RF transmitter inside a metal enclosure, re. reflections

[genekuli]

wavelength is .3 m to 6m, a 3m waveguide can do up to the 6m λ, so it will reflect all the frequencies concerned

 

[alan123hk]

Summary:: on an receiving antenna, that a 1W RF transmitter source might induce 1mW. But inside a RF reflective (metal) enclosure, this same scenario might induce more than the 1mW on a receiving antenna because of the duplication of apparent radiation sources of the reflections? maybe it might induce near 1W?

wavelength is .3 m to 6m, a 3m waveguide can do up to the 6m λ, so it will reflect all the frequencies concerned

If everything works perfectly, you can really input 1W and the system has no loss, then the load can of course receive 1 W.

However, the actual situation may not be as ideal as expected. For example, if the impedance does not match, you may find that the input port of the transmitting antenna and the output port of the receiving antenna have a larger reactive impedance. It's actually hard for you to inject the energy into the transmitting antenna and extracted the energy from the output port of the receiving antenna, and multiple reflections may not necessarily be superimposed and enhanced, or they may cancel each other out.

I believe that the question you mentioned already involves the field of waveguides and microwave engineering. This is a rich and difficult subject. Generally speaking, only microwave engineers and scholars who specialize in this field can have an in-depth understanding of its detailed design and operation, and most people only know some very basic knowledge.

 

[sophiecentaur]

only microwave engineers and scholars who specialize in this field can have an in-depth understanding of its detailed design and operation,

Too right. You have a cavity that's supplied and loaded in two regions (not even points) with loops or plates. Knowing just what you can approximate that to and then doing the right numerical analysis requires a special specialism, imo. It takes transmission line theory to a whole new level.

 

[genekuli]

I was hoping someone would or could take a guestimate to approximate the increase in efficiency of adding the enclosure. from other replies i gathered that the reflected energy available to the Rx antenna might be 1000 time the amount of the original incident 0.01W from the 1W Tx source due to reflections. giving it a ~1W Rx power.
i just wonder if this is in the ball park?

 

[DaveE]

I was hoping someone would or could take a guestimate to approximate the increase in efficiency of adding the enclosure.

Nope. You're not going to get a good simple answer to this here. The enclosure might help, or it might hurt. It all depends on the details. Anyone that really needed to optimize this configuration would rely greatly on theory and computer modelling.

 

[DaveE]

Also, optimization over 4.5 octaves is really hard. Efficient systems are narrow band.

 

[genekuli]

so for those frequencies that do not have destructive interference.
would the ballpark figure be between 0.5W to 1W on the rx from the 1W tx source inside the RF reflective box?

 

[sophiecentaur]

the reflected energy available to the Rx

This is the problem. "Available" means that the receiving antenna will be perfectly matched to the impedance of the standing wave impedance at every frequency. That ain't going to happen so you would have to specify your problem with far more detail. What happens at one frequency is hard to say because a receiving antenna will not be isotropic but could you even assume that the incident energy, off-beam is all reflected back into the resonator?

I think you'd be in with some sort of a chance of a solution for light but not for a coherent situation. Your suggested factor of 1000 would be basically assuming that. But the loading of a cavity will affect its Q factor, which would limit the 'magnification'. You will never get more out than you put in so the only power available can only be the input power.

Bottom line is whether you want the max Energy Density at points inside the cavity (perhaps the 1000X figure) or how much Power can you get out - which will necessarily be equal or less than Power In.

 

[alan123hk]

This is the problem. "Available" means that the receiving antenna will be perfectly matched to the impedance of the standing wave impedance at every frequency. That ain't going to happen so you would have to specify your problem with far more detail. What happens at one frequency is hard to say because a receiving antenna will not be isotropic but could you even assume that the incident energy, off-beam is all reflected back into the resonator?

I think you'd be in with some sort of a chance of a solution for light but not for a coherent situation. Your suggested factor of 1000 would be basically assuming that. But the loading of a cavity will affect its Q factor, which would limit the 'magnification'. You will never get more out than you put in so the only power available can only be the input power.

Bottom line is whether you want the max Energy Density at points inside the cavity (perhaps the 1000X figure) or how much Power can you get out - which will necessarily be equal or less than Power In.

This comment based on engineering and physics is very reasonable.

 

[DaveE]

I think to understand the complexity of this problem better, you should learn about the "interference of waves". All of the different paths that the radiation from one antenna to the other don't result in the simple addition of the radiation. Some of those paths add up but just as many will cancel because of the phase difference between the radio waves. If you change the frequency of the waves each sum will change because the phase shift created by the path length changes with the frequency. Of course, your enclosure will have many, many different paths for the radiation to follow from source the receiver.

