Low Noise Amplifier Souce Impedance effect of antennae
In a receiver.
I understand that the first amplifier after the antennae should have low noise figure and high gain. The noise figure contribution of the remaining stages will be reduced by the gain of the first low noise amplifier. Hence the first amplifier plays a major roll in determining the overall NF of the system.
To achieve a low noise figure an optimum source impedance must be presented to the first amplifier. However, if the receiver were say a hand held pager, the impedance presented by the antennae would change as it moves close to different foreign objects.
How on Earth can the impedance for optimum noise figure then be maintained?
The only way I can think of is to use a resitive match which is probably a bad idea.
You are probably right but in real receiver-antenna situations, one has to find a balance in the complete communication system.
In a "handheld pager" or similar device, the use of "extremely" low receiver noise figure has no real meaning: a simple whip antenna will introduce various interfering signals as it is wideband. To design a good low-noise receiver - antenna system, consider rather an example of a satellite TV receiver.
In this case, the parabolic dish antenna translates a weak signal into a low-noise down converter located in its focus. This arrangement brings the low-noise preamplifier as close to the antenna as possible. The antenna also contributes by a noise power collected by its side lobes, often picking up the noise radiated by the Earth and vegetation around.