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| Subject: | LF: Measuring field strength |
| From: | Dick Rollema <[email protected]> |
| Date: | Mon, 07 May 2007 13:59:49 +0200 |
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| Reply-to: | [email protected] |
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Again with some errors removed.... To All from PA0SE Mal, G3KEV wrote: The loop must not be loaded. Most SLM's have a possibility for a high impedance input. That may be a problem with a receiver used as a SLM. On the other end the impedance of the loop is so low on 500 kHz, in my own one turn loop 2.7 ohms, that even when loaded with 50 ohms the output of the loop hardly differs from the open circuit voltage. Keep the coax between loop and SLM or receiver as short as possible and convenient. You can't trust the S-meter of a receiver. So that has to be calibrated. The loop itself does not have to be calibrated. There is a simple equation that gives the relation between field strength and voltage induced in the loop. I tried to include the equation into an earlier message but that was rejected. The equation comes out of Terman and includes Greek letters. I will rewrite it in a way that hopefully is acceptable to the censor. e = (2 * 3.1 * E * N * A * cos phi) / L in which: e = voltage induced in loop in volt. E = field strength in V/m N = number of turns in loop A = area of the loop in square metres phi is angle with respect to plane of loop. L = wavelength in metres. When measuring the loop must be turned for maximum signal; that makes cos phi = 1. I prefer to use one turn, so N = 1. Using more turns increases the voltage induced. But the capacitance between the turns could corrupt the result. Also the reactance of the loop increases, raising its impedance and making it more difficult to avoid loading the loop. Usually the voltage induced in one turn is high enough to be read on a SLM. There is no need to screen the loop because of its low impedance. I made my own loop as a square with sides of 1 metre. That makes A = 1. Afterwards I found that I could only just get the loop into the car. So better do some measurements on the doors of your car before deciding on the size of the loop. Measuring field strength is definitely not a precision business. There are so many disturbing factors that it is of no use to aim for highest accuracy. Hopefully the people at your Ofcom realise that as well. The best way is to take many readings on different locations, in different directions and different distances, as Jim, M0MBU, has done on 137 kHz and use the average value as the result. . Also you have to keep out of the near field of the antenna. A distance of 2 wavelengths from the transmitter usually is enough. Finally an anecdote. When I started measuring FS on 137 kHz I looked for places around my home at some 4 kilometres distance or more where the field would not be disturbed by overhead lines, underground cables, large metal fences etc. A road with street lighting was obviously unsuitable because there was certainly an underground cable. I finally choose a nice parking lot near the border af a lake. Walking around with my portable instrument showed that the reading did not change so the field was reasonably undisturbed. I then made measurements with the one turn loop and the W & G SPM-12 SLM, run off the car battery. Sometime later I read that the place was a meeting point for homosexuals. I then realised there were indeed always some cars with a man inside. What must have been their thoughts about my mysterious doings? Happy measuring! 73, Dick, PA0SE |
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