Re: [Amps] 10dB and propagation

[David Kirkby <david.kirkby@onetel.net>]

Ian White G3SEK wrote:

> Yuri wrote:
>
> > You guys might try to poke fun at this. But there are many factors 
> > contributing to signal levels received at the other end. Antenna 
> > pattern, local ground conditions, terrain and propagation conditions. 
> > VHFers know about ducting on VHF between W6 and KH6, you drive up the 
> > hill and you can find spot where your signal (few watts) will hit it 
> > and you QSO with KH6. You go few hundred feet up or down and you lose 
> > it.
> Sure, all of that is true... but it all goes strictly dB for dB. There 
> is no support for tropo ducts behaving in a non-linear fashion.
>
> Let's not confuse the variable *importance* of a dB under different 
> circumstances (which is obviously true) with any suggestion that the 
> propagation medium itself is non-linear.
>
> A non-linear medium would have to mean that your signal was directly 
> affecting the ionization density or the refractive index of the 
> troposphere. As I already said, that's wishful thinking at amateur 
> power levels. 


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).


Proof (for the mathmatically minded).
----------------------------

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