Can a Tiny Antenna Transmit Long Wavelength EM Waves?

[lucas_]

Can you transmit em wave with antenna much smaller than its wavelength? For example. ELF antenna is very long. Can you make one small enough to fit in the pocket by the device constructing the long wavelength part by part?

 

[Drakkith]

Can you transmit em wave with antenna much smaller than its wavelength?

You can, but the power transmitted by the antenna in that wavelength range would be minuscule.

 

[lucas_]

You can, but the power transmitted by the antenna in that wavelength range would be minuscule.

How does one do that?

 

[Drakkith]

How does one do that?

Just have your transmitter set to the frequency you want to transmit on. If it goes low enough you can transmit on any frequency you desire. But like I said, you'll run into the issue that you're transmitting so little power that you can't detect the transmission. Especially if your receiving antenna is also very small compared to the carrier wave wavelength.

 

[lucas_]

Just have your transmitter set to the frequency you want to transmit on. If it goes low enough you can transmit on any frequency you desire. But like I said, you'll run into the issue that you're transmitting so little power that you can't detect the transmission. Especially if your receiving antenna is also very small compared to the carrier wave wavelength.

Any illustrations? This works for cell phones too, for example, one the size of a matchbox only with equally tiny antenna?

 

[marcusl]

It's not enough to shrink your antenna, you also need to match its impedance to that of your transmitter. This is usually 50 ohms.
To see instructions and illustrations, get a copy of the ARRL Antenna Handbook.

 

[Paul Colby]

Another thought is to try using a small antenna to couple to objects in its surroundings increasing the electrical size of the radiator. Thus the person carrying the antenna becomes part of a radiating structure for example. Also, for near field coupling where the receiver is less than a wavelength distant things may be easier if that is consistent with the application. You might try some designs out using a small signal generator along with a receiver of some kind.

 

[davenn]

Can you transmit em wave with antenna much smaller than its wavelength? For example. ELF antenna is very long. Can you make one small enough to fit in the pocket by the device constructing the long wavelength part by part?

Any antenna less than a 1/4 wavelength at the frequency of interest is going to be horrifically inefficient
and as @Drakkith said, output is going to be miniscule.

Any illustrations? This works for cell phones too, for example, one the size of a matchbox only with equally tiny antenna?

Yeah, but cellular phones work on UHF to low end microwave bands 900 MHz to ~ 2 GHz and 1/4 wave
antennas ( or multiples there of) are reasonably east to construct and produce very good efficiency.Dave

 

[Drakkith]

Any illustrations? This works for cell phones too, for example, one the size of a matchbox only with equally tiny antenna?

Try this book.

 

[marcusl]

OP indicates being a beginner ‘B’ while Balanis’s book is at advanced undergrad/grad level.

 

[berkeman]

Can you transmit em wave with antenna much smaller than its wavelength? For example. ELF antenna is very long. Can you make one small enough to fit in the pocket by the device constructing the long wavelength part by part?

Here's a related thread that may also help:

https://www.physicsforums.com/threads/does-the-size-of-an-antenna-matter.960025/#post-6088051

 

[lucas_]

My nephew was asking me last week whether if Antman was the size of Ant. He could still send radio waves in his radio or cell phone. so if the antenna in Antman pocket is merely 0.1mm long. Can he send a frequency that uses the antenna of normal size cellphone? What should he do to send a radio if he has ant level labs too? This is a serious question and just want to be able to answer it correctly. Thank you!

 

[Drakkith]

Can he send a frequency that uses the antenna of normal size cellphone?

Yes, but not very well. The signal would be extremely weak.

What should he do to send a radio if he has ant level labs too?

It's easy. If the antenna is significantly smaller than the wavelength of the signal, then the power radiated will be very weak. Cell phone signals are roughly a few dozen centimeters in wavelength, so if the antenna is less than about a quarter of that size it won't be able to efficiently transmit the signal.

