Why can't antennas be made smaller and smaller?

[alan123hk]

Why can't antennas be made smaller and smaller?
Assuming we implement the following measures : -

1. use room temperature superconductor to eliminate the ohmic loss of antenna
2. reduce the thermal noise of antenna by lowering its temperature
3. Perfect impedance matching network by means of using superconductor and temperature control in the same way...etc

Then why can't we make antenna as small as we wish ?

 

[Drakkith]

Antennas only couple efficiently with an EM wave when the antenna is about 1/4 the size of the wavelength of the wave. So if your wifi router's antenna is significantly smaller than about 25 millimeters (1/4 the wavelength of a 2.5 GHz signal), then it can't send or receive wifi signals very well.

 

[Baluncore]

Assuming we implement the following measures : -
1. use room temperature superconductor to eliminate the ohmic loss of antenna
2. reduce the thermal noise of antenna by lowering its temperature
3. Perfect impedance matching network by means of using superconductor and temperature control in the same way...etc
Then why can't we make antenna as small as we wish ?

1. What is this mythical room temperature superconductor? Meanwhile, ohmic losses are not important with antennas. It is the feedline where ohmic losses occur when there is a high SWR with circulating line current.

2. The thermal noise of the antenna is not important. The noise of the first RF amplifier is very much greater. The background noise of the real world is also very significant. Communication signals being received are now often spread spectrum signals, with many other signals in the same band. Separation of signals is based on knowing the spreading sequence and using that to extract the wanted data stream.

3. It is all very well matching the line to the antenna feed-point. But if the antenna is going to radiate it must also be matched to the impedance of free space = 376.73 ohms. Small antennas have a simple dipole pattern without the directional gain possible from antennas that are longer or wider than half a wavelength. An antenna is really a transducer that interfaces the transmission line to space.

“as small as we wish ?”. We only need to make the antenna the same size as the device connected to the antenna. The housing can be the antenna.

 

[tech99]

Why can't antennas be made smaller and smaller?
Assuming we implement the following measures : -

1. use room temperature superconductor to eliminate the ohmic loss of antenna
2. reduce the thermal noise of antenna by lowering its temperature
3. Perfect impedance matching network by means of using superconductor and temperature control in the same way...etc

Then why can't we make antenna as small as we wish ?

In principle, an antenna can radiate all the transmitter power even when it is reduced in size below lambda/2. The problem comes that the radiation resistance rapidly becomes very small, whilst at the same time the reactance becomes large. This then requires the addition of an opposite reactance to cancel it. The added reactance then has ohmic losses. A further problem is that the Q of the system, equal to X/R, now becomes very high, leading to very narrow bandwidth and reducing the information rate. As a consequence, it is very difficult to obtain good efficiency with small antennas. However, for some applications it is acceptable. For example, MF reception using a ferrite rod, long wave transmission and some mobile phones.
Note the comment by baluncore about noise temperature. The noise temperature of an antenna is primarily set by its surroundings, or the volume of space it is looking into, and the overall noise limitation is often the receiver front end.
Impedance matching does not have to be perfect unless a feeder with high attenuation is used, for which case it will exacerbate the losses. Consider the simple case of a 20 volt battery having 10 Ohms internal resistance connected to (a) a 10 Ohm load and (b) a 20 Ohm load. Compare the power delivered in the two cases.

 

[berkeman]

Why can't a bell be made smaller and smaller?

http://www.stthomaschurchbells.com/images/gallery/asia-church-bells.jpg

 

[alan123hk]

Hello gentlemen, thank you all for your replies.
I would like to add the following comments.

I want to ponder about the fundamental limitations of small antennas so that certain assumptions have been made
For example, I don't even know the so-called room temperature superconductor really exists and just assumed this thing is available for the sake of convenience of discussion. I also know that it's no use to reduce the temperature of antenna if the background noise temperature is higher than that of the antenna itself,..etc.

Let's concentration on the discussion of the fundamental limitations of small antennas,.

I found an article on the web that said : -
"There is a fundamental theoretical limit to the bandwidth and radiation efficiency of electrically small antennas. Attempting to circumvent these theoretical limits can divert resources in an unproductive manner to tackle a problem which is insurmountable"

Can the limit of antenna efficiency be overcome by using zero resistance conductor if it really exists ?

It seems that the limit of bandwidth is irrelevant since we can still use an extremely small antenna to transmit a narrow band signal (or low bit rate in digital) according to the related formula which quantitatively describes the relation between the minimum Q and physical dimensions of the antenna, namely it is still useful for many applications even this limit of bandwidth is insurmountable.

Thanks and regards.

 

[Drakkith]

Can the limit of antenna efficiency be overcome by using zero resistance conductor if it really exists ?

Did Baluncore not already answer this in post #3?

It seems that the limit of bandwidth is irrelevant since we can still use an extremely small antenna to transmit a narrow band signal (or low bit rate in digital) according to the related formula, namely it is still useful for many applications even this limit of bandwidth is insurmountable.

