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eeemcee
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at a point they become plain waves, how much will they be attenuated when 'received' by a spiral log periodic antenna Vs another dipole? Would an H field created by a loop antenna have less attenuation?
Assuming a gain of 0 dBd for the LP antenna the loss is 3dB.eeemcee said:at a point they become plain waves, how much will they be attenuated when 'received' by a spiral log periodic antenna Vs another dipole? Would an H field created by a loop antenna have less attenuation?
No. The radiation fields from a dipole and a loop are the same once we get further than about a sixth of a wavelength from the antenna. E and H are always in a fixed ratio.eeemcee said:Would an H field created by a loop antenna have less attenuation?
The simple rule is that the transmitting and receiving antennas need to have the same polarisation. If we use circular polarisation at both ends that is fine provide they both have the same direction of rotation. If we mix opposite polarisations, such as H and V, or RH and LH circular, there is a large loss, maybe 30dB. If we mix circular and linear polarisation we find only half the power is received, giving 3dB loss.eeemcee said:That's why I'm confused: for instance, television transmitting and receiving antennas are usually horizontal here but vertical in the UK, AM broadcast antennas are usually vertical are these for convenience or effectiveness? Spiral Logs are also right or left handed spiraled. Then there's FM home antennas that a sre horizontal but vertical on a vehicle-- does it matter?
The gain of these antennas is dictated by their length, so we do not expect much difference between circular and linear polarised versions. If someone is offering more gain with less length, then be sceptical. As mentioned previously, communication between linear and circular polarised antennas will result in a loss of 3dB.eeemcee said:A yagi log periodic has a gain of 7dB, does a circular have the same? it's much more compact, does it's gain Vs it's different polarity
result in a gain of only 4 dB when receiving/emitting from/to a monopole?
This is only true in free space. Near buildings and trees and so on multiple pathways and conducting structures mix things up quite a bit. How much electric field component ends up in the same direction as the induced currents in the antenna depends on some pretty complicated stuff in the environment.eeemcee said:at a point they become plain waves
How does a building do this?Paul Colby said:This is only true in free space. Near buildings and trees and so on multiple pathways and conducting structures mix things up quite a bit. How much electric field component ends up in the same direction as the induced currents in the antenna depends on some pretty complicated stuff in the environment.
mainly technical reasons ... If the main TV transmitter is horizontally polarised, then any small translator transmitters used toeeemcee said:That's why I'm confused: for instance, television transmitting and receiving antennas are usually horizontal here but vertical in the UK, AM broadcast antennas are usually vertical are these for convenience or effectiveness?
As @tech99 said ... be careful thereeeemcee said:A yagi log periodic has a gain of 7dB, does a circular have the same? it's much more compact, does it's gain Vs it's different polarity
result in a gain of only 4 dB when receiving/emitting from/to a monopole?
Baluncore said:Half wave dipole gain is 2.15 dBi.
A dipole antenna is a type of antenna that is commonly used in radio and telecommunications. It consists of two conductive elements, typically metal rods, that are oriented in opposite directions and connected to a radio frequency (RF) transmission line.
When an alternating current is applied to the dipole antenna, it creates an oscillating electric field. This electric field then induces a magnetic field that is perpendicular to it. The resulting electromagnetic field is polarized in a specific direction, depending on the orientation of the dipole antenna.
Polarisation refers to the orientation of the electric and magnetic fields in an electromagnetic wave. In a polarised field, the electric and magnetic fields are aligned in a specific direction, while in an unpolarised field, they are randomly oriented.
Polarisation is important in dipole antennas because it affects the direction and strength of the electromagnetic waves that are transmitted and received. By controlling the polarisation of the electromagnetic field, dipole antennas can be used to transmit and receive signals in specific directions, making them more efficient and effective in communication systems.
Yes, the polarisation of an electromagnetic field can be changed after leaving a dipole antenna. This can be achieved by using a polarisation filter, which can selectively block or pass certain orientations of the electromagnetic field. Additionally, the polarisation of an electromagnetic field can also be changed through reflection and refraction in certain materials.