Impedance of a half-wave dipole antenna

[Averagesupernova]

Nowhere in this thread have I seen the word transducer. Any good transducer will absorb the electrical energy supplied to it and produce most of that energy as a different type. Think of the antenna as a transducer.
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If you know anything about antenna theory you will know that a dipole has maximum current and minimum voltage at the center and the opposite out on the ends. At what points we attach our transmission line to along the dipole antenna will determine the impedance the transmission line sees due to the ratio of voltage/current at that point. That's about as simple yet accurate as it can be put. I know enough about antennas to be dangerous. ;)

 

[sophiecentaur]

Any good transducer will absorb the electrical energy supplied to it and produce most of that energy as a different type.

.. . . . . .or the inverse, in the case of a receiving antenna or a microphone.

 

[xopek]

I can see why that could make you cross. However, 'we' have been trying to present you with a number of opinions about the meaning of Resistance.

Yes I understand that and that's helpful.

A piece of solid carbon with wires at each end is a very straightforward example of resistance which will present the same value of resistance over a wide range of frequencies and currents. That resistance value here is just a description of Energy Transfer; Electrical to Thermal. The same thing happens in a radiator of EM waves.

Yes, makes sense, I am clear on that. As long as energy is absorbed, it then gets converted to another form of energy, thermal, or EM waves, or whatever. And for the energy to be absorbed, the resistance has to be active, otherwise some of it will be reflected. It may or may not be a bad thing depending on the "device" in question.

There is another example which might help you and that is an electric motor. When you are lifting a load with an electric motor, you will be able to measure the V and the I and it will present a Resistance to the power supply. There is (ideally) no thermal dissipation in the motor (or in the wires of an antenna) and the resistance that's presented will vary with the load (or the gearing mechanism / motor speed). These are similar in essence to the matching network on an antenna.

But an electric motor is an inherently inductive device. So it is almost purely reactive. It converts electrical energy to mechanical energy via the induced EM field (Lorentz force). If I understand correctly, it is seen as a varying "resistance" because of the back EMF induced in the windings that opposes the current flow. The current increases under the load since the motor stalls and that reduces the back EMF. So it is basically a speed-dependent voltage source. Sounds nothing like a matched, resonant antenna that has a purely active resistance and therefore can absorb energy. But I agree, we can add a motor to our collection of "black boxes" with some V/I relationship (and also out of phase). In that sense it is not different than an antenna, I guess.

However, this is offtopic, but I am not clear on one thing: since a motor is an inductive load, most of the energy will be reflected back to the source, will it not? But hasn't that same energy been transferred to the mechanical energy that makes the motor rotate, via the Lorentz force? Has it not been "absorbed"? Does the Lorentz force do work? I don't think it does, since force seems to be applied orthogonally to the motion, but I may be thinking of the magnetic field as opposed to the electric field. I may need to ask in the physics section.

I think you are needing too much 'carbon' in your mental model of resistance.

Not at all, I stated multiple times that I understand that not everything is a "resistor" and gave some examples. But at the same time I am not afraid to open the black box to take a peek inside it. That way I can learn more about it.

 

[Averagesupernova]

No motor that is purely reactive will ever develop any torque. The more you mechanically load a motor the more real power is taken from the source. The motor example is a pretty good one.

 

[Baluncore]

And for the energy to be absorbed, the resistance has to be active, otherwise some of it will be reflected.

The "reactive" component of the impedance I understand, but "active" resistance, what is that?

But an electric motor is an inherently inductive device. So it is almost purely reactive.

Not so. The motor has a magnetising inductance, but the loaded motor appears resistive and transmits real power to the shaft.

However, this is offtopic, but I am not clear on one thing: since a motor is an inductive load, most of the energy will be reflected back to the source, will it not?

Obviously not. The motor equivalent circuit is a winding inductance, in parallel with a resistance. The current in the reactive inductance circulates energy, it does not transmit it. The real transmitted energy is in phase.

 

[hutchphd]

Does the Lorentz force do work?

The Lorentz Force certainly does work. The magnetic portion $\vec v \times \vec B$ does not. But there are time varying magnetic fluxes hence rortational electric fields that most certainly do. That is how motors work. Did you look at Feynman and the LC coax to infinity? All "resistive" real loads.

 

[xopek]

The "reactive" component of the impedance I understand, but "active" resistance, what is that?

