maanothing

Joined Sep 27, 2019
2
Is it possible that a radio wave could be direct current only? I know that dc fields exist , but could a radio be recitifed at it's output to produce a signal with no negative or inverting component to the em wave?

cheers

Joined Jan 15, 2015
4,906
No, not based on the definition of a radio wave.
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz to as low as 30 hertz. At 300 GHz, the corresponding wavelength is 1 mm, and at 30 Hz is 10,000 km. Wikipedia

Another definition is:
Radio wave definition, an electromagnetic wave having a wavelength between 1 millimeter and 30000 meters, or a frequency between 10 kilohertz and 300000 ...

Ron

crutschow

Joined Mar 14, 2008
24,111
An EM (radio) wave requires a change in the current/voltage versus time generating the wave.
DC does not do that.

OBW0549

Joined Mar 2, 2015
3,147
Is it possible that a radio wave could be direct current only?
NO.

Glenn Holland

Joined Dec 26, 2014
698
On the subject of very long electromagnetic waves, there is a phenomenon called "Alfven Waves" which are composed of a magnetic field embedded in a plasma.

These waves are commonly found in stars like the Sun and their wavelength is often thousands of miles. Accordingly, they can have a frequency of only one cycle in many hours and sometimes one cycle in a couple of days. Alfven waves are commonly associated with the formation of sunspots and the triggering of solar flares.

Mathematical models indicate the wavelength essentially approaches that of a D.C. signal as the frequency approaches zero. That's as close as you're going to get to producing a "D.C." radio wave.

Mark Hughes

Joined Jun 14, 2016
404

DickCappels

Joined Aug 21, 2008
6,096
I've spent quite a few months experimenting with small antennas connected to the output pins of microcontrollers to transmit the edges of pulses up to several meters. These were strings of pulses at rates ranging up to a few tens of kHz. They were pulsating DC when then went into the antenna, but they were AC when the arrived at the receiver because the coupling was capacitive and the DC component was lost. You can transmit "almost DC".

jpanhalt

Joined Jan 18, 2008
8,493
@DickCappels
I hope you will tell us more when you are ready.

John

MrAl

Joined Jun 17, 2014
6,968
Hello,

[The following is based on a straight wire antenna. See last paragraph for a circular antenna]

To obtain radiation from an antenna you must accelerate an electron within that antenna. What this means in terms of DC current is the current level itself must change. This is a 'double' change not just a single change. However, most applications use signals that have this property naturally because the current level itself must change in order to maintain a true AC signal.
So normal AC signals like sine waves and square waves cause radiation because the electrons are either constantly being accelerated (and decelerated) or they are done so abruptly.

This means that since a DC current does not change speed at all it can not radiate (in a straight antenna). A sine signal has constant change of change (acceleration) so it can radiate constantly. A square wave has abrupt change of change so it can radiate also.

To look at this another way, an electron at rest in an antenna does not radiate. An electron that has constant speed (velocity) also can not radiate. It's only when the electron speed changes (acceleration) does the electron radiate.
DC current in basic theory is comprised of electrons that move with constant speed, thus it does not radiate. Digging a little deeper, since the electrons are actually jumping around we might get tiny bits of radiation but the directions will be random and probably mostly cancel out although there will be noise. Since the noise is low level and uncontrollable, it is of little use in a transmitting application.

To look at this in terms of units of current, if you have a constant 1 amp flowing in an antenna there will be no useful radiation. If you suddenly change that to 2 amps, you will get radiation. If you then maintain the current at 2 amps you will not get anymore useful radiation. If you drop it back down to 1 amps you will again get useful radiation for a short time. If you do this repeatedly (1 amp then 2 amps then 1 amp, etc.) then you will get a constant stream of radiation.

Here is something to think about though...
If we have a round (circle) antenna 1 meter in diameter and pump a 1 amp DC current though it, do we get radiation?
The electrons are traveling at constant speed, however, they are also turning constantly and that means that they have angular acceleration.
My guess is that there could be radiation however it is probably very tiny because of the size of the electrons relative to the diameter of the antenna. It's like a race car riding around a track a million miles in diameter...the track looks mostly straight to the car so there is very little angular acceleration and thus in the case of the electron, very little radiation.

