobstructed blowing up the central portion of Okanagan Lake. The mast with the rotator and antennas tore loose from bolts on the side of the house. It was reinstalled a week later.

Richardson, J., and Knueth, W. (1998) Revisiting the Radio Doppler Effect from Forward-scattered Meteor Head Echoes. WGN 26:3. p 123.

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