 

[tech99]

If the container is not resonant at the frequency of operation, the transmitter and receiver antennas will have difficulty doing their job. This is because the reflected energy will create large impdances in series with the antennas.
If the container is resonant, then the transmitting and receiving antennas will need to be moved around to find the best position. It should then be possible for nearly all the transmitted power to be received.
In this situation I think the radiation resistance of the two antennas will severely damp the resonances of the container. However, if the two antennas are just lightly coupled into the container, then intense fields will occur as a result of energy being stored due to resonance.
If we were to probe the container under resonant excitation, we would find very intense electric fields and magnetic fields, but in different positions and in phase quadrature. If we then inserted a receiving antenna, the stored energy would be supplied to the antenna over several cycles.

 

[genekuli]

This is the problem. "Available" means that the receiving antenna will be perfectly matched to the impedance of the standing wave impedance at every frequency. That ain't going to happen so you would have to specify your problem with far more detail. What happens at one frequency is hard to say because a receiving antenna will not be isotropic

i was hoping to use the same antenna design for tx and rx, so as to match the two antennas. and of course match the lead and transmitter.

 

[genekuli]

so how about from the 1W tx in the box.
i might be able to have 0.5W incident on the rx antenna?
0.5W loss for distractive interference and imperfect reflections, etc.
would that be in the ball park?

 

[DaveE]

for those frequencies that do not have destructive interference.

It's not just the "frequencies that do not have destructive interference", it's all of the different paths that the radiation can follow. At any given frequency, some will be constructive, some destructive. Then if you change the frequency, you will get a different answer.

 

[sophiecentaur]

0.5W incident on the rx antenna

The word "incident" is not really appropriate imo and this is the whole point. If the Tx and Rx antennae are perfectly matched then there will be no standing wave . Transmitting antennae are not fed the way you seem to imply, the antenna is matched to the feeder to eliminate reflection at the feed point but what impedance would the antenna be working into? A transformed version of the Impedance of (looking into) the receiving antenna. A transmitter would be very inefficient if it dissipated half of its RF power - which is what a 50Ohm source does for you.
You are basically talking in terms of a pretty badly designed feeder between `Tx and Rx. Why would you do it that way?
There are cavity resonator filters that have very low insertion loss by arranging the coupling in and out properly - also Klystron cavities do a good job of extracting the RF power carried by the electron beam. But this is not what the OP is trying to discuss.

 

[genekuli]

The word "incident"

i used that borrowed terminology to exclude the antenna's losses

 

[genekuli]

it seems that i will have to build it (the aforementioned set up, i.e. tx & rx in a metal box) and take measurements. as the theory seems too dependent on confounding factors.
but i would appreciate some wild guesses from anyone willing as to the maximum percentage of the source's tx power i will likely detect with the rx (locating the best sweet spot with identical antennas, all hardware matched with minimal VSWR)

 

[hutchphd]

After you do this, what will you have? Why are you doing this? This is all very well known and rather complicated physics. Just wondering...

 

[DaveE]

it seems that i will have to build it (the aforementioned set up, i.e. tx & rx in a metal box) and take measurements. as the theory seems too dependent on confounding factors.
but i would appreciate some wild guesses from anyone willing as to the maximum percentage of the source's tx power i will likely detect with the rx (locating the best sweet spot with identical antennas, all hardware matched with minimal VSWR)

It's not the theory that's the problem, it's the confounding details. They don't go away when you build it, they just get much harder to adjust. Good luck with that. You'll get a result, but I doubt that you'll learn much from it.

You have yet to address any of those confounding details in your post, so, no, your very unlikely to get a guess from us. Honestly, it could be anything.

 

[alan123hk]

it seems that i will have to build it (the aforementioned set up, i.e. tx & rx in a metal box) and take measurements. as the theory seems too dependent on confounding factors.
but i would appreciate some wild guesses from anyone willing as to the maximum percentage of the source's tx power i will likely detect with the rx (locating the best sweet spot with identical antennas, all hardware matched with minimal VSWR)

Generally, some basic and appropriate conditions should be set as much as possible before making an estimate, so that a meaningful estimate can be made after the possibility is narrowed.

I have a small suggestion. Maybe you can consider setting some conditions yourself, and then explain in detail how to estimate the power received by a receiving antenna that conforms to the laws of physics, and then let others see if this is reasonable.

 

[sophiecentaur]

it seems that i will have to build it (the aforementioned set up, i.e. tx & rx in a metal box) and take measurements. as the theory seems too dependent on confounding factors.
but i would appreciate some wild guesses from anyone willing as to the maximum percentage of the source's tx power i will likely detect with the rx (locating the best sweet spot with identical antennas, all hardware matched with minimal VSWR)

I don't think you have given the context of all this. Afaics, you seem to be proposing to replace a perfectly good piece of Feeder / transmission line / waveguide with your thought experiment box. But to what end? You seem to want to discuss this as if the Tx and Rx antennae were in free space (hence the term "incident" but they are not in that condition. They are very closely coupled elements that are assumed to work as perfectly as possible. That is to say, with minimal insertion loss.

