In many youtube videos about building low power radio transmeters , I have seen a piece of regular wire or extension cord used as a antenna. But , it seems like telecommunication antennas have specific geometry (like dipole antenna , dish antenna etc).But why do we have to follow specific geometry for designing antenna?
Is it because using an extension cord as a antenna have much more power losses? If it does have , how do we calculate it?
A wire antenna that doesn't have a specific geometry may be called a random wire antenna. Such an antenna will correspondingly have a random performance with a random radiation pattern. In these cases, typically we care more about getting any signal out at all, rather than doing it efficiently or in a specific direction. A random wire antenna might be cut to length for a specific frequency, or it may be a more or less random length with an antenna tuner between it and the radio to match the impedance. (Even a random length wire may need to be cut within a range of lengths for some tuners to be able to tune it.)
A dipole's most important characteristic is that it is close to a half wave long. It's geometry isn't critical beyond that, but variations in geometry will change characteristic impedance and radiation pattern (and possibly polarization) at least. Typically a dipole is depicted as two quarter wave segments that are colinear. Practically, we may want to actually have them in an inverted V pattern for best impedance, but other patterns work as well, and may or may not affect radiation pattern significantly. A dipole has some directionality, but typically the concern with a dipole is polarization (for vhf) and elevation angle of the radiation pattern (for vhf and hf). For HF dipoles, height above ground may affect elevation angle of the radiation pattern more than the antenna's internal geometry.
A dish is not an antenna, but may be part of an antenna system. A dish is a reflector, and typically has an antenna near the focus of the dish with a radiation pattern that covers the dish; the dish shape is critical to focusing RF energy on the antenna, and the antenna's shape is important for optimizing the aiming of the energy at the dish. The dish is typically parabolic but may be spherical or hyperbolic, and the curve may be in 2 or 3 dimensions. The ideal antenna for the dish can be any number of models with a hemispherical or one sided conical radiation pattern. (Although, a 2d parabolic dish could use a dipole exactly at the focus, parallel to the axis of curvature.)
Other antennas, for example, yagi, moxon, patch, etc., are directional antennas, and their geometry is critical to forming their directionality. The directionality also is coupled with making the antenna high gain.
An antenna that is small in relation to the wavelength always has a dipole radiation pattern. Electric dipole or magnetic dipole or a combination of both. For transmit small antennas have a narrow bandwidth because they have to be power matched to the transmitter. Currents become very large in transmit so efficiency suffers. Small antennas may have a wide bandwidth for receive by use of special low noise amplifiers. As size grows antennas may get all sorts of interesting properties. Directivity (yagis) wide bandwidth (logperiodic) or both (parabolas) There are also interesting antennas with very long wires. There is no need to make an antenna "resonant" other than that the matching network needed to match it to a 50 ohm transmitter may become simpler or even superfluous.
A non resonant extension cord as a antenna will need an antenna tuner for transmit because the impedance will be far from 50 ohms. In case the wire is thin there will be ohmic losses and if the wire is covered with thick PVC there will be dielectric losses. On receive losses do usually not matter at all since external noise is attenuated by losses to the same extent as the desired signals. External noise is typically much weaker than the receiver noise below 30 MHz. With high losses in the upper HF range one might need an antenna tuner also on receive.
There are at least 3 reasons for using "special geometries" for RF antennas: resonant frequency, impedance match, and pattern.
If the antenna is of a length and geometry that is resonant at the frequency of interest, then the antenna will be more efficient capturing energy from the airwaves at the frequency of interest.
If the antenna geometry has a matched feedpoint impedance at the frequency of interest, then it can pass power to the feedline or feedpoint with a lower likelihood of resistive losses (depending on the feed line or wire type).
If the antenna geometry at the frequency of interest is sized, shaped, and angled to better capture signals in the polarization and direction of interest, and potentially less well positioned and angled to capture RF noise, then the signal transferred to the receiver will have a better S/N ratio (less noise that could completely cover up the signal(s) of interest). Very short wires (relative to wavelengths) might capture more household RF noise than any interesting HF radio signals.
