[alancalverd]

No, a simple dipole aerial does not use standing waves.The signal strength is maximal when the dipole is a quarter wave long.

[Bored chemist]

No, a simple dipole aerial does not use standing waves.The signal strength is maximal when the dipole is a quarter wave long.

Why?

[alancalverd]

Because the potential difference between the poles is maximised.  https://en.wikipedia.org/wiki/Dipole_antenna

[Bored chemist]

Because the potential difference between the poles is maximised.

I didn't ask you to restate it, I asked you to explain it.

[wolfekeeper]

How are you going to measure the distance?

You don't have to in #3. You can measure wavelength two ways - dispersion angle from a diffraction grating or peak amplitude of a dipole aerial - with the two instruments effectively at right angles, simultaneously.

Uh huh. How you going to measure the pitch of the diffraction grating or the size of the aerial, using a one-way measurement?

There isn't actually any way to measure one-way speed of light. If you could do that, then the speed of light would not be 'c' and you'd be able to establish a universal rest frame. It's always 'c' in any measurement and you can't establish that.

[Bored chemist]

Uh huh. How you going to measure the pitch of the diffraction grating or the size of the aerial, using a one-way measurement?

Hang on.
If you take that view then you can't measure the one way speed flight, but you also can't measure the one way speed of a Ford Fiesta, or anything else.
You can't even measure its length.
So "measuring the speed of..." or even "measuring..." becomes meaningless.

It turns out that you can't measure anything except the speed of light, and you don't need to do that because it's c.

[wolfekeeper]

Essentially the whole point of relativity, which is generally not well articulated, is the observation that all possible measurements involve things travelling in two directions, so it doesn't matter if the speed of light is different in different directions, because matter will rearrange itself to cancel out any variation leaving you to measure the speed of light as 'c' in all cases.

Obviously that doesn't count for Ford Fiestas, because they go so much slower than the speed of light, although there is a fun puzzle which is somewhat related which goes:

"I drove a lap of a circuit in my Ford Fiesta at 60 mph, how fast do I have to go in the second lap to double my speed?"

[Bored chemist]

Obviously that doesn't count for Ford Fiestas, because they go so much slower than the speed of light

So does Alan with his tape measure and dipole.

[Bored chemist]

"I drove a lap of a circuit in my Ford Fiesta at 60 mph, how fast do I have to go in the second lap to double my speed?"

If you can do it, you can do it by staying still.

[evan_au]

a simple dipole aerial does not use standing waves.The signal strength is maximal when the dipole is a quarter wave long.

The dipole radiated output is maximised when there is a resonance between the incoming frequency and the length of the dipole*
- At resonance (a standing wave effect), the voltage on the dipole is maximised, and the far-field (radiated) electromagnetic field is the strongest (for a given input voltage).
- At the end of the dipole conductor, there is an abrupt change in impedance, which reflects electrical energy back towards the feed point (and back down the feed wire to the transmitter)
- Amateur radio enthusiasts use a Standing Wave Ratio meter to tell when the impedances are best matched between the transmitter and the antenna, with minimum reflected power from the antenna.
See: https://en.wikipedia.org/wiki/SWR_meter

*In calculating the size required for the dipole elements, you don't use c=299,792,458 m/s, but you apply an "end effect" factor of around 96%, as electromagnetic signals travel slower in conductors.
See: https://www.electronics-notes.com/articles/antennas-propagation/dipole-antenna/length-calculations-equation-formula.php
- This article also has a nice graph of current and voltage in a dipole, which reveals a snapshot of the standing wave...

[Bored chemist]

a simple dipole aerial does not use standing waves.The signal strength is maximal when the dipole is a quarter wave long.

