Wireless routers vs. microwave ovens

[kevinisfrom]

Of course the polarizations need to be aligned. Fortunately, aligning the antennas in one polarization is a lot simpler than shifting the phase of the multiple Tx waveforms...

The problem with your proposal is much worse. Do you know how antenna array beam forming and beam steering works? You use destructive interference at the beam angles where you want little/no power, and constructive interference where you want the most power. But there is no magic going on here -- you still are transmitting full power out of all of the antennas in the array, and losing all of the power at the angles where you are using destructive interference to cancel the Tx waveform.

You do get some addition of the power in the main lobe(s), but you still are wasting all of the power at the angles where you are generating destructive interference. It's not like the power at those angles somehow magically gets transferred to the angles where you have constructive interference.

Antenna arrays are not about generating more power. They are about sacrificing power in order to generate a Tx power pattern that is more focused.

https://en.wikipedia.org/wiki/Antenna_array

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On destructive interference using phased arrays. Why not use horn antenna (or an array of horn anttennas), assuming you just want a forward pointing beam without steering capabilities, or just move the entire horn antenna array to steer?

 

[berkeman]

On destructive interference using phased arrays. Why not use horn antenna (or an array of horn anttennas), assuming you just want a forward pointing beam without steering capabilities, or just move the entire horn antenna array to steer?

Cost and wavelength are part of the decision process. Horns and parabolic antennas work best at higher frequencies, so for fixed narrow-beam transmission they would be good choices for those higher frequencies. To steer higher frequency beams you can move a whole array and/or use phase shifts to the elements to do the steering. Some interesting variations include this in-nose movable array that uses both mechanical movement and phase-shift beam steering:

https://www.translatorscafe.com/uni.../radar-unambiguous-range/?f=2&fu=kHz&mobile=1

 

[sophiecentaur]

On destructive interference using phased arrays. Why not use horn antenna (or an array of horn anttennas), assuming you just want a forward pointing beam without steering capabilities, or just move the entire horn antenna array to steer?

Despite the fact that they found the CMBR with a large 'hog horn' you don't see a lot of them these days.
This is getting further and further off topic. There are many different forms of radar, depending on the application. High Power of long distances, short wavelength or large antennae for good resolution, small for cheapness etc. etc.. When it's possible, it's an obvious advantage to have no moving parts but 360 degree rotation is hard but not impossible without a motor.

If you want an example of an over the top system for over the horizon radar then this link is one. It used HF bands and was an attempt at getting signals back from very distant targets (Soviet Bombers and Missiles!). It was a Cold War system. I visited the site once, in the early 80's (iirc) but the antenna was dismantled by then. There was a vast D shaped concrete base and traces of a radial array of HF Log Periodic antennae. Needless to say - no moving parts. I heard that one big problem was with 'egg insulators' which used to arc over. There were tales of fires in the rigging of passing ships too. (Lat and Long are 52.106043, 1.579825 on Orfordness Suffolk)

The equipment building was very James Bond with a communications room with traces of all the gear that a NATO defence control would be expected to have - not much more than a bare room at the time but very creepy. I actually designed an MF antenna which was erected on the same site for transmissions to Eastern Europe and I felt I was in really good company. I think my antenna is still there and you can see it on Google Maps. Two rows of three MF monopoles (à la Yagi).

 

[berkeman]

Even for phased arrays? I thought there were lower power RF sources for each antenna in the array? Like the giant phased arrays you see on battleships.

You need a single RF source for a phased array, since you do the beam forming and steering with variable phase shifts from that reference source. You may have amplifiers (with variable phase shift capability) for each antenna, depending on the size and overall power of the array.

 

[kevinisfrom]

You need a single RF source for a phased array, since you do the beam forming and steering with variable phase shifts from that reference source. You may have amplifiers (with variable phase shift capability) for each antenna, depending on the size and overall power of the array.

So a single RF source which generates an RF AC current, then gets amplified through amplifiers, and then that amplified AC current is radiated as RF EM radiation? Is the RF source just an electronic signal generator?