 

[lucas_]

Yes, but not very well. The signal would be extremely weak.
It's easy. If the antenna is significantly smaller than the wavelength of the signal, then the power radiated will be very weak. Cell phone signals are roughly a few hundred centimeters in wavelength, so if the antenna is less than about a quarter of that size it won't be able to efficiently transmit the signal.

In short. Ant man won't be able to send any radio signal no matter how advanced is his technology at his Ant size labs. And the best he could do is use paper and walk on it with shoes of ink (in other words, just writing)?

 

[Drakkith]

In short. Ant man won't be able to send any radio signal no matter how advanced is his technology at his Ant size labs. And the best he could do is use paper and walk on it with shoes of ink (in other words, just writing)?

Well, he could simply transmit at a higher frequency. A 20 GHz signal would have a wavelength of about 1.5 cm, and an efficient antenna could be 1/4 that size, or about 3-4 mm. A 40 GHz signal would need an antenna of about 1.5-2 mm, and an 80 GHz signal would need an antenna about 0.75-1 mm long.

 

[lucas_]

Well, he could simply transmit at a higher frequency. A 20 GHz signal would have a wavelength of about 1.5 cm, and an efficient antenna could be 1/4 that size, or about 3-4 mm. A 40 GHz signal would need an antenna of about 1.5-2 mm, and an 80 GHz signal would need an antenna about 0.75-1 mm long.

Brilliant. Let us say he further decreased the size to Planck scale (in one of his movies and and let's ignore the quantum rules for now). What is the frequency and wavelength corresponding to the Planck scale antenna. If this signal was transmitted and you are one foot away from it. Can the signal hurt you?

 

[Drakkith]

Brilliant. Let us say he further decreased the size to Planck scale (in one of his movies and and let's ignore the quantum rules for now). What is the frequency and wavelength corresponding to the Planck scale antenna. If this signal was transmitted and you are one foot away from it

A Planck scale antenna would need to transmit in the far gamma-ray range to be efficient and you essentially cease to have a communications antenna as we know it.

Can the signal hurt you?

Yes, gamma rays can hurt you.

 

[lucas_]

A Planck scale antenna would need to transmit in the far gamma-ray range to be efficient and you essentially cease to have a communications antenna as we know it.
Yes, gamma rays can hurt you.

Radio waves being non-ionizing are not harmful. Why can't you transmit using higher frequency ionizing waves (like x-ray) that with similar all around reach like radio waves? Is it due to lack of power? But if there was sufficient source of power, could you send gamma waves as radio waves to the surrounding?

 

[davenn]

Is it due to lack of power?

NO, it's dangerous

 

[Drakkith]

Why can't you transmit using higher frequency ionizing waves (like x-ray) that with similar all around reach like radio waves?

You can. You just can't encode or receive receive information from them like you can with radio and microwaves. At such high frequencies the wave oscillates too quickly for electronics to use as a carrier wave. You would be forced to do something more like Morse code, where the radiation is pulsed on and off and information is encoded in the pattern of on-offs.

But if there was sufficient source of power, could you send gamma waves as radio waves to the surrounding?

You could send them out, but they would be gamma rays, not radio waves.

 

[lucas_]

NO, it's dangerous

Ok.
In ordinary antenna, the electrons move up and down which transmit EM field in all directions.

Why is that in x-rays, the electrons moving up and down in tinier antenna doesn't transmit x-ray waves in all directions.

Don't worry. Won't do it.

 

[Drakkith]

Why is that in x-rays, the electrons moving up and down in tinier antenna doesn't transmit x-ray waves in all directions.

You can't really make electrons oscillate back and forth that quickly. The frequency is simply too high.

 

[lucas_]

You can't really make electrons oscillate back and forth that quickly. The frequency is simply too high.

What is the maximum frequency where you can still move the electrons back and forth in the antenna? Is there any java applet how the threshold look like, where the electrons can no longer move fast enough, so it's stuck at a ceiling frequency?

 

[lucas_]

You can. You just can't encode or receive receive information from them like you can with radio and microwaves. At such high frequencies the wave oscillates too quickly for electronics to use as a carrier wave. You would be forced to do something more like Morse code, where the radiation is pulsed on and off and information is encoded in the pattern of on-offs.You could send them out, but they would be gamma rays, not radio waves.

