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| To: | <[email protected]> |
|---|---|
| Subject: | LF: Re: E-Field Probe calibration question |
| From: | "James Moritz" <[email protected]> |
| Date: | Thu, 1 Apr 2010 16:48:19 +0100 |
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| Reply-to: | [email protected] |
| Sender: | [email protected] |
Dear Andy, LF Group,I haven't done quantitative tests due to the difficulty of using such an antenna in the Brookmans Park RF environment, but in some quick checks I did, output from an active whip certainly depends on height above ground and the presence/absence of a ground plane (not to mention presence of trees, other wires, etc.). I suggest you ask the manufacturers how they calibrate it, and try to replicate that... A loop is much less susceptible to these things, it seems. I tried my measuring loop (measuring DCF39 during daytime, which is usually a very stable field strength here), and there was no more than about 0.1dB difference between on the ground and 10m in the air. However, when mounted on a metal vehicle, the presence of the vehicle distorts the H field locally - I found that my 30cm square loop measured about 2-3 dB higher when on the roof of the car, compared to the same height on a tripod. The exact amount depended on whether the body of the car was aligned in the direction of the signal source (higher apparent FS) or at right angles. Mounting the antenna on a 1m mast on top of the car reduced this effect to about 0.2 - 0.3dB (although it gets even funnier looks ;-)). The formula given by SM6LKM can be adjusted to give the loop EMF for an equivalent E-field strength, assuming E/H = 377ohms: V = 2.1*10e-8*fNAEAssuming the loop is small compared to a wavelength, and operating a long way below its self-resonant frequency. Cheers, Jim Moritz 73 de M0BMU |
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