KA1GT Troposcatter and Link Budget Calculator

Download link for program Scatter2.6.1.exe

 

This program estimates path loss in the 50MHz to 24GHz region via either line of sight (radio line of sight) or troposcatter mechanisms. It doesn't address the case of single or multiple "knife edge" diffraction or propagation via anomalous mechanisms such as ducting or ionospheric reflection (at lower frequencies). It also doesn't take into effect Fresnel zones, which will increase path loss when a line of sight path just grazes the ground or some other non "knife-edge" obstacle (add about 6dB in that case). A perfect duct would have 1/2 the free space loss in dB (e.g. 160 dB FSL would be 80dB for a perfect duct). The program also does a link budget calculation based on the predicted path loss, TX output, Rx noise figure, Antenna noise temperature, Tx and Rx line losses and Rx bandwidth.

Knife edge diffraction is a more complex case and I may put together a program to estimate simple single knife edge diffraction losses. Typically they will be somewhere between the free space loss and the scattering loss of a similar length path. The loss is fairly easy to calculate for the ideal case, but real paths rarely, if ever, approach the ideal case and often are far from it. Still, a minimum path loss can be calculated.

The program does calculate path loss for a perfect duct, but perfect ducts don't exist, so look on that number as the absolute minimum possible loss via ducting. It's included for interest rather than any practical value.

5 methods of troposcatter loss estimation are used. All are empirical relationships based on experimental data. That means a number of troposcater links were measured and either graphical or numerical methods were developed by their authors that fitted to the experimental numbers. None of the methods are calculations based on first principles and a theoretical model of tropospheric scattering. In some cases the range of scattering angles and frequencies considered were quite limited, so scattering calculations based on limited experimental data might be significant extrapolations of the data. An article which discusses some of troposcatter models on which this program is based appeared in the Winter 1991 issue of "Communications Quarterly" on pages 119 to 127. If you want a copy, email me (see below).

For the normally graphical methods, I fitted the plotted data to a numerical (usually high order polynomial) relationship so that things could be numerically calculated rather then being read off graphs or nomograms.

Experimentally it has been observed that the methods of Yeh, Collins and CCIR tend to be most reliable. The ITT numbers are often quite good over a limited range. The Rider numbers are included for completeness and because they are easy to calculate, though they are the least reliable. An average of the Yeh, CCIR and Collins numbers (when available) might be a good starting point.

None of the methods have an explicit term for atmospheric absorption (due to the tail of absorptions due to Oxygen and Water vapor). The program calculates that, and you can use that number if you wish. It's very important over long paths at 24GHz. You might say that there is an atmospheric absorption coefficient inherent in the empirical relationships because they are based on experimental data, but much of that data is at relatively low frequencies at which the atmospheric absorption is very low. Since absorption is a complex function of frequency, simple extrapolation doesn't work.

The effects of Aperture to Medium coupling loss are explicitly included in the Yeh and CCIR methods. This related to the observation that signal strength of a scattered signal does not increase as fast as you'd think as antenna gains go up when the gains are high (and beamwidth narrow). The Yeh estimation includes a scattering angle factor while the CCIR method doesn't. The Yeh values are probably about 2dB high for low gain antennas, but for consistency are included in the Yeh scattering loss.

There are also effects of the N value (which is a function of the radio frequency refractive index of the atmosphere) on scattering loss which are dealt with in different ways by the different methods. None explicitly include the vertical N profile but are generally based on the input of the N value at sea level (typically around a value of 310).

All the loss predictions are for the average yearly median troposcatter path loss. On a day to day basis the actual path loss can be more or less than this median loss, depending on temperature, humidity, passage of weather fronts, time of day etc. Just because the prediction suggests that the loss is too great for communication does not mean that it will be on any particular day at any particular time.

Note that under extreme conditions (e.g. very large or very small scattering angles) some of the methods may give results that are not consistent. However if two of three of the methods give a similar number and one or more of the methods give a result that is wildly different, the ones that agree are likely to be a better estimate of the actual loss.

The theory behind the program is covered in some detail in the Troposcatter article on this website.

Comments on this program (especially bug reports) are welcome. Email to bobatkins@hotmail.com

Download link for program Scatter2.6.1.exe

A few possibly useful books:

         

And while I have your attention...

Stuff For Sale

[All as of February 2014]

I will be moving QTH in the Spring of 2014 and so some stuff has to go. I don't really want to let go of any of it, but I don't think I have much of a choice! Anyway, here's a partial listing:

  • 6-tube ring amplifier for 1296. The OZ9CR design, built by OZ9CR himself. Comes with a nice adjustable filament and bias supply including metering. Good for 400-500W if you get it tuned right. Comes with tubes and blower (but no guarantee on the tubes), You suply the HT.
  • 12 ft stressed parabolic dish (K2RIW design). I have the central hub (that's the hard part), plus the ribs. I don't think I have the screening but it might be hidden somewhere! If it's not, the screening is the easy part.
  • HP-141 Spectrum Analyzer. The one with the round CRT display. Used to work 100% but last time I was using it the display died. Could be the CRT but I haven't taken it apart to check it out. However it does have the IF section and an HP8554 plugin (100kHz to 1250MHz) which are presumably working fine (plus an HP8553 low 110MHz plugin and a spare IF section)
  • High voltage PSU, or at least the components. This was the power supply I used on 432 EME with my K2RIW amplifier. Not 100% sure of the voltage and current rating, but I'm guessing 2Kv at at least 1/2amp. Hasn't been used in 20 years, but I assume the transformer is still good. Can't guarantee the electrolytics. It's heavy. Construction quality leaves something to be desired, but it did the job for EME.
  • 600-0-600 transformer (1200v CT). Weight 77lbs. 6awg primary, 15awg secondary. Looks like it should be good for maybe 5A at 1200v?
  • This one is hard to part with. My K2RIW EME amp I do have the bias supply but it's a bit of a rat's nest constructionwise. Comes with the blower and tubes. Last time on the air it worked fine, so I've no reason to suspect it won't do so again. Puts out in the region of 500W with a 2kv PSU.
  • I might let one of my 1296 loop yagis go if you twist my arm.
  • Couple of dead TWT amplifiers for parts. Not light.
  • Miscellaneous bits of waveguide and various microwave parts, e.g. 1-2GHz, 2-4Ghz and 4-8GHz isolators/circulators. Ask, I may have one!

Prices are negotiable (especially on the heavy stuff!)

Some of this stuff would be pickup only since it's either heavy or bulky. I'm in NJ, just off Rt78, exit 36. You can reach me via email at bobatkins@hotmail.com. There's also a chance I might be at the conference in Manchester CT on April 13th 2014.

Amazon can be a useful source of Hardware