Backscatter Concept
and Setup
A Simple Procedure and Calculator
Typical meteor scatter (ms) communications uses the trail of ionized partials resulting from the reentry of typically small meteors to reflect RF energy back to earth. When the trail is between two users, it would be referred to as forward scatter and for most amateurs, the generally agreed on optimal range is about 600 to 900 miles, with even modest stations. Much of the basics and limitations of ms are set in simple geometry and spherical geometry due to the curvature of the earth and the layer of the atmosphere where the ionization occurs.
The ionization stream that meteors produce can be very useful to those not on a straight line as well. Reflections from the trail can be bounced back and also tangentially producing back and side scatter. Backscatter can be useful and dependable for communications, particularly on 6m. The trick is to knowing where to point. The earth’s curvature along with the typical altitude of “burns” (about 100,000 feet) coupled with the beam width of typical antennas suggests a general solution is not difficult to visualize. What is needed is a common point equidistant from two stations at the right distance. That distance can be derived from ½ of the optimal forward scatter of 600 to about 900 miles. Thus we would like a common intersecting point about 400 miles from each for a “round trip” distance equal to the optimal forward scatter distance.
Use of the Excel spreadsheet is simple and very straightforward. It has been scanned many times by antivirus programs. Macros must be enabled for the many macro user defined functions (UDF) to work. A number of the UDF’s assume input being passed from fixed and absolute cell references and also returns some intermediate results to fixed and absolute cells. To prevent problems, the sheets are locked and password protected. Should someone wish, I will provide the password on request.
If anyone is skilled at developing Java scripts or other standalone programs and would take the existing work and expand it, please let me know. The ultimate would be a pop up or html page to do the calculations and possibly display a simple plot. The math and text string manipulations involved is not that complex but mostly just tedious.
Figure 1. Sample of using a great circle map centered on one location to illustrate the concept of backscatter and how the intersecting radii are found. Locations are approximate in the figure but close. The blue circle around each station is drawn at a 400-mile radius. Given the relatively large 3db beam width of normal ham antennas, pinpoint accuracy is not needed.
Figure 2.
Sample display of the spreadsheet with the same information as displayed in the
map. The program takes the two locations and determines the two intercepts at
400 miles from each operator. Six
character grids are required for input and given for results along. The azimuth
an is given for both operators to both optimal intercept points.
One of the two points should be agreed on and both operators point
towards it. The selected intercept
may be dictated or influenced by noise or geographical features such as
mountains. The azimuth and distance (400 miles) given by the
spreadsheet will agree very close with WSJT if you enter either intercept grid
into WSJT in place of the To Radio grid.
For those trying backscatter for the first time, there are several things to consider and know ahead of time. Backscatter pings will be of shorter duration than an equivalent forward scatter one. Thus, better results will occur with the generally better early morning random meteors than during the middle of the day. The approach works well on six meters but coupled with the shorten pings from backscatter and also from higher frequencies, it is difficult but not impossible on two meters. Great patience will be required for those attempting 2m backscatter.
Bruce, N5SIX