In reciever system, attenna is connected to ground? why not just open it?

 

In reciever system, attenna is connected to ground? why not just open it?

Good evening nowich,

Welcome to AAC! We hope you continue to ask questions!

Antennas are usually grounded so that they aren't subtle to static build up, or lightning. Usually that's more for the towers, the towers are grounded but the antennas aren't. What kind of antenna would be grounded right? Or, with the simple QRP FM transmitters, they just have a simple antenna wire. That's all you need anyway, since you're usually inside and the antenna wouldn't have hardly any chance of a lightning strike outside. Look at the attachment for a visual aid.

Remember! For grounding antenna towers, you want the shortest path to ground. Notice that I didn't have the ground connect to the top of the tower but at the bottom.

 

Antennas are often grounded as part of the matching circuit.

If you imagine a quarter-wavelength long piece if wire or PCB track, one end can be connected to ground and it will form a resonator.

(Think of holding or clamping the end of a piece of springy wire hard against a solid object. Pluck the other end and it bounces back and forth for a while - it resonates).

The antenna wire has zero impedance at the grounded end, and theoretically infinite impedance at the other end. At some point a bit away from the ground end, it will have 50 Ohms impedance which is a standard for a lot of radio devices, so the connecting track near the grounded end gives the ideal match to the antenna.

To go back to the springy wire example, if you touch the tip while it's still bouncing back & forth, its stops quickly. If you touch it slightly away from where its clamped, there is a small amount of movement with a lot of power behind it, and touching it does not damp the resonance much - it's a better 'match'.

Example:
Note the antenna track here is on the right, grounded at the top left and zigzag to allow it to fit in a smaller space. The feedpoint tap is clearly visible.

http://focus.ti.com/lit/an/swra117d/swra117d.pdf

 

If you imagine a quarter-wavelength long piece if wire or PCB track, one end can be connected to ground and it will form a resonator.

Yes this is true but it is also worth bringing out the point that when we are talking waves and antennae we are talking standing waves.

With any standing wave there is a point of zero amplitude and a point of max amplitude. (amplitude = radio frequency voltage ).

We ground the point of zero voltage. However we only do this for the same reason we ground anything - because we want to fix the voltage at that point relative to the outside world.

Some antennae are not grounded.

Some (eg dipoles) are not themselves grounded but work 'against an RF ground' (the earth or the body of a vehicle for instance). This phrase means that the antenna acts as if there was a mirror image antenna in the ground, with a zero at their common point.

 

Thanks for everybody!
The answers inspire me a lot!

 

Why Is transmitter base cables secured In ac wave form position.