My Understanding 1 - Antenna is just a piece of a metal conductor. "Antenna is an OPEN CIRCUIT, and not a SHORT CIRCUIT". Is my understanding correct that the Antenna is an open circuit and not a short circuit? Or is it different in different cases.
This is not correct. There's a simple counterexample and a harder answer.
The simple counterexample is that, whereas "dipole", "monopole", or "whip" antennas consist of one or two separate conductors, loop antennas consist of a single conductor — a loop — that is connected to the radio at both ends. (Antennas for AM broadcast receivers are commonly loop antennas — either wound on a ferrite rod inside the receiver, or an external wire loop that you might have seen packed in with a "receiver" in the home-theater sense.)
The longer story is that in order to understand antennas you need to understand impedance. Impedance is a generalization of resistance — any component with resistance also has impedance — that is an important tool when working with radio-frequency circuits that can resonate, or, more generally, that react differently to different frequencies.
Given the concepts of impedance, we learn that an antenna (or any other component) has an impedance that is different depending on the frequency your are considering. When we talk about something being an open or short circuit as measured with a multimeter, we're talking about its impedance as measured with DC applied to it. DC has a frequency — zero hertz. On the other hand, if we measure an antenna with a tool that applies a radio-frequency signal, we will find (if it is a good antenna) an impedance that is neither zero — an "RF short circuit" — nor infinity — an "RF open circuit".
So, antennas can be designed such that they "look like" open circuits or short circuits to our usual understanding or wires, but still perform well as antennas. Both designs have their uses.
Antenna being a just a single piece of wire/metal conductor, it can Either TRANSMIT or RECEIVE at a single time. Since, Antenna comprises of only one conductor, it can transmit or receive. In other words - Antenna is always a Half-duplex system, right?
False. Radio waves don't care. The foundation of this is the superposition principle, which is true of the electromagnetic field whether it is in space or interacting with a radio or a transmission line or even just being the flow of current in an ordinary electric circuit. The principle tells us that you can have waves going both ways and they do not interfere with each other.
Now, in practice, in order to do full-duplex you have to either:
- use a device called a diplexer or duplexer (which is usually made of a pair of very good filters) to prevent the receiver from picking up the very strong signal from the adjacent transmitter,
- use very different frequencies, or
- do half-duplex but switch between transmitting and receiving very fast so it feels like it's full duplex (WiFi devices do this, for example).
But none of these are about properties of the antenna; rather, they are due to practical limitations on designing transmitters and receivers. (And also the fact that if you transmit on the same frequency you're receiving, objects in the environment will reflect the signal back to the receiver. Which is great if you're building a radar, but not if you're building a communications device.)
I am just understanding that for proper reception of antenna, the antenna length should be of half wavelength or quarter wavelength for proper reception of the transmitted signals. Why? - Without diving deep, I am just understand it as RESONANCE.
Sort of. My advice would be, again, to study impedance.
The impedance of an antenna changes depending on the frequency you are calculating or measuring with. If you use just the wrong frequency for the length, or just the wrong length for the frequency, your antenna will be an "open" or "short" circuit for RF purposes, and will therefore not transfer any signal into your receiver or out of your transmitter. So, you need to avoid those lengths. The quarter-wavelength is, in a sense, the length that is most distant from the open circuit condition, so it is the one that is the easiest to actually get signal into or out of.
(For loop antennas, you need to be distant from the short circuit condition, but it's basically a mirror-image situation, the same structure with different numbers. Also, for all of these lengths, there are longer lengths that are also usable, but we often use the shortest length because small is convenient and because the longer lengths have additional problems.)
(There's some complications that I'm skipping over because I think it wouldn't be useful to talk about "system impedances", say, until you know what an impedance is.)