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Old 25-Aug-2013, 1:32 AM   #7
GroundUrMast
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Join Date: Oct 2010
Location: Greater Seattle Area
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Fortunately, a simple installation does not require us to be concerned w/ impedance, since using a BALUN to convert from the 2 wire antenna output, to a coax input, changes the impedance for us, automatically.
I agree. I know of no consumer grade hardware that provides for adjustment of output impedance. You'll find that most OTA hardware is configured to operate at 75 ohms... Done, move along, nothing more to look at there... And the few examples of 300 ohm hardware interfaces neatly with the aid of the matching transformer you mentioned already. Again, no provision for fine tuning the impedance... Just a fixed transformation ratio by design.

Still, as an academic exercise, I'm certainly happy to see people take interest in the technology.

I'd like to offer a different analogy, that I think addresses the concept of output impedance more directly...

In a plumbing or hydraulic system, pressure is analogous to voltage in an electrical system. Resistance to flow of fluid and resistance to flow of electron current are related concepts. Volume of fluid per second is comparable to electrical current.

Consider a pump that supplies water or hydraulic fluid to a system... With the pump running and the flow stopped by a closed valve you would expect the outlet pressure of the pump to be at maximum... True? Then, in a real world application, as you open the valve, and as fluid begins to flow... you expect the output pressure to drop somewhat. That's because the internal parts of the pump are limited in size and have some resistance to the flow of fluid. There is something inside the pump the impedes flow, and so as the flow rate increases, so does the pressure drop.

That 'internal' resistance in a pump is analogous to the resistance and reactance inside any real electrical devise that can supply voltage and current. We refer to that combination of resistance and reactance as 'impedance' and when we are referring to an impedance that affects the output of an electrical power source, it's referred to as 'output impedance'.

Batteries, generators, amplifiers, antennas, splitters, etc. all can deliver voltage and current to a load connected to their output port... they also have integral resistance and reactance. So each will have an output impedance which in some cases will be constant and in cases such as a battery, will increase as they discharge.

The goal of the impedance lesson 1 was to set the stage for showing that when the impedance of a source such as a generator, antenna or amplifier is matched by the impedance of a load, maximum power transfer can occur. Conversely, when impedance is not matched, inefficient power transfer takes place.
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Last edited by GroundUrMast; 25-Aug-2013 at 1:45 AM. Reason: sp.
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