You mentioned here it was possible to send gamma rays like radio waves in antenna to the surrounding (I assumed in circles), but in comment one message later you said it was not possible "You can't really make electrons oscillate back and forth that quickly. The frequency is simply too high.". Kindly clarify what is the case. Thank you!

 

[Drakkith]

What is the maximum frequency where you can still move the electrons back and forth in the antenna?

I believe it is somewhere in the far IR range. I'm not sure of the exact frequency.

You mentioned here it was possible to send gamma rays like radio waves in antenna to the surrounding (I assumed in circles), but in comment one message later you said it was not possible "You can't really make electrons oscillate back and forth that quickly. The frequency is simply too high.". Kindly clarify what is the case. Thank you!

You can send out gamma rays into the surrounding environment, just like you can do with radio waves. The fundamental principles behind their propagation are identical since they are both EM waves. The difference is in how they are generated. You can't generate gamma rays by oscillating electrons back and forth, so you can't generate them with a conventional antenna. You need something like nuclear decay.

 

[lucas_]

I believe it is somewhere in the far IR range. I'm not sure of the exact frequency.

So this is the reason why visible light can't be emitted by antenna?
no exception?

You can send out gamma rays into the surrounding environment, just like you can do with radio waves. The fundamental principles behind their propagation are identical since they are both EM waves. The difference is in how they are generated. You can't generate gamma rays by oscillating electrons back and forth, so you can't generate them with a conventional antenna. You need something like nuclear decay.

How about chest x-ray machine. How do they direct the x-ray? nuclear decay since antenna not possible?

 

[Drakkith]

So this is the reason why visible light can't be emitted by antenna?
no exception?

Yes, that's right. No exceptions as far as my limited understanding goes.

How about chest x-ray machine. How do they direct the x-ray? nuclear decay since antenna not possible?

I think they shoot electrons into a target plate, and the sudden deceleration of the electrons emits x-rays.

 

[davenn]

What is the maximum frequency where you can still move the electrons back and forth in the antenna?

In what would be called "conventional" antenna, say, a dipole of tube, wire or PCB track, the frequency
isn't overly high, around 60 - 70 GHz. That is around the frequency used by collision avoidance radar
in cars etc.

From there and up into the 100's of GHz and low THz, the RF emission is usually generated within
semiconductor devices eg. special diodes and radiated out through the body of the diode.

This is a very long way below IR frequencies.

Once you get to IR, Visible Light and UV light. The generation process for EM changes and it has to
do with injecting energy into the atoms of a material, say, the tungsten filament if a light globe.
This causes some of the electrons in the atoms to jump to higher energy levels and when the electron
returns to its normal energy level, it emits a photon ( packet of energy) of light. The frequency of the
emitted photon is dependent on the energy level it has.

As we move up into X-rays the generation process changes yet again.

Basically, as @Drakkith said, one way X-rays are generated is to fire a beam of electrons at a target.
The target in medical X-ray machines is usually a piece of tungsten. The electrons in the beam are
accelerated to very high speeds using very high voltages.

X-rays are generated when the beam of electrons give up some of their energy when they interact with
the electrons surrounding the nucleus of the tungsten atom or from within the nucleus. This is called

Bremsstrahlung emission ( go google it and learn more).​

​

Here's an old X-ray tube I photo'ed in a museum recently

see the similarities with the diagram above
the angled target etc

There is another X-ray emission process called K-shell emission

( go google​

it ​

and learn more).​

Gamma rays are just another step up from the energy levels of X-rays photons. There is no defined ​

boundary ​

between them. Tho many physicists use the generation method as the definition. ​

It's just as the X-ray photon energy level rises, we start calling them Gamma ​

rays instead.​

The difference in generation is that the gamma photons are generated from within the nucleus of ​

an unstable atom undergoing nuclear radioactive decay.​

I have a number of minerals in my rock and mineral collection that are Gamma Ray emitters​

They make a Geiger counter go crazy ​

​

​

Dave

 

[sophiecentaur]

This thread has drifted off topic (just a bit!) but there is something that needs to be cleared up about what's been written about the OP. For instance:

Any antenna less than a 1/4 wavelength at the frequency of interest is going to be horrifically inefficient
and as @Drakkith said, output is going to be miniscule.