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MrAl

Joined Jun 17, 2014
6,968
I've spent quite a few months experimenting with small antennas connected to the output pins of microcontrollers to transmit the edges of pulses up to several meters. These were strings of pulses at rates ranging up to a few tens of kHz. They were pulsating DC when then went into the antenna, but they were AC when the arrived at the receiver because the coupling was capacitive and the DC component was lost. You can transmit "almost DC".
Hi,

I did something similar back in the 1970's but used TTL gates to generate the RF signal.
The idea was to generate a sub harmonic of an unused FM radio station so that the harmonics would reach that high (like 90MHz or something from a 10MHz square wave, utilizing the 9th harmonic).
I was able to get out a mile or two maybe three. What i used to do was set up the experiment, then ride out in the car with the FM radio on and see how far i could go.
The antenna was nothing more than a wire.
I look at this as a sort of success because of the distance obtained and the very low power used to transmit (no transistors just a TTL gate and TTL crystal oscillator).

This leads me to believe that with a 5v uC we should get similar results with more controllability.

SLK001

Joined Nov 29, 2011
1,543
I've spent quite a few months experimenting with small antennas connected to the output pins of microcontrollers to transmit the edges of pulses up to several meters. These were strings of pulses at rates ranging up to a few tens of kHz. They were pulsating DC when then went into the antenna, but they were AC when the arrived at the receiver because the coupling was capacitive and the DC component was lost. You can transmit "almost DC".
"Pulsating DC" is not DC. The actual spectrum of "pulsating DC" is humongous.

SLK001

Joined Nov 29, 2011
1,543
'Nuff said!

jpanhalt

Joined Jan 18, 2008
8,493
@MrAl said:
Here is something to think about though...
If we have a round (circle) antenna 1 meter in diameter and pump a 1 amp DC current though it, do we get radiation?
The electrons are traveling at constant speed, however, they are also turning constantly and that means that they have angular acceleration.
My guess is that there could be radiation however it is probably very tiny because of the size of the electrons relative to the diameter of the antenna.
It would seem that if radiated waves depended on acceleration of electrons traveling as a continuous (non-pulsating) current in a circular piece of wire that drift velocity would be important.

Unfortunately, that velocity is quite slow (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html) compared to reversals caused by AC of a few kHz (or sharp edges).

Using that calculator for drift velocity, 10A in 24 awg (0.81 mm dia) copper wire has a drift velocity of 1.425 ^10-3 m/s. A ring with an effective path length* of 1.425 mm and would produce 2 reversals/loop, i.e., each second. Acceleration (v^2/r) = 5 mm/sec^2. That would require a very small loop for anything that was not mHz.**

*I am assuming the mean path that should be used would be something like the magnetic path length of a toroid and not the simple average of OD and ID x π. The formula for magnetic path length is here: http://www.mhw-intl.com/assets/CSC/CSC Design Formulas 2011.pdf However, regardless of the model, the path would be short.

**Assume the wire is 0.8 mm diameter and one can make a loop with an OD of 2.4 mm and ID of 0.8 mm. (I am ignoring deformation of the wire as it is formed.) The mean path is 4.58 mm. A 1.425 mm mean path would require an ID on the order of 45 microns.

Janis59

Joined Aug 21, 2017
970
The question may have re-wording to - how much time will take to walk fro NY to LA with walking speed zero point zero zero. The answer is - zero bits in eternity.
The DC fields like H field or E field may exist separately, however they are attenuating in centimeters not a kilometers, this is first, but second, if any transfer a bit per second You havent a DC but modulated signal with side frequencies around a carrier what are AC. Transfer a 30 bps and get a 30 Hz AC.

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nsaspook

Joined Aug 27, 2009
6,720
It would seem that if radiated waves depended on acceleration of electrons traveling as a continuous (non-pulsating) current in a circular piece of wire that drift velocity would be important.

Unfortunately, that velocity is quite slow (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html) compared to reversals caused by AC of a few kHz (or sharp edges).