Active resistance, hmm sounds like something from criminal code.
But I am sure you know that I meant "real" resistance R. "Active" is by an analogy with "active power" as opposed to reactive power, I guess. Can't you say that? The true or real, or actual or active power.

Not so. The motor has a magnetising inductance, but the loaded motor appears resistive and transmits real power to the shaft.

Obviously not. The motor equivalent circuit is a winding inductance, in parallel with a resistance. The current in the reactive inductance circulates energy, it does not transmit it. The real transmitted energy is in phase.

Thanks. I may be thinking of an unloaded motor with a low power factor. That reminds me of a transformer in an idle mode that only has a low magnetizing current flowing in the primary.

 

[Baluncore]

Active resistance, hmm sounds like something from criminal code.
But I am sure you know that I meant "real" resistance R. "Active" is by an analogy with "active power" as opposed to reactive power, I guess. Can't you say that? The true or real, or actual or active power.

I do not understand what you mean by active resistance or active power.

You can say what you want, but it will just confuse others and yourself. If you arbitrarily assign words and meanings to things that already have different names and meanings, then you will be living in a world by yourself. Meaningful discussion is not possible without well-defined terms.

The opposite of active is passive.
https://en.wikipedia.org/wiki/Passivity_(engineering)
An active circuit demonstrates some form of power gain. You are suggesting that a passive component, like a resistor, could have some form of power gain. That is self-contradictory.

 

[sophiecentaur]

But an electric motor is an inherently inductive device.

No - as discussed above.

But I am sure you know that I meant "real" resistance R.

I don't and I don't think you do either - or at least you have not actually identified any difference between 'your' real resistance and the resistance which appears when power is transferred.
You should pause and ask yourself could you have been the first person in history to have spotted that there are two 'kinds of' resistance and everyone else has got it wrong. Alternatively, you could just have failed to see the fact that those kinds of resistance do not differ in any way - once you dig deep enough. They are equivalent.
Consider a form of Turing test with resistors instead of a computer. Take a black box with two terminals, connected to an unknown device. It measures as a pure resistance, at a particular frequency. How do you know anything more than that about what's inside, to tell you if it's a piece of carbon or nicely tuned dipole? What more does your measured V/I tell you that could reveal the 'true' nature of that resistance you measured or where and how the power is being dissipated?

 

[edrienbi]

73 + j42 means it has an active resistance plus reactance. But this "active" resistance is not a DC resistance, right? We can't just measure 73 ohm with a multimeter since the circuit is open. What closes it then? Let's say the reactive part is zero. How do we measure 73 ohm? By measuring current and voltage? But what causes resistance and what kind of resistance is that since it is only measurable as a ratio of AC current to AC voltage but cannot be measured as a DC resistance?

The multimeter is using a DC source (Battery) to apply its voltage on the load to be measured and have a current through it then by measuring the current effect on its sensor to decide the value of load resistance .But for the antenna or any device which used in high frequency applications as antenna the reactive part effect will appear and the resistive part response with HF input will be different as DC input , so the measurement of such impedance needs AC measuring equipment to apply an AC input to have the ac response of the load.

 

[anorlunda]

Just to flog a dead horse. A complex impedance can also be described as having a magnitude and angle. It is not necessary to say that it has real and imaginary parts.

@xopek , it sounds like you dug a mental hole trying to visualize complicated (I won't say complex) stuff in your head, and by taking the meaning of words like resistance too literally.

 

[xopek]

I don't and I don't think you do either - or at least you have not actually identified any difference between 'your' real resistance and the resistance which appears when power is transferred.

I think it is a reading comprehension issue. I've never said there were "two types of resistances when power is transferred". I was referring to R in R + jX. R is called resistance. It is not "my real resistance". It is THE resistance when power is transferred. That's all. What I actually said/implied was that the term impedance had two parts: the real part (I called it "active" unfortunately which apparently threw you off) and the imaginary part (the reactance, when power is NOT transferred).

You should pause and ask yourself could you have been the first person in history to have spotted that there are two 'kinds of' resistance and everyone else has got it wrong.

Again, I've never said there were two kinds of resistance. You made this up.

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Consider a form of Turing test with resistors instead of a computer. Take a black box with two terminals, connected to an unknown device. It measures as a pure resistance, at a particular frequency.

You said "pure resistance". I might as well ask what do you mean by that? That's the same thing I meant by "real resistance". Which I accidentally called "active resistance" and then corrected myself and that made you think I thought there were "two kinds of resistance".