If you are after a ball-park figure for Insertion Loss, or perhaps variation of insertion loss across the band. Looking at manufacturers' figures for insertion loss of cavity filters then you seem to be able to buy stuff with better than 2dB loss and the best figures I could find (after a cursory search) was about 0.1dB for a low loss cavity band pass filter design. But that would assume the cavity has been designed optimally and you don't specify your situation so it could be a lot worse than that.

Bottom line would be that the VSWR would define the minimal insertion loss so you could work with that idea.

 

[genekuli]

After you do this, what will you have? Why are you doing this? This is all very well known and rather complicated physics. Just wondering...

it is this "garage research" which has actually been responsible for a lot of our modern inventions. for me it is to go beyond theory and have fun exploring, looking for interesting findings.

 

[anorlunda]

it is this "garage research" which has actually been responsible for a lot of our modern inventions. for me it is to go beyond theory and have fun exploring, looking for interesting findings.

That only works well when you study the theory first. Our member @phinds said it best.

Thinking outside the box only works well when you first understand what's IN the box.

 

[sophiecentaur]

it is this "garage research" which has actually been responsible for a lot of our modern inventions

Have there been any successful "inventions" from you, using this method?
Without some solid technical basis, the law of monkeys and typewriters tends to apply.

Apple and others were successful, not because they started in a garage but because their founders had done a lot of homework first.

 

[nsaspook]

it is this "garage research" which has actually been responsible for a lot of our modern inventions. for me it is to go beyond theory and have fun exploring, looking for interesting findings.

I hope eventually you get satisfying answers. RF engineering is a mature field in applied science so don't expect to find much that's actually new.

 

[DaveE]

to go beyond theory

Beyond theory, or around theory? Experimental data isn't very useful without a hypothesis to compare it to.

 

[Charles Link]

What you @genekuli are trying to do basically occurs inside a microwave oven. Even the food itself can be highly reflective , and the power levels are much higher than what would be radiated by the transmitter in free space. I can't presently quantify this, but a google of this topic might show some interesting results.

 

[genekuli]

it's just some fun, playing around with interesting stuff, I'm not much into nightclubs as an alternative and i don't drink, so moving antennas around a metal room and watching the spectral analysis is what i can do instead, actually i find it fascinating

 

[genekuli]

I hope eventually you get satisfying answers. RF engineering is a mature field in applied science so don't expect to find much that's actually new.

thanks. it's just some fun, playing around with interesting stuff, I'm not much into nightclubs as an alternative and i don't drink, so moving antennas around a metal room and watching the spectral analysis is what i can do instead, actually i find it fascinating

 

[alan123hk]

it is this "garage research" which has actually been responsible for a lot of our modern inventions. for me it is to go beyond theory and have fun exploring, looking for interesting findings.

Although you seem to be an amateur and are far from the advanced level of research, I still admire your passion for science.

When I was young, I was also very interested in electromagnetic phenomena and tried various small experiments. If you are very interested in natural sciences, you might as well try to learn related basic theories and mathematical tools step by step, which will help you understand deeply and reach a higher level, and you may become a scientist in this field in the future.

As for the experiment you are going to conduct now, you may find many different phenomena. For example, because electromagnetic waves have a polarization direction in addition to the direction of propagation, and in most cases, the antenna itself also has great directivity and sensitive to polarization as well, so when you change the direction of the transmitting antenna or the receiving antenna, you will find that the signal strength of the receiving antenna will change greatly.

Another situation you will observe is that even if the distance between the two antennas is the same and their direction remains the same, the received strength measured in an open space and in a metal box may differ greatly. This is because when the size of the metal box and the wavelength of the electromagnetic wave are close to the order of magnitude, a certain degree of standing wave and resonance are usually formed in the closed metal box, therefore, the impedance of the antenna port will change, and the field distribution will be very different from the open space.

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/waveguides/

 

[sophiecentaur]

don't expect to find much that's actually new.

but, if you do make the most of already available knowledge, you will find loads to interest and satisfy you. Even Newton acknowledged the idea of standing on the shoulders of giants.

 

[cmb]

Summary:: on an receiving antenna, that a 1W RF transmitter source might induce 1mW. But inside a RF reflective (metal) enclosure, this same scenario might induce more than the 1mW on a receiving antenna because of the duplication of apparent radiation sources of the reflections? maybe it might induce near 1W?

wavelength is .3 m to 6m, a 3m waveguide can do up to the 6m λ, so it will reflect all the frequencies concerned

The output will never exceed the input.

If there is a transmitter and receiver that receives , as it were, 'repeated' transmissions, then the impedance will decrease, like having multiple resistors receiving a current. Therefore, the effect of the box is that it can act as an impedance matching feature.

However, the output can be substantially less than the input, thus can act as an attenuator.

Those two things, each acting in their own way, will lead to you having made a band pass filter.

As if by magic, youtube provides the direct demonstration;-



HTH

 

[nsaspook]

but, if you do make the most of already available knowledge, you will find loads to interest and satisfy you. Even Newton acknowledged the idea of standing on the shoulders of giants.

This poster has been on several sites and we have already been in several conversations about his various, somewhat esoteric questions similar to this one.