To: | Ian White G3SEK <g3sek@ifwtech.co.uk> |
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Subject: | Re: [Amps] 10dB and propagation |
From: | David Kirkby <david.kirkby@onetel.net> |
Date: | Mon, 07 Feb 2005 01:46:31 +0000 |
List-post: | <mailto:amps@contesting.com> |
Ian White G3SEK wrote:Yuri wrote: I missed the start of this, so don't know exactly how it started, but I think I know what people are saying - mostly rubbish. It *IS* quite possible the transmitter power is influencing the transmitting antenna, or the accuracy of the power metre (diode saturated, so reading incorrectly). At high powers power the diodes in power metres might saturate. And the scales are always compressed at the high end. This is the most likely explanation of what appears to be happening. A ferrite balun could saturate at high power. It might be mis-tuned, so actually works better when saturated then when not saturated. (Although to get a 3dB change, you would need need to loose 500W somewhere, which would get a bit hot). But this is a possible (but unlikely) explanation. I don't believe there is ANY transmitter on earth (amateur or pro) that could influence the ionosphere by causing a non-linear behavior in that. The following will show this is a bit unlikely. If I have not made any mistakes (and I've not double-checked it), the electric field between the terminals of a 9V PP3 battery would be 380x higher than the electric field at the bottom of the D-layer if you used a 10MW transmitter with a 20dB gain antenna. Can you honestly imagine a 9V battery causing this non-linearity - yet that would give 380x the field strength!!! And I assumed a 10MW transmitter and a 20dB gain antenna, which I think you would accept are generous. (I later see someone mention troposphere, which is lower, I can't be bothered to redo the calcuation. The field streghts would be higher, but the result still the same).
Assume you transmit 10 MW with a 20dB gain antenna, so have an ERP of 1 GW. Very ambitions, but lets assume that. Let's comute the E field (V/m) at the bottom of the D layer (around 75km). The surface area of a sphere surrounding the antenna is 4*pi*r^2 or 7 * 10^10 square metres. Hence power density = Power / area = 10^9/(7*10^10)= 14mW/m^2. You can relate the electric field to the power density and impedance of free space (377 Ohms) http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part6/page4.html E=sqrt(Power density * Zo) = 2.3V/m. Now the terminals of a 9V battery are about 1 cm apart, giving an electric field of 900 V/m, or some 380x that of 10MW transmitter. [I realised later someone mentioned troposphere which is lower and so will give higher fields, but I think the point is made. There is *NO WAY* you will influence the atmosphere.] -- Dr. David Kirkby, G8WRB Please check out http://www.g8wrb.org/ of if you live in Essex http://www.southminster-branch-line.org.uk/ _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps |
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