A MF Ground Wave Field Strength Calculator and a Cyclomotive Force (CMF) Calculator for Vertical Antennas

by VK5SL

First Posted - 7 July 2011

Some time ago I wrote an XL based field strength calculator principally for MF broadcast applications based on the work of Summerfield and Norton with additional earth curvature corrections added for our particular application. Since then I have had the need to write a similar sheet for calculating an MF services CMF based on field strength measurements over the first ten to fifteen kilometres from the transmitter site. I though that this sheet might well have some application to those working on five hundred kilohertz.

The sheet is largely self explanatory and contains several worked examples based on actual operating broadcast services. However if you, or anyone else that wishes to use the sheet has any questions, please don’t hesitate to contact me. Incidentally in all of my XL sheets the data inputs are in red with all other colours being the calculated outputs. Secondly many of my sheets make use of complex notation and other XL computational add-ins which may not be installed on the program users computer. To install these add-ins go to "Tools down to Add-Inns" and tick all of the available boxes.

The attached XL file contains a list of general terrain constants and the CMF and original field strength calculators.

The Ground wave field strength calculator is quite straight forward to use, data inputs are in red.

The Summerfield / Norton propagation engine is a direct lift from the nineteen forty three edition of the Radio Engineers Handbook by Terman with the addition of earth’s curvature corrections to facilitate the use of the calculator beyond plain earth assumptions. Overall the calculator mimics the later CCIR curves to a fair degree of accuracy and has been very useful. Incidentally the specific distance calculator simply allows you to predict the field strength at any wanted distance from the transmitter.

The CMF calculator sheet uses the Summerfield / Norton engine without modification. To use the calculator you simply input the Frequency, expected CMF, Conductivity / dielectric constant and measured field strength / distance values into the sheet. The CMF, conductivity and dielectric values are then manipulated until the attenuated field strength curve best matches the measured field strength data with the object of reducing the RMS delta to “1”. (When I get some time I’ll make this near automatic.)

Incidentally within the CMF calculator you’ll see the use of the term “CMF” as a data input. In Australia, the Australian Communications & Media Authority (ACMA) licences MF broadcast services in terms of Cymomotive force rather than antenna power.

The CCIR defines CMF as being:

“The product formed by multiplying the electric field strength at a given point in space, due to a transmitting station, by the distance of the point from the antenna. This distance must be sufficient for the reactive components of the field to be negligible; moreover, the finite conductivity of the ground is supposed to have no effect on propagation. The cymomotive force (c.m.f.) is a vector; when necessary it may be expressed in terms of components along axes perpendicular to the direction of propagation. The c.m.f. is expressed in volts; it corresponds numerically to the field strength in mV/m at a distance of 1 km.” If necessary the CMF value can be derived directly from the “Ground Wave Path Loss Calculator”.

I’ve also attached a vertical pattern and gain calculator for “practical” MF radiators of various heights and cross sections which I wrote in the very early nineties, before I had access to an easily manipulated front end for NEC. It assumes a near perfect earth i.e., 120 radials, but may be of some use. As with the other sheets, it needs the complex notation addin etc to be present.

I’m rather envious of your MF allocations but I hope that with a little nudging we might be allocated similar. As a point of interest many years ago I was measuring the impedance of one of our larger MF masts down the bottom of the band using a GR 1606B (before the OIB-3) and being quite amazed at the level of some of the US stations I could here on the bridge detector.

Best regards, Paul Lawson VK5SL, Adelaide Australia.

Email address: Lawson (dot) paul (at) internode (dot) on (dot) net Telephone: 08 - 83442686