Does the aerial [antenna] not need to be positioned perpendicular?

[homeworkhelpls]

TL;DR

I'm confused, i thought that for microwaves / radio waves the metal grille (aerial) needs to be perpendicular to the transmitter in order for a signal to be received. But here they are in the same orientation?

Shouldnt the aerial be in perpendicular orientation for all em waves? including radio

 

[Baluncore]

The closely spaced metal grille is reflecting the electric polarisation parallel with the wires of the grille. Only the orthogonal component can pass through.

 

[homeworkhelpls]

what is orthogonal and what is the electric polarisation from the grille. All i know is free electrons from the grille cancel out the electric field in the same propagation?

 

[Baluncore]

what is orthogonal and what is the electric polarisation from the grille.

A horizontal wire antenna transmits a horizontally polarised electric wave, with a vertically polarised magnetic wave. Those waves are orthogonal, which means at 90 degrees.
https://en.wikipedia.org/wiki/Polarization_(physics)

All i know is free electrons from the grille cancel out the electric field in the same propagation?

The current induced in the grille by the EM wave, cancels the forward wave. The energy must go somewhere, so it is reflected, back towards the transmitter.

 

[homeworkhelpls]

A horizontal wire antenna transmits a horizontally polarised electric wave, with a vertically polarised magnetic wave. Those waves are orthogonal, which means at 90 degrees.
https://en.wikipedia.org/wiki/Polarization_(physics)The current induced in the grille by the EM wave, cancels the forward wave. The energy must go somewhere, so it is reflected, back towards the transmitter.

Ok i get the first part, how is the forward wave cancelled by the current and what is the forward wave (magnetic/electric field), and what does the energy returning to the transmitter contribute to the question?

 

[Baluncore]

...how is the forward wave cancelled by the current and what is the forward wave (magnetic/electric field),...

When an EM wave is incident on a perfectly conductive sheet, the magnetic component induces a perpendicular (+90°) current in the sheet. That current in turn generates a magnetic field, again perpendicular (+90°), making a total of a +180° rotation. The sum of the incident and reversed magnetic fields into the sheet is zero. But the 180° component is reflected with the energy. Good conductors make good mirrors.
If the conductive sheet is a grille, only EM waves, polarised with the electric component perpendicular to the wires, can pass through. EM waves with electric field component parallel to the wires will be reflected.

and what does the energy returning to the transmitter contribute to the question?

Conservation of energy requires the energy be reflected. If it gets back to the transmitter, it will sum to the transmitted wave, with a phase determined by the distance between the transmitter and reflector.

 

[homeworkhelpls]

Ok i get the reflection part and most of the EM wave passing through the grille makes sense, but

in this example, EM waves are parallel to the wires and not reflected, however the EM waves perpendicular to those of the transmitter are. I might be confusing wave propagation and the electric field propagation of the wave.

 

[Baluncore]

I might be confusing wave propagation and the electric field propagation of the wave.

Both the transmit and the receive antennas need to be oriented in the same way. The electrically conductive dipole elements of the antenna define the orientation of the electric field, which simply names the antenna polarisation.

The current, that flows in the dipole element, produces the perpendicular magnetic field.
The magnetic component of the EM wave; H, is proportional to the current in the dipole element.
The electric component of the EM wave; E = H * Zo; where Zo = 376.73 ohms.
https://en.wikipedia.org/wiki/Impedance_of_free_space
As the EM wave propagates through space, the E and H fields remain in phase.
E and H, and the direction of propagation, are all mutually perpendicular.

The H field of the EM wave induces an electric current in the receive antenna element.
That current becomes the electrical signal that reaches the tuned receiver.

 

[homeworkhelpls]

Thanks it makes more sense now, by the way how do you even learn all this apart from attending lessons I mean, can you help me in a general context become as good as you in problem solving or applying context to questions, and if so what’s your best studying advice?

 

[Baluncore]

... by the way how do you even learn all this apart from attending lessons I mean, can you help me in a general context become as good as you in problem solving or applying context to questions, ...

You ask an impossible question, just what I like.
I am not an expert, I am an impostor. My brain is an empty vacuum that sucks in ideas, which then evaporate. I do not believe I know anything, so I question everything, and argue with myself. I can only see what I feel is wrong, all else is transparent and so can be ignored.

Forget the lessons, they define the syllabus. Read hundreds of books and articles about the same topic, so you get hundreds of interpretations, and so gradually grow one better understanding.

Take on research that has a very steep learning curve, so you discover concepts that will be useful later. When I fail to find an answer or solution, I put it aside, but keep wondering in the background about the solution constraints.

Work at many jobs to get a wide experience of all the things that interest you, ignore the remuneration. Try to design, make or fix things, where no one else will try. After a while, your success rate will improve. Go back to first principles, try to work out why things failed.

