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Author Topic: Would shaking an antenna affect its performance?  (Read 3405 times)

manyquestions

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Would shaking an antenna affect its performance?
« on: 16/06/2015 21:24:01 »
I've recently been reading up on antennas and standing waves, my question is this, if I feed a monopole antenna with an electromagnetic signal at a given frequency and trim the antenna length until it becomes resonant, then I remove the signal take hold of the antenna and shake it mechanically at the same frequency as the electromagnetic signal I applied earlier, would the antenna still be resonant at its trimmed length with the mechanical vibration?, if no, which is my feeling, is there any correlation between the two forms of resonance? thanks, john
« Last Edit: 17/06/2015 07:54:24 by chris »

Colin2B

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Re: RESONANCE
« Reply #1 on: 17/06/2015 03:46:13 »
............ then I remove the signal take hold of the antenna and shake it mechanically at the same frequency as the electromagnetic signal I applied earlier, would the antenna still be resonant at its trimmed length with the mechanical vibration?, if no, which is my feeling, is there any correlation between the two forms of resonance? thanks, john

John
Welcome to the forum and thanks for the question, which is not one which many people would think to ask.
The answer as you have guessed is no.
As you will have discovered in your studies, the radio monopole frequency is generated by the electrical current moving the electrons in the antenna so the they generate radio waves. These being electromagnetic radiation, their relationship between wavelength and frequency is determined by the speed of light,as you will know.
With mechanical vibrations, the relationship between wavelength and frequency is determined by the speed of sound in the the rod. The speed of sound is in turn determined by Young's modulus for the material and it's density. As with radio antenna there are various modes of vibration and you might find it interesting to research them, they depend on how you support the rod. A good text on musical acoustics will guide you, or you should find something on the web.
The correlation between the two types of radiation, radio and mechanical, is that the frequencies are dependant on the length of the rod.

PmbPhy

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Re: RESONANCE
« Reply #2 on: 17/06/2015 03:46:32 »
I've recently been reading up on antennas and standing waves, my question is this, if I feed a monopole antenna with an electromagnetic signal at a given frequency and trim the antenna length until it becomes resonant, then I remove the signal take hold of the antenna and shake it mechanically at the same frequency as the electromagnetic signal I applied earlier, would the antenna still be resonant at its trimmed length with the mechanical vibration?, if no, which is my feeling, is there any correlation between the two forms of resonance? thanks, john
When you mechanically shake the antenna you're shaking the entire metal lattice of positive nuclei and negative electrons. So their fields cancel each other out. When you feed in a signal its only the electrons which are vibrating.

PmbPhy

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Re: RESONANCE
« Reply #3 on: 17/06/2015 03:50:12 »
Quote from: Colin2B
With mechanical vibrations, the relationship between wavelength and frequency is determined by the speed of sound in the the rod.
You're assuming that he shook the rod from one end and it takes the signal traveling at the speed of sound to get to the other end. However he made no statement on how it's shaken. For example; a mechanical or electronic device could be attached continuously throughout the length of the antenna so that when it's shaken the rhythm is maintained throughout the entire antenna.

Colin2B

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Re: RESONANCE
« Reply #4 on: 17/06/2015 03:54:14 »
I've just noticed that PmbPhy's reply has 'collided' with mine. He makes a good point as to why mechanical vibration doesn't generate radio waves. Conversely, I'm not aware that electrical stimulation results in mechanical vibration other than in materials like piezo electric. Anyone aware of other effects?

Colin2B

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Re: RESONANCE
« Reply #5 on: 17/06/2015 03:59:58 »

You're assuming that he shook the rod from one end and it takes the signal traveling at the speed of sound to get to the other end. However he made no statement on how it's shaken.
No, I didn't assume one end, but I did assume it was supported in some way and as I said, the frequency depends on the mode of vibration
Must confess I didn't think of shaking the whole rod uniformly! Would it set up a natural resonance?
Certainly a bar dropped on the floor will resonate as it bounces, unsupported. That needs some thought!

manyquestions

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Re: Would shaking an antenna affect its performance?
« Reply #6 on: 29/06/2015 20:44:27 »
hi to both colin2b and pmbphy, firstly my appologies for my late reply, i came down with some sort of bug after posting and couldnt get rid of it, on the mend now though, anyway i found both answers great to be honest, thanks also to colin for the warm welcome too what a great forum, i will be reading again now my my mind is back in gear so may just have another question i cant answer! thanks again to you both, john

Colin2B

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Re: Would shaking an antenna affect its performance?
« Reply #7 on: 29/06/2015 21:59:39 »
......may just have another question i cant answer! thanks again to you both, john
You might want to think about what shaking means. As you can see PmbPhy and myself put different interpretations on it.
For example, if you put a dipole at the top of a flexible mast and that mast rocks side to side, then if you look at the polar plot for a dipole you will see that signal strength reduces at the extremes of the rocking compared to when it is vertical. Shaking as described by PmbPhy would not have any effect!

jccc

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Re: Would shaking an antenna affect its performance?
« Reply #8 on: 29/06/2015 22:36:08 »
shake the antenna, it will produce gravitational wave at same frequency.

if you shake it at radio frequency, it will be fun.

chiralSPO

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Re: Would shaking an antenna affect its performance?
« Reply #9 on: 29/06/2015 23:43:04 »
jccc, enough with the EM is gravitational waves! you can pursue that line of thought in your own threads, where it will be hotly debated, but don't pollute other threads with this nonsense.

jccc

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Re: Would shaking an antenna affect its performance?
« Reply #10 on: 30/06/2015 00:37:02 »
shake antenna, will affect performance.

i noticed during storm or windy weather, tv screen is unstable.

Colin2B

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Re: Would shaking an antenna affect its performance?
« Reply #11 on: 30/06/2015 07:07:05 »
shake antenna, will affect performance.

i noticed during storm or windy weather, tv screen is unstable.
This is a good and valid contribution jccc.
The reason is the same as I mentioned above, but the TV Yagi array has a very directional, ie narrow,  sensitivity on its polar plot so it is susceptible to smaller movement than a dipole antenna.

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Re: Would shaking an antenna affect its performance?
« Reply #11 on: 30/06/2015 07:07:05 »