 

[kevinisfrom]

Cost and wavelength are part of the decision process. Horns and parabolic antennas work best at higher frequencies, so for fixed narrow-beam transmission they would be good choices for those higher frequencies. To steer higher frequency beams you can move a whole array and/or use phase shifts to the elements to do the steering. Some interesting variations include this in-nose movable array that uses both mechanical movement and phase-shift beam steering:

https://www.translatorscafe.com/uni.../radar-unambiguous-range/?f=2&fu=kHz&mobile=1

But since the phased array is 'wasting' the energy in areas of destructive interference, seems like the horn or parabolic antennas are better if you don't need active steering. Why are horn antennas better for high frequencies? And which frequencies are considered high?

Sorry for all the rabbit hole of questions, I should probably start another thread.

 

[kevinisfrom]

The problem with your proposal is much worse. Do you know how antenna array beam forming and beam steering works? You use destructive interference at the beam angles where you want little/no power, and constructive interference where you want the most power. But there is no magic going on here -- you still are transmitting full power out of all of the antennas in the array, and losing all of the power at the angles where you are using destructive interference to cancel the Tx waveform.

I was curious about phased arrays power amplifies and was watching a youtube video on it. They were showing the following slide:

The video mentioned that the Effective Isotropically Radiated Power is the gain per element * power per element * number elements^2

They said if you have an element that is a 100W power amplifier, and you have 100 elements, the EIRP is 1MW.

This made me think, if there was destructive interference and that power is wasted, how is it that the power radiated per element stacks so easily without considering all the loses from the destructive interference?

 

[sophiecentaur]

without considering all the loses from the destructive interference?

There are no "losses" For every Watt that doesn't turn up in one direction there will be an extra Watt put into the main beam.

 

[sophiecentaur]

Take the simple case of two dipoles, separated by half a wavelength and fed cophasally with equal amplitudes. In the line of centres, there will be cancellation and along the normal there will be twice the amplitude - corresponding to four times the power of a single element. Total power radiated over a sphere will be twice that of one dipole. These are basic ideas which you need to take on board first.

 

[kevinisfrom]

There are no "losses" For every Watt that doesn't turn up in one direction there will be an extra Watt put into the main beam.

Isn't this contrary to berkeman's post?

You use destructive interference at the beam angles where you want little/no power, and constructive interference where you want the most power. But there is no magic going on here -- you still are transmitting full power out of all of the antennas in the array, and losing all of the power at the angles where you are using destructive interference to cancel the Tx waveform.

You do get some addition of the power in the main lobe(s), but you still are wasting all of the power at the angles where you are generating destructive interference. It's not like the power at those angles somehow magically gets transferred to the angles where you have constructive interference.

Antenna arrays are not about generating more power. They are about sacrificing power in order to generate a Tx power pattern that is more focused.

 

[sophiecentaur]

@kevinisfrom this is a complicated topic and you have to get the basics first before you can draw the sort of conclusions you have been doing.
it’s easy to be misled about things when you read glib statements on line. It’s not surprising that EE is such an intense course compared with many others.
You need to read around more, before you try to make too many unjustified ‘connections’ between the isolated correct bits of information that you have.

 

[etotheipi]

How would the microwave change after it is absorbed in a material? I was thinking that the amplitude would be attenuated and that's where the transfer of energy occurs, but assuming a perfect dielectric, can you have absorption but no attenuation?

I know it's a little late, but I came across a very nice question about microwave ovens https://www.ipho2019.org.il/wp-content/uploads/2019/07/Theory-2-Question.pdf (and https://www.ipho2019.org.il/wp-content/uploads/2019/07/Theory-2-Solution.pdf). Part B deals with what I think you are interested in, and provides a simplified model for looking at dielectric absorption of microwave radiation at different depths (specifically, treating the water as having an average dipole moment per unit volume). Perhaps you will find it interesting

 

[sophiecentaur]

Isn't this contrary to berkeman's post?

We are saying the same thing. Energy is conserved so you cannot get any extra or any less, overall. If there is any resistive loss then the dissipated energy has to be included in the total/