Dear LF Group,
Some comments on G3JKV's recent E-mail -
I calculated the ERP on the basis of the measured antenna current
and estimated radiation resistance of the two antennas. Part of the
reasoning behind this was that the antenna efficiency would not be
an issue; the radiated power is just I squared times the radiation
resistance, and the calculated radiation resistance is a function
only of the antenna geometry, and not it's losses. In principle at
least, the only result of improved efficiency due to the Decca earth
mat would be to reduce the amount of transmitter power required to
produce the measured value of antenna current. So antenna
efficiency did not actually enter into the calculation of ERP, and the
presence or absence of the Decca earth would not have affected
the result of the calculation.
Having said that, clearly the ERP calculations are wrong, because
there was a significant difference in signal strength, and so by
definition ERP, between the two antennas, where the calculations
said they should be the same. Unfortunately, my field strength
measuring equipment is not accurate enough to say for certain that
this was because the small antenna was producing more ERP than
it should, or that the Decca antenna was producing less than it
should. However, it is probably easier to believe the former, since
the assumptions on which the calculations are based are more
nearly met by the Decca antenna than the small inverted L.
I don't know if this experiment has much to tell us about the
presence or absence of horizontally polarised signals; however,
the ferrite rod antenna I used to measure field strengths showed
the difference in field strength of roughly 4dB observed by nearly
everyone else, and should not have been sensitive to horizontally
polarised signals (ie. vertical H field).
Cheers, Jim Moritz
73 de M0BMU
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