Apparently, I spent my first 10 years taking things apart, then the next five putting broken things back together, so that by the age of 15, my constructive profit had finally exceeded my destructive loss.

Understand and practice critical thinking, then analyse the way the original designer was thinking. That can develop into a form of art appreciation.

Technology is not your enemy, work with it, not against it. Become part of the problem by showing empathy and care for the technology, in the same way you would a domestic animal. Become a horse, dog, and technology whisperer.

Don't believe it, I am probably wrong. Just be yourself.

 

[tech99]

The H field of the EM wave induces an electric current in the receive antenna element.
That current becomes the electrical signal that reaches the tuned receiver.

As far as I know, a magnetic field cannot cause electrons to move. It is the accompanying electric field which does this.

 

[Baluncore]

As far as I know, a magnetic field cannot cause electrons to move. It is the accompanying electric field which does this.

You might be right, but in the up-close near-field analysis of conductive antennas, the electric field is best ignored, it is the current that counts.

Currents and magnetic fields are more directly understood through Lenz's law, without the electric field. Good conductors make good mirrors. The magnitude of the current at the surface of the mirror exactly cancels the incident magnetic component into the mirror. Introducing the electric field gradient into the reflection mechanism from a conductor, over-complicates the analysis.

We accept that an eddy current flows in a circle. We should not need to consider the ouroboros of a circular voltage gradient, or apply Kirchhoff's voltage law to a mirror.

 

[homeworkhelpls]

You ask an impossible question, just what I like.
I am not an expert, I am an impostor. My brain is an empty vacuum that sucks in ideas, which then evaporate. I do not believe I know anything, so I question everything, and argue with myself. I can only see what I feel is wrong, all else is transparent and so can be ignored.

Forget the lessons, they define the syllabus. Read hundreds of books and articles about the same topic, so you get hundreds of interpretations, and so gradually grow one better understanding.

Take on research that has a very steep learning curve, so you discover concepts that will be useful later. When I fail to find an answer or solution, I put it aside, but keep wondering in the background about the solution constraints.

Work at many jobs to get a wide experience of all the things that interest you, ignore the remuneration. Try to design, make or fix things, where no one else will try. After a while, your success rate will improve. Go back to first principles, try to work out why things failed.

Apparently, I spent my first 10 years taking things apart, then the next five putting broken things back together, so that by the age of 15, my constructive profit had finally exceeded my destructive loss.

Understand and practice critical thinking, then analyse the way the original designer was thinking. That can develop into a form of art appreciation.

Technology is not your enemy, work with it, not against it. Become part of the problem by showing empathy and care for the technology, in the same way you would a domestic animal. Become a horse, dog, and technology whisperer.

Don't believe it, I am probably wrong. Just be yourself.

What an answer!

 

[sophiecentaur]

Conservation of energy requires the energy be reflected. If it gets back to the transmitter, it will sum to the transmitted wave, with a phase determined by the distance between the transmitter and reflector.

It's worth mentioning that this reflected wave will spread out from the reflector and its amplitude will be very much reduced. Even in the extreme case of an infinitely wide reflector, the field strength, by the time it gets back to the transmitter, will be 1/4 of the value at the reflector. Reflected waves are seldom much of an issue to the transmitter except in cases like a Yagi antenna, in which there are several elements near the 'driven' element. Here, the reflections affect the actual impedance ' seen by' the transmitting amplifier.

As far as I know, a magnetic field cannot cause electrons to move. It is the accompanying electric field which does this.

When you are dealing with a free electron and a static electric or magnetic field on its own, this is a more relevant fact, I think. When an EM wave passes an electron, both fields will affect the electron's motion. The E field effect is sort of 'obvious' but a varying H field will induce a current which is also electron motion. So it's not 'that simple'.

 

[rcgldr]

Aircraft radio control transmitters, the older 72 Mhz ones using 4 to 5 foot long antennas on transmitters, and 3 to 4 foot long antennas on the models worked regardless of orientation of transmitter or aircraft. Range was at least 1/2 mile.

 

[sophiecentaur]

The received signal level will depend on (in a simple ideal model) the component of the incident E field along the wire. To get an exact null or even a very low signal, the misalignment needs to be just right. So most orientations give a received signal.
See how tight the direction of a mf portable ferrite aerial needs to be for a reception null. This is a magnetic aerial of course but the same principle applies.

 

[tech99]

It fascinates me to watch the radio control enthusiasts pointing the antenna at the aircraft! This is the direction for minimum signal - I expect the ground reflection saves them.

 

[Baluncore]

This is the direction for minimum signal - I expect the ground reflection saves them.

