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Ermi wrote: Rich did a good job showing how inaccurate field strength readings at VHF can be…
To clarify, the important point from my comments is that even though the FSM may be accurately measuring the field at its receive antenna location, at VHF and above that field is not a reliable indicator for determining the ERP launched by the transmit antenna over a terrestrial path toward that receive antenna.
… Valentin Trainotti says …. the accuracy of the field strength meters themselves can cause errors of +/- 2 dB or more. He says that, for this reason, it is difficult to measure how well a short monopole installation compares to a perfect quarter wave monopole.
However a good FSM should have no difficulty in measuring the difference between two fields to within a tenth of a decibel, which accuracy probably is sufficient for the application.
A well-designed antenna less than 1/10 wavelength long is likely to be at least 1.5 dB below the quarter wave monopole, but the exact difference is very difficult to measure.
John Kraus in Chapter 6 of ANTENNAS, 3rd edition shows a gain difference of 0.4 dB between a 1/4-wave monopole and a very short monopole, with both using a perfect ground plane. This difference in gain is related to the slightly different elevation pattern shapes of these antennas, not because this short antenna system has more loss.
Of interest here are the 1937 experiments of Dr George Brown et al of RCA Labs. From their IRE paper describing this work, “It was found that the antenna shown in Figure 31 (G = 22 degrees) gave a field strength only 8.5% less than the antenna shown in Figure 20 (G = 99 degrees).”
Here G is used to denote the electrical height of the monopole in degrees, so the 22-degree radiator is less than your 1/10-wave condition (36 degrees). The ground system used in this comparison consisted of 113 radials each of 0.41 wavelengths.
The groundwave field from the 22-degree radiator with this very good, but less-than-perfect ground plane was 0.77 dB less than from the 99-degree radiator. Some of the 0.37 dB increase in Kraus’ value of 0.4 dB results from the fact that a 99-degree radiator has more h-plane gain than a 90-degree (1/4-wave) radiator (related to the shape of its elevation pattern). The rest of the difference results from I^2R losses in the radial ground system.
The actual field measured by Dr Brown 3/10 of a mile from a 1/4-wave monopole using the radial ground described above was 98.5% of the maximum field possible for a perfect radiator of that height over a perfect ground plane. For the 22-degree radiator it was about 93.1%. The reduction in groundwave relative field for the shorter radiator is due to its elevation pattern shape, and its lower radiation resistance compared to other resistive losses in the antenna system.
Probably Valentin Trainotti would find that this comparison of theoretical vs measured data shows very good agreement, and that measurement of groundwave fields at MW frequencies will give quite accurate results when done with due care.
The linked NOUO contains field strength readings to seven places. The accuracy implied by readings with so many places is very deceptive.
True. An inaccurate calculation/measurement can be shown with great precision — however that value is still inaccurate.