Using that calculator for drift velocity, 10A in 24 awg (0.81 mm dia) copper wire has a drift velocity of 1.425 ^10-3 m/s. A ring with an effective path length* of 1.425 mm and would produce 2 reversals/loop, i.e., each second. Acceleration (v^2/r) = 5 mm/sec^2. That would require a very small loop for anything that was not mHz.**

*I am assuming the mean path that should be used would be something like the magnetic path length of a toroid and not the simple average of OD and ID x π. The formula for magnetic path length is here: http://www.mhw-intl.com/assets/CSC/CSC Design Formulas 2011.pdf However, regardless of the model, the path would be short.

**Assume the wire is 0.8 mm diameter and one can make a loop with an OD of 2.4 mm and ID of 0.8 mm. (I am ignoring deformation of the wire as it is formed.) The mean path is 4.58 mm. A 1.425 mm mean path would require an ID on the order of 45 microns.
Very good points about using wire. With accelerated charged in vacuum the effect of Cyclotron radiation is detectable down to one electron.
https://physics.aps.org/articles/v8/36

Janis59

Joined Aug 21, 2017
970
Cyclotron must be installed for job of sensor??

Janis59

Joined Aug 21, 2017
970
Actually, even if at Your country "Lenin`s" bulbs are still in use and not condemned, even for them may use a ferrite ring, where in middle is going the wire and thin wire makes some 60-100 turns controlled by opamp, making a signal proportional to the current. No current --> no signal. So, after stays Schmitt trigger and voila!

jpanhalt

Joined Jan 18, 2008
8,493
@nsaspook
Yes, well aware of that. Brought back memories of kids (like myself) building accelerators for science fair.

MrAl

Joined Jun 17, 2014
6,968
It would seem that if radiated waves depended on acceleration of electrons traveling as a continuous (non-pulsating) current in a circular piece of wire that drift velocity would be important.

Unfortunately, that velocity is quite slow (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html) compared to reversals caused by AC of a few kHz (or sharp edges).

Using that calculator for drift velocity, 10A in 24 awg (0.81 mm dia) copper wire has a drift velocity of 1.425 ^10-3 m/s. A ring with an effective path length* of 1.425 mm and would produce 2 reversals/loop, i.e., each second. Acceleration (v^2/r) = 5 mm/sec^2. That would require a very small loop for anything that was not mHz.**

*I am assuming the mean path that should be used would be something like the magnetic path length of a toroid and not the simple average of OD and ID x π. The formula for magnetic path length is here: http://www.mhw-intl.com/assets/CSC/CSC Design Formulas 2011.pdf However, regardless of the model, the path would be short.

**Assume the wire is 0.8 mm diameter and one can make a loop with an OD of 2.4 mm and ID of 0.8 mm. (I am ignoring deformation of the wire as it is formed.) The mean path is 4.58 mm. A 1.425 mm mean path would require an ID on the order of 45 microns.
Yes and we see why DC is not used for RF transmission
It's ok with me if you want to go four Euclidean dimensions theoretically and just quote the diameter of the loop and of the wire. We can see right away that the radiation has got to be so small it wont be of any use, at least not by any applications i can think of at the moment.

So the drift velocity in said wire is only 1.4mm per second? Gee they sure are slow pokes.
Dont think we'll be seeing any radiation anytime soon.
Interesting though that when they reverse that could be a big change in acceleration and if it is done enough times per second we get plenty of radiation.
Just thinking a bit further, if the small loop of wire does radiate anything it would have to be detected as a very low frequency wave. Depending on the placement of the receiving antenna it may not even be a sine wave but in the right place it may approach that of a sine wave. That would mean we can get a very low frequency sine wave (or something like it) just by passing DC though a loop of wire.
How about if we go to a much finer diameter wire?
This is interesting to think about even if we would ever use it, but then you never know, with today's small circuits getting smaller and smaller maybe a tiny loop on a chip somewhere could communicate with another chip just by using a DC current. It would turn into a VCO i guess as the DC current would change the frequency.
Just thinking a bit more for now.

nsaspook

Joined Aug 27, 2009
6,720
It's not the electrons reversing that matter for radiation, it's the electric field of their charge, the finite speed of information and the retardation between cause and effect of electric field changes across space.

https://www.feynmanlectures.caltech.edu/II_26.html

If nothing changes like in a steady state DC field, then there is no cause and effect to transmit to the universe via radiation.

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