While the frequency is low band VHF, and the operator is short when measured in wavelengths, a short whip antenna pointed towards the receiver will have a minimum dipole energy in that direction, but the image in the ground will reinforce the signal towards the horizon, by almost a factor of two. Null times two is still close to zero.

If the whip antenna is perpendicular to the receiver, (side on), the dipole radiation will be greater, but the inverted ground image will almost completely cancel the transmitted signal towards the horizon.

The thing that saves them is the human body that remains (hopefully) perpendicular to the ground plane. The vertical operator is coupled by hand capacitance to one end of the short dipole whip antenna. That makes an inverted 'L' antenna, with vertical polarisation, that reinforces in the direction of a receiver near the horizon. It is not the whip that radiates energy towards the receiver, it is the operator's body.

 

[rcgldr]

It is not the whip that radiates energy towards the receiver, it is the operator's body.

I've seen exceptions to this. A radio control glider with the transmitter trimmed so the glider turns large radius circles, while high up. The transmitter is set down, and the "pilot" walks away for a while, returns back to find a thermal and get the glider upwind and higher, then set down the transmitter again. I've also seen this done with a motorized assist glider set with just enough power to hold altitude while circling in near zero wind condition, and the transmitter set down on a wooden table and timed to see how long it could take before having to grab the transmitter again. It was a bit over 20 minutes.

 

[Baluncore]

I've seen exceptions to this. A radio control glider with the transmitter trimmed so the glider turns large radius circles, while high up.

That makes an inverted 'L' antenna, with vertical polarisation, that reinforces in the direction of a receiver near the horizon. It is not the whip that radiates energy towards the receiver, it is the operator's body.

I was referring there to low-angle radiation, towards the horizon, which is also usually the maximum-range situation. When the glider is high above, the ground plane only partly cancels the signal, but the range is usually much less, so signal reduction by a few dB is less critical.

The human body and the whip antenna are perpendicular, which makes an inverted 'L' antenna. The advantage of an 'L' antenna is that it transmits both H and V polarisation, but they do not cancel, because they are not in phase. They produce radiation that is, in effect, elliptically polarised.

 

[tech99]

The case of a vertical antenna over the ground is very interesting. There is some good information in a paper by Bullington of Bell Systems*. Over perfect ground, or sea water, the ground reflection is aiding at low angles, whereas for a low horizontal antenna it is cancelling. Over imperfect ground, the vertical antenna suffers cancellation in the same way as the horizontal antenna, but in addition, it radiates a surface wave. For this reason, for low antennas, vertical is better than horizontal, even at VHF. There is a height of a few metres below which vertical will retain its advantage. At greater heights, both are equal. The cancellation effect means that over ground, for most practicable heights at VHF and below, the received power decreases with the fourth power of distance, rather than the expected inverse square law.

For the case of an inverted L antenna, the antenna conductor is resonant and supports a standing wave. Therefore, the phase of the antenna current is almost the same everywhere. The uplead will radiate vertical polarisation and the top will radiate horizontal polarisation. Each will propagate as described above. When viewed from the side, we see a tilted polarisation, and because the radiation from the two portions is essentially in phase, it appears to me that it will be linear polarised.

The model control antenna is likely a quarter wave monopole, probably shortened using inductive loading. If the transmitter is placed on a table, there is no ground plane to which the ground terminal of the transmitter can be connected. In such a case, the capacitance of the transmitter case will be in series with the antenna. This capacitance will be very small and will place a large capacitive reactance in series with the antenna system, which will greatly reduce the antenna current, and hence the radiation.

*Radio Propagation for Vehicular Communications, IEEE Transactiions in Vehicular Technology, Vol VT26, No. 4, Nov 1977.

 

[Baluncore]

Over imperfect ground, the vertical antenna suffers cancellation in the same way as the horizontal antenna, but in addition, it radiates a surface wave. For this reason, for low antennas, vertical is better than horizontal, even at VHF. There is a height of a few metres below which vertical will retain its advantage. At greater heights, both are equal. The cancellation effect means that over ground, for most practicable heights at VHF and below, the received power decreases with the fourth power of distance, rather than the expected inverse square law.

It seems here you are referring to communication with other ground based receivers, not to receivers overhead.

For the case of an inverted L antenna, the antenna conductor is resonant and supports a standing wave. Therefore, the phase of the antenna current is almost the same everywhere.

That all assumes the antenna is short, and is cut to be resonant as a bent half-wave dipole. Since the hat in this model is the RC transmitter, which was tuned to be self-resonant, that cannot be the case. The human body is not so much part of the same conductor, as it is a capacitive-coupled counterpoise with a hand-capacitance induced quadrature phase shift.
The bend in an inverted 'L' is an impedance mismatch, that makes it two different radiators. The hat can be efficiently excited as a horizontal dipole against the tower. Since the distance between each part of the tower and ground is changing, it is best modelled as a leaky tapered transmission line, which makes it a poor resonator.

