Dear LF Group,<br><br>As an attempt to increase the available data on fields radiated from LF antennas,I enlisted the aid of Dave Lauder, G0SNO, to perform some field strength measurements on my LF signal. Dave is a colleague at the University of Hertfordshire, and writes the Radcom EMC column. Thanks for giving up your spare time, Dave. <br><br>I sent some test signals on 136.0kHz from my home QTH, whilst <br>Dave measured the field strength at U of H's Telecomms lab, 4.2km <br>away. We used a Schaffner-Chase HLA6120A wideband active <br>loop antenna, in conjunction with an HP 8591EM EMC spectrum analyser. The loop antenna actually senses the magnetic (H) field component of the radio wave, but since the distance from the transmitter is great enough (nearly 2 wavelengths) to be considered to be in the far field, the electric field strength (E) can be calculated from the relationship E/H = 120pi. The calibration data for the antenna is programmed into the analyser, w hich does the calculations to give the measurement directly in terms of the equivalent electric field, in volts per metre.<br><br>I measured my antenna dimensions as accurately as possible with <br>it in two configurations, (1) at 6.1m average height, then propped <br>up with telescopic poles to (2), 8.1m. Transmit power was around <br>100W, and I adjusted power for an antenna current of 1.4A in both <br>cases. The equivalent E field strength was 141uV/m for (1), and 195uV/m for (2).<br><br>Using the text book formula for radiation resistance of a top-loaded <br>vertical, ie. Rr = 160. pi^2.h^2/lambda^2, and 1.4A current gives <br>radiation resistance and total radiated power:<br>(1) - 12milliohms, 24mW<br>(2) - 21milliohms, 42mW<br><br>To get to ERP, according to the definition in the BR68 regulations <br>booklet, we must take directivity of the short monopole vs. <br>directivity of a half wave dipole into account. The ideal monopole <br>has directive gain over the dipole , since it is over a ground plane; <br>no power gets radiated downwards, therefore there is twice as <br>much power radiated above the ground plane. According to the text <br>books, the directivity of a short, top loaded monopole is 3.0, whilst <br>that of a half wave dipole is 1.64. The gain of the monopole over a <br>dipole is therefore 3/1.64, or 1.8.<br><br>Therefore, the ERP of the vertical is the radiated power x1.8, or:<br>(1) 43mW<br>(2) 76mW<br>(must get on with that 1.3kW amplifier!)<br><br>Again referring to the text books, the field stength in mV/m due to a <br>given ERP is 7.sqrt(ERP)/r, where r is the distance in km. For <br>4.2km then, the field strengths should be:<br>(1) 340uV/m<br>(2) 460uV/m<br><br>Comparing these calculated field strengths with the measured <br>values shows that the actual field stength is about 7.5dB down on <br>the calculated field strength. Increasing the height of the antenna <br>gives an increase in field strength of 2.8dB, close to t hat predicted <br>by the radiation resistance formula.<br><br>Possible causes of the missing 7.5dB could be losses in ground <br>wave propagation over imperfect ground. However, the curves in <br>Terman's Radio Engineering show that over this distance, <br>additional losses are insignificant even with poor grounds.<br><br>The other possibility is the vertical directional pattern of the <br>antenna. An ideal short monopole over perfect ground has field <br>proportional to the cosine of the elevation angle, ie. maximum at <br>ground level and zero straight up. As others have discussed, <br>imperfect ground results in a null in the radiation pattern at ground <br>level. If the maximum radiation occurs at 20 degrees elevation, you <br>would have to position the measuring antenna at a height of 1400 <br>metres at 4.2km distance in order to measure it. <br><br>The results seem to be in reasonable agreement with the opinions <br>a few weeks ago resulting from PA0SE and DK8KW's <br>m easurements, ie, that people were several dB short of the ERP <br>they thought they had. However, these measurements are made <br>over longer distances, and the data in Terman's book shows that <br>beyond a certain distance, the field strength of the ground wave <br>falls off faster for an imperfect ground, and becomes inversely <br>proportional to the square of the distance, compared to a perfect <br>ground, where it is just an inverse-distance law. The point where <br>this occurs depends on the ground constants, but is about 30km for <br>very poor ground, and 300km for sea water. There is also the <br>question of at what point ionospheric propagation becomes dominant.<br><br>Hope this gives some food for thought,<br><br>Cheers, Jim Moritz<br>73 de M0BMU<?color><?param0100,0100,0100><br><br>
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