The dipole radiated output is maximised when there is a resonance between the incoming frequency and the length of the dipole*
- At resonance (a standing wave effect), the voltage on the dipole is maximised, and the far-field (radiated) electromagnetic field is the strongest (for a given input voltage).
- At the end of the dipole conductor, there is an abrupt change in impedance, which reflects electrical energy back towards the feed point (and back down the feed wire to the transmitter)
- Amateur radio enthusiasts use a Standing Wave Ratio meter to tell when the impedances are best matched between the transmitter and the antenna, with minimum reflected power from the antenna.
See: https://en.wikipedia.org/wiki/SWR_meter

*In calculating the size required for the dipole elements, you don't use c=299,792,458 m/s, but you apply an "end effect" factor of around 96%, as electromagnetic signals travel slower in conductors.
See: https://www.electronics-notes.com/articles/antennas-propagation/dipole-antenna/length-calculations-equation-formula.php
- This article also has a nice graph of current and voltage in a dipole, which reveals a snapshot of the standing wave...

Spoilsport, ;-)
I was waiting for Alan to try to answer in a way that didn't use the word "resonance" (even though the page he cited mentions it a lot.).

It's pretty obvious that the only way the signal "knows" what the antenna impedance might be is that it goes to the end + bounces back.

Without that  there's no way to build up the voltage to a maximum.

[wolfekeeper]

FWIW Veritasium has a video about this exact topic:

[CliffordK]

FWIW Veritasium has a video about this exact topic:

It's Impossible

Just because something hasn't been done doesn't mean it can't be done.

When I was growing up there was a discussion about the limits of light microscopy, at around the wavelength of light (1/2?). 

Today there are multiple methods that break that limit.  Or, perhaps it is not as much breaking the limit, as finding ways around the limit.

[wolfekeeper]

"I drove a lap of a circuit in my Ford Fiesta at 60 mph, how fast do I have to go in the second lap to double my speed?"

If you can do it, you can do it by staying still.

You can't do it. In order to double your average speed, you'd have to go infinitely fast on the second lap.

[Bored chemist]

"I drove a lap of a circuit in my Ford Fiesta at 60 mph, how fast do I have to go in the second lap to double my speed?"

If you can do it, you can do it by staying still.

You can't do it. In order to double your average speed, you'd have to go infinitely fast on the second lap.

OK, so that's what the first clause of my answer
If you can do it, you can do it by staying still.
refers to.
But there's a cop-out.
What about if you can do it?
Is there a way it might not be a trick question requiring infinite speed?
And the answer is yes, sort of.
If the question was set by someone unsure of the  difference between speed and velocity.
It's a lap of the track, so the net distance traveled is zero (you end up in the same place as the start).
So the average velocity of the car is zero. (Whatever the speed might be).
And you can double that by standing still.

So, if the person who asks  can be talked into muddling velocity and speed, then you can do it.

BTW, did you really think I couldn't do basic arithmetic?

[wolfekeeper]

The answer is still that you can't do it. The term 'speed' is defined in the normal way, and is not simply the overall change in the position vector divided by the time.

[gem]

Hi all

Is there any method that you can think of that could achieve this? Have a stab at it.

Similar to the sodium lamp, I believe it would be possible to derive the speed of light using the red/blue shift due to interaction of light with a gravitational field, for example
 https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experimenthttp://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/gratim.html

[Bored chemist]

I'm just trying to unscramble those two links.

Hi all

Is there any method that you can think of that could achieve this? Have a stab at it.

Similar to the sodium lamp, I believe it would be possible to derive the speed of light using the red/blue shift due to interaction of light with a gravitational field, for example
 https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment

http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/gratim.html

[gem]

Opps thought I had left a gap,
Thank you BC

[jeffreyH (OP)]

Construct a device in the shape of an equilateral triangle. Place two lasers at corner A and targets at corners B and C. Have a second laser at corner B and a clock at corner C. Fire the two lasers at corner A simultaneously. One at corner B and the other at corner C. When the light is detected at corner C start the clock. When the light is detected at corner B fire the laser at corner B towards corner C. When the light from corner B is detected at corner C stop the clock. This device will work in any orientation to measure any discrepancy in the expected value for the speed of light.