When viewed from the side, we see a tilted polarisation, and because the radiation from the two portions is essentially in phase, it appears to me that it will be linear polarised.

A conclusion, based on the false assumption that the 'L' is short and tuned to λ/2 in free space. An inverted 'L' antenna, usually stands above an imperfect ground plane, and will often be driven against it's image in a surface ground wire, like a λ/4 vertical with a hat. In the case of an RC transmitter, the whip antenna and the human body are independent resonators, with hand held reactive coupling.

This capacitance will be very small and will place a large capacitive reactance in series with the antenna system, which will greatly reduce the antenna current, and hence the radiation.

But the RC transmitter was tuned to be self-resonant, so low capacitance will not greatly reduce antenna current, it will tend to maintain it.
A hand capacitance of 1 pF at 100 MHz has a reactance of about -1600, which is not insignificant. A park bench may have a steel frame, which will make a more interesting parasitic element.
The current in the human body will be significantly less than in the whip antenna, but will certainly not be zero, so it cannot be discounted when there is so much more power radiated than is needed for communication.

 

[tech99]

You are quite correct that I was considering a ground level receiver, rather than the aeroplane.

 

[sophiecentaur]

Aamof, the radiation pattern of such a system is anyone's guess. The only way to be sure of what you were getting would be to do a pattern measurement with a receiver in a drone. At least we have those available these days; not many years ago it would take a helicopter to measure HRP and VRP.
The alternative is 'faith' and the fact that a human operator will wave the antenna about a bit if the plane starts to misbehave.

 

[Baluncore]

Aamof, the radiation pattern of such a system is anyone's guess.

That is a defeatist attitude. Everyone is entitled to an opinion, and As A Matter Of Fact, some people's predictions turn out to be better, than other people's guesses.

Experience, and the scientific analysis of antennas, can lift one's understanding, a couple of steps further above the black-art of antennas. The climb up those steps is long, slow, and expensive. We learn only by doing, making mistakes, and then realising and understanding those mistakes. That feedback loop involves taking many different viewpoints, while discussing the subject at every opportunity, in constructive ways.

The description of the radiation pattern comes down to the level of abstraction of the model, and an ability to compute, imagine, or "see" the expected RF currents on the conductive surfaces, in 4D. But, the model is always limited by the definition, and so must always differ somehow, from the undeniable real world.

 

[sophiecentaur]

That is a defeatist attitude.

My attitude is based on experience with HF curtain arrays - which are designed 'properly' and not just suspended somewhere near the ground. The pattern and actual gain are very dependent on the ground impedance, which is why it's important to do what you can in the way of earth mats. RC flyers don't have that luxury.

The description of the radiation pattern comes down to the level of abstraction of the model, and an ability to compute, imagine, or "see" the expected RF currents on the conductive surfaces, in 4D. But, the model is always limited by the definition, and so must always differ somehow, from the undeniable real world.

There's no doubt that experience is a big help. However, as with hi-fi and performance cars, people often want to treat the limited models of systems as gospel. Any predictions should be taken with a pinch of salt, once the major factors in an RC link have been taken care of, it's down to coding, transmitter and receiver design (those are more under the control of the users).

 

[Baluncore]

Any predictions should be taken with a pinch of salt, once the major factors in an RC link have been taken care of, it's down to coding, transmitter and receiver design (those are more under the control of the users).

The RC operator's vision is part of the control feedback loop, they must estimate the orientation, airspeed, and airflow over the model. I believe the deep and narrow nulls, where communication is momentarily lost, go quite unnoticed by RC operators, as the model will fly through, or fall out of a null.

The RC operator is more likely to blame themselves or turbulence, for any deviant model dynamics, rather than a null in the radiation pattern. That is a sensible behaviour, since the nulls are sparse, and so very narrow in space and time.

 

[sophiecentaur]

The RC operator's vision is part of the control feedback loop, they must estimate the orientation, airspeed, and airflow over the model. I believe the deep and narrow nulls, where communication is momentarily lost, go quite unnoticed by RC operators, as the model will fly through, or fall out of a null.

The RC operator is more likely to blame themselves or turbulence, for any deviant model dynamics, rather than a null in the radiation pattern. That is a sensible behaviour, since the nulls are sparse, and so very narrow in space and time.

I agree entirely. Many excellent RC operators know virtually nothing about EM and that's my point. The equipment works well enough, despite some very approximate theory. It's Darwinian; survival of the fittest systems but in total ignorance of how life / EM works.