Plastic Bottle as an Antenna!?
I was recently interviewed on how a plastic bottle can radiate. My reply was that it has sudden bends/turns and accelerating charges radiate. I was countered with the statement that the bottle was plastic. I replied that even human body radiates. So what is the real deal here? Can any object act as an antenna? If so how?
you should have asked more questions before answering, so as not to look foolish or impetuous.
All objects radiate energy, proportional to their heat. That is how radiometers work, for instance.
Or did he mean using a plastic bottle as an antenna? you could have talked about the bottle/air interface, and at some angles of incidence it would form a transmission structure due to the differing dielectric constants, like a fiber optic cable does.
You could have talked about filling the bottle with things, or putting a conductive surface or wires along the bottles outer surface....and on and on.
Or maybe just stick a candle in the middle and be illuminated with radiated light.
Some plastics can retain a high-V static charge. In a sense this can radiate (and be detected from a distance), although it is not the same thing as EM radiation.
Plastic bottle or metal can?
http://www.turnpoint.net/wireless/cantennahowto.html
http://www.instructables.com/id/Chea...oster-that-is/
Interesting!
The interviewer took a sprite bottle and asked me how this thing can radiate? Initially I was like... whaaaaaat?
Anyways, now I do understand. But what is with this TIR phenomenon and how does it help in radiation?
Thank you biff44
That is good! Never thought about that!
Also what about me taking a plastic bottle, attracting some +ve or -ve charges and moving back and forth as fast as I can?
was it a normal sprite bottle, or diet sprite?
Let me rephrase the question.
How can I say an object is radiating or is behaving as an antenna?
We have lots of antenna structures like spherical, Yagi-Uda, Parabolic, Dipole, Slot, Bow Tie, Wearable, PIFA and so on. But how can I say just by mere inspection that this is an antenna?
Normal Sprite (Plastic One), NOT diet Sprite (Tin can).
I'm under the impression that your question is somehow missing a foundation.
You are asking about an transmitter antenna, there will a generator and a port (feed point) where you connect antenna and generator. Optionally a cable or waveguide inbetween.
In case of the plastic bottle, where's the feed point? What kind of connection do you consider?
Leave the initial discussions.
I am rephrasing the question to: How can I verify an object is radiating or is behaving as an antenna?
We have lots of antenna structures like spherical, Yagi-Uda, Parabolic, Dipole, Slot, Bow Tie, Wearable, PIFA and so on. But how can I identify just by mere inspection that this is an antenna?
I fear you are just generalizing the vagueness of your original question.
Any dielectric or conducting object can act as an antenna under circumstances. A reasonable question could be what's an effective antenna with specific properties (frequency, bandwidth, directivity, room restrictions), what are general antenna design principles.
This is true, every structure carrying RF current generates an electromagnetic field and can radiate RF power to some extent.
But most of the time the Antenna Engineers look for metal (conductor) structures when is about designing antennas.
About antenna radiation theory: when an AC current flows through a conductor, electric and magnetic fields are created around the conductor.
If the length of the conductor is very short compared to a wavelength (<< λ/4), the radiated electric and magnetic fields will decrease near the conductor.
As the conductor is lengthened, the intensity of the radiated EM fields enlarges and part of the energy goes into the space.
When the length of the conductor approaches a quarter of a wavelength (λ/4) at the frequency of the applied AC signal, most of the energy will escape into the free-space in the form of electromagnetic radiation.
I would ask: where/how is the signal input?
The rest is only a matter of radiation efficiency. If you ask an EMC engineer, he will tell you about radiation from wires, supply networks and other conductors that are not designed as an antenna, but radiate to some degree. We usually call it an antenna if the purpose is to radiate, but there is certainly some degree of radiation from all conductors.
IMO this is a great interview question. It is open ended and allows you to show many of the important skills you need in a real job.
Some of the things an interviewer could discover from this are:-
- Your ability to elicit requirements from a non-technical customer. The customer hasn't given you enough information. You need to find out more.
- Ability to explain complex phenomena in a clear and easy to understand way.
- Show your breadth of knowledge. i.e. how many ways can you make it radiate?
- Knowledge of manufacturing techniques. I.e. how could you embed an antenna in bottle.
- Identifying problems. It might need to radiate when full as well as empty. Would this create problems?
- Thinking outside the box. Maybe you could embed an RFID tag that changed its code dependent on the fluid level.
There is so much scope in this question for you to impress the interviewer, and you are free to twist your reply to almost any subject that you are well versed in.
So long as you don't just give a "I don't know!" reply you are on to a winner with this one.
Unscrew button, push the bottle fast and hard, it will then radiate an infra wave if empty, or else will it radiate Sprite. (Done that mistake)
Drop a filled bottle from a high building. It will emit a shock-wave in ground.
Blow in top of a empty bottle and it will radiate sound waves.
Rubbing it against nylon-clothes and discharge against ground and it will radiate EM waves due the spark.
The spark will also emit visible light.
Put the bottle on fire and it will radiate heat, gases and a lot of particles, even ionizing particle radiation. PET-bottles do burn, but rather weak.
Using a plastic bottle more or less filled with a conducting liquid can be used as a tunable antenna. It is a US patent by Kingsley/Antenova.
With my own little experience, i dont think its ever possible to radiate artificial radio wave with a non-conductive object.
Wrong!
Take a pithball about the size of a marble between your thumb and forefinger. If you don't know
what a pithball is, or can't find one, a hunk of plastic foam or a ping pong ball will be fine. For
lack of a better name we will refer to any of these articles as a pithball. Rub the pithball on your
carpet to give it a good electrical charge. Now wave the pithball back and forth in the air over a
six-inch distance as fast as you can. The pithball is sending out electromagnetic waves! Let's say
you are achieving a rate of 10 cycles of motion per second. If you have placed in the corner of
the room a sufficiently sensitive megameter-wave receiver, it will detect a signal when tuned to a
frequency of 10 hertz, or to a wavelength of 30 million meters. If you could vibrate your hand
fast enough you might even be able to carry on radio communication in this fashion!
though, i dont know what a pithball is, but did you know that your hand and your body as whole can conduct(partially)/store electric charge?
This Seem Controversial, Again, An Objecj That Cannot Conduct/Store Electric Charge Cannot Be Used As An Aerial(e.g. for fm/am transmitter).
Dielectric waveguides and antennas do exist. Just a matter of frequency, suitable permittivity and impedance matching. See e.g. https://en.wikipedia.org/wiki/Dielec...onator_antenna or a profound RF engineering text book.
You can't... except if you mean a "reasonably good" antenna.
I like this excerpt:
It's from the Preface to Phased Array Antenna Handbook, First Edition (R. Mailloux, Artech House, 1994).
Regards
Z