Electromagnetic fields are polarised when leaving a dipole antenna....

[eeemcee]

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?

 

[tech99]

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.

 

[tech99]

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.
The radiation in both cases comes from electrons in the conductors which are accelerated back and forth by the transmitter.
If we get very close to these antennas we can see strong local fields coming from the large currents and voltages. As the antennas are resonant, a lot of energy is stored in these fields. The reactive energy in E and H is always equal, but in the case of a loop it is common for the E energy to be mainly stored within a capacitor, so we don't see a large reactive E field.
.

 

[eeemcee]

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 are horizontal but vertical on a vehicle-- does it matter?

 

[tech99]

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 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.
So why choose a particular polarisation?
1. To reduce interference between stations using close frequencies (or even the same frequency).
2. To allow the use of low antennas, where we can utilise a surface wave over the Earth or sea. This is the case for AM broadcasting and also for portable VHF sets.
3. To avoid fading when communicating with satellites. We use circular polarisation, because the atmosphere rotates polarisation in an unpredictable way.
4. To avoid problems with the mechanical mounting of an antenna. For instance, there is a problem mounting a vertically polarised Yagi antenna on a vertical pole.
For communication above about 30MHz using high antennas, we do not usually see differences in average path loss between polarisations.

 

[eeemcee]

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?

 

[tech99]

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?

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.
Be careful about the gain of a monopole, by the way - this is a tricky question.

 

[Paul Colby]

at a point they become plain waves

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.

 

[tech99]

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.

How does a building do this?

 

[Paul Colby]

It's quite simple. The iron bars[1] reinforcing in concrete have currents induced by the incident field. These currents will and do reradiate. Now the polarization (component of the scattered E-field) will lie along the current induced. This can and is any old which a way.

[1] and pipes and power line/door bell wires. Even dielectrics like wood will have some induced polarization current which is a current which will reradiate quite nicely at some frequencies.

 

[davenn]

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?

mainly technical reasons ... If the main TV transmitter is horizontally polarised, then any small translator transmitters used to
cover geographical shadow areas will be vertically polarised to maintain a signal separation that different polarisations
afford. This even allows for the low power transmitters to often be on the same frequency as the main high power transmitter.
The house in a geographic shadow zone and also getting the signal on a different polarisation means it's likely to only receive the small low power translator signal.

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?

As @tech99 said ... be careful there

a log period Yagi doesn't have a gain of 7dB ...
Firstly, an antenna gain is mainly a function of how many elements it has. a 3 element yagi has approx 6dBi, a 10 element Yagi, 10dBi
A wide band yagi generally has a gain of approx 6dBi ... namely that of a 3 Yagi on any frequency within the range of the log-periodic's bandwidth

Secondly, we never refer to an antenna gain as just plain xx dB gain, as it is meaningless.
The gain of an antenna ALWAYS has a reference to something. The 2 common references are to an isotropic radiator gain = 0,
so we can say that a given antenna, a Yagi, loop, etc has a gain of xx dBi gain over an isotropic radiator.
The other is the gain over ( as referenced to) a dipole, so you antenna can have a gain of xx dBd. From memory a
dipole has a gain of approx 2.3 dBi (gain over an isotropic radiator)

Also from memory, there is something like 25 - 30 dB difference between horizontal and vertical polarisation. As a result, it gives very good isolation.
On long distance HF propagation, this isn't really an issue/factor as the multiple reflections off the ground and ionosphere really mess
with the polarisation and the signal being received can have very mixed polarisations ... part of the reason for signal fade-outs.
But for shorter range VHF, UHF and up into the microwave bands, polarisation compatibility between TX and RX antennas is critical.
And I personally have proved that time and time again with my ham radio experiments on frequencies from 144MHz through to 24GHz.

My figures should be pretty close, am sure @tech99 will adjust where necessary
My old memory aint what it used to beDave

 

[Baluncore]

Half wave dipole gain is 2.15 dBi.

 

[davenn]

Half wave dipole gain is 2.15 dBi.

cheers, I was in the ball park ... didn't have time to google it

 

[tech99]

Thank you, Paul, I just wanted to get your line of thought.
I think there are about six ways that depolarisation can occur on a path.
1) As you describe, a re-radiating structures that is tilted.
2) Reflections coming from a mirror which is to one side but above or below the horizon will create a geometrical twist.
3) Ionospheric paths alter polarisation.
4) Reflector antennas as used for microwaves tend to have cross-polarised side lobes lying close to boresight. These usually lie in planes at 45 degrees to the principle planes. If the antenna is not pointing accurately, these can create sensitivity to the opposite polarisation.
5) If a reflected ray arrives on one of these side lobes, cross polarisation is degraded.
6) The reason for these side lobes is that for geometrical reasons, the currents in the dish do not flow truly vertical or horizontal but are bent like lines of longitude. On axis, the cross polarised radiation hopefully cancels. If the incoming wavefront does not illuminate the whole reflector uniformly, however, perhaps due to a ground reflection, cancellation will not occur.