It all depends on the type of antenna in question; not all antennae are dipoles. A (slightly less than)quarter wave monopole on a big ground plane or a (slightly less than) half wave dipole is easy to 'match' the output of a transmitter to. The process of radiating energy into space from an antenna presents itself as a Resistance (the only way that RF energy can actually be transferred). This resistance is about 377Ω and, just as you need to match any AC source to a load, to launch power efficiently, you need a 'Matching Transformer' of some sort. A half wave dipole is just one structure that acts like the right transformer and gives you a pure resistance of 73Ω at one frequency (it resonates at this particular length so there is no Reactance involved. The 73Ω is called the Radiation Resistance. At other frequencies there are extra Reactance components and the 73Ω goes down and down as the antenna is shorter. This reduction is pretty extreme and soon the resistance of the antenna and feeder structure becomes relevant (like high resistance mains leads) but there is also a big Reactive component and that can be 'tuned out' by a matching network. BUT it can only be done over a very limited bandwidth. If you really want to radiate from a small structure and if you are prepared to use a narrow bandwidth then there is no fundamental lower size limit. The BBC 200kHz Transmitting antenna at Droitwich is only 218m high for a wavelength of 1500m and it belts out 150kW [Edit not 15kW!] to cover most of the UK.

Mostly, we treat receiving and transmitting antennae as behaving the same way but the Ferrite Rod receiving aerial senses the Magnetic field and it can be a tiny fraction of the wavelength of the received signal. Transmitting is a bigger problem. This guy describes a small transmitting loop antenna. Radio Hams can be very inventive at times.

 

[davenn]

It all depends on the type of antenna in question; not all antennae are dipoles. A (slightly less than)quarter wave monopole on a big ground plane or a (slightly less than) half wave dipole is easy to 'match' the output of a transmitter to.

very slightly can be brought into line with some matching but the OP was taking about significantly
so, my and other comments were not out of line

The BBC 200kHz Transmitting antenna at Droitwich is only 218m high for a wavelength of 1500m and it belts out 150kW [Edit not 15kW!] to cover most of the UK.

Maybe you really meant that the TX is producing 150kW .. The ERP would be less

just did a quick calc. for 200kHz an electrical 1/4 wave is ~ 318m, so 218m isn't a big leap in faith

It's within the realms of "close enough", when noting my previous comments
A little matching will do the trick

 

[nsaspook]

So this is the reason why visible light can't be emitted by antenna?
no exception?

View attachment 246298How about chest x-ray machine. How do they direct the x-ray? nuclear decay since antenna not possible?

Visible light can be received by fairly conventional antennas at the nano-scale so if we can't emit visible light from an antenna it's not because of a physics limitation on EM antenna dipole radiation at that wavelength.
https://www.me.gatech.edu/featured_colarectenna

 

[DaveE]

NO, it's dangerous

And expensive.

 

[sophiecentaur]

A little matching will do the trick

And a massive amount of copper in the ground mat, of course. That would count as the antenna, I suppose.
The VLF transmitting station at Rugby is another example of a notionally 'small' antenna in terms of wavelength. It has a long history of different installations, from as low frequency as 16kHz. One of the arrays transmitted on 60kHz (5km wavelength) and the antenna consisted of a number of 260m masts. That MSF signal did go a long way! The actual size of the array would be open to interpretation but it certainly was hard work going lower than a quarter wave.
But these small arrays did actually work and provide(d) usable services.

 

[Mister T]

My nephew was asking me last week whether if Antman was the size of Ant. He could still send radio waves in his radio or cell phone.

Ha ha! He's ready to learn about scaling.

http://www.lr-web.dk/Lru/microsites...ojekt_5.9_Scaling_the_Physics_of_Lilliput.pdf