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| To: | <[email protected]> |
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| Subject: | LF: Re: VLF dj8wx on 8970.022Hz |
| From: | "James Moritz" <[email protected]> |
| Date: | Tue, 8 Mar 2011 21:00:13 -0000 |
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Dear Uwe,One can make a very approximate, order-of-magnitude estimate of ERP and voltage from the info you have:- The effective height of a heavily top-loaded vertical such as your antenna will be a bit less than the average physical height of the conductors - assume about 12m. The radiation resistance Rrad = 160 * pi^2 * (Heff/lambda)^2 at 9kHz is therefore 200 micro-ohm - it will be reduced if there are trees, buildings etc. close to the antenna. The antenna current will be slightly less than the current in the radials, due to the distributed capacitance of the loading coil, etc., at max power let us say 2.7A. ERP will then be 1.8 * (2.7)^2 * 200u = 2.7mW. The exact capacitance will depend on the spacing and diameter of the antenna wires, etc., but I estimate roughly 1200pF based on your drawing. The reactance at 9k is therefore 15kohm. With 2.7A antenna current, the antenna voltage would be 40kV RMS. There are major possible sources of error when trying to measure the antenna voltage in the way you describe - even at 9kHz, the effect of stray capacitance will be very serious - a 10M resistor has the same magnitude of impedance as 1.7pF of stray capacitance, so only a short length of exposed wire will cause a large change in voltage at the scope input. Also, as you have seen, the strong E field can couple significantly to the internal wiring of the measuring equipment unless it is very well screened. Any potential divider must also be well screened, and take into account the time constants due to resistance and stray capacitance. Cheers, Jim Moritz 73 de M0BMU |
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