How do you know anything more than that about what's inside, to tell you if it's a piece of carbon or nicely tuned dipole?

Exactly. The impedance of a "nicely tuned dipole" doesn't have an imaginary/reactive part. It is not a capacitive or reactive load.

What more does your measured V/I tell you that could reveal the 'true' nature of that resistance you measured or where and how the power is being dissipated?

If power is being dissipated/transferred, then I know it is a purely resistive load. Not capacitive or inductive.
Would you at least agree there is a difference between "pure resistance" (your term) and reactance?
Can we call it "two kinds"?

Yes, yes, I understand it is one and the same resistance when power is being dissipated. There are no two kinds of resistance.

Maybe I should make this my signature since no matter what I say, I've been pigeonholed by the experts as someone who thinks that "there are two kinds of resistance" :)

 

[sophiecentaur]

Would you at least agree there is a difference between "pure resistance" (your term) and reactance?
Can we call it "two kinds"?

You have brought in a totally new idea here. Impedance is a complex quantity (R + jX). R and X are in the same units. What could you mean by "two kinds?"
x,y co ordinates in space are distinct but they are not 'two kinds' .

By "pure Resistance", I simply meant R + j0. However, every component has a finite value of X.

I seriously think that you are now trying to avoid being wrong by changing the goal posts. Just go and study the subject again and come to terms with the system.

 

[hutchphd]

And to further flog that horse,

so the measurement of such impedance needs AC measuring equipment to apply an AC input to have the ac response of the load.

For the original question I would ask what is the (half) wavelength for a DC source? That's a very big antenna. For the coax LC line the answer is clearer and the measurement would be, for that geometry, 47 ohm. That is why you terminate such a cable with 50ohm resister.

Yes, yes, I understand it is one and the same resistance when power is being dissipated. There are no two kinds of resistance.

The original question

We can't just measure 73 ohm with a multimeter since the circuit is open. What closes it then? Let's say the reactive part is zero. How do we measure 73 ohm? By measuring current and voltage? But what causes resistance and what kind of resistance is that

seems to imply otherwise. Hope this has helped. Read the Feynman chapter again...you always learn something

 

[xopek]

I do not understand what you mean by active resistance or active power.

Wow. Seriously? Active power is a textbook term. Active, reactive, apparent, and complex power.
Active power: power is dissipated/transfer of energy occurs. It does work.

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

You can say what you want, but it will just confuse others and yourself. If you arbitrarily assign words and meanings to things that already have different names and meanings,

I don't arbitrary assign words and meanings.

Meaningful discussion is not possible

Yeah I noticed that.

without well-defined terms.

The opposite of active is passive.

Words can have many meanings. If you want to play the word game, the opposite of active is also reactive.

https://en.wikipedia.org/wiki/Passivity_(engineering)
An active circuit demonstrates some form of power gain.

The word "active" in "active circuit" is not the same as active" in "active power". I didn't come up with this term. Active power is the opposite of reactive power.

You are suggesting that a passive component, like a resistor, could have some form of power gain. That is self-contradictory.

Sorry the word active confused you so much. Perhaps you are more used to the term "real power" as the opposite of "reactive power". And that's all because I accidentally said "active resistance" instead of "real resistance" R.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

Words can have many meanings. If you want to play the word game, the opposite of active is also reactive.

No, we don't like to play word games. We try to use standard terminology as much as possible, especially when trying to help inexperienced folks learn new concepts that they are asking about.

And that's all because I accidentally said "active resistance" instead of "real resistance" R.

Yes, that was one source of the confusion. There were also others.

73 + j42 means it has an active resistance plus reactance. But this "active" resistance is not a DC resistance, right? We can't just measure 73 ohm with a multimeter since the circuit is open. What closes it then? Let's say the reactive part is zero. How do we measure 73 ohm? By measuring current and voltage? But what causes resistance and what kind of resistance is that since it is only measurable as a ratio of AC current to AC voltage but cannot be measured as a DC resistance?

In any case, I think we've addressed your questions from your Original Post, so I'll close this thread now. If you have more questions about how antennas work or about other topics, please do a forum search to see if you can find the answer to your questions, and if not feel free to start a new thread. It's always best to post links to the reading you have been doing when starting a new thread in the technical forums.

Thanks to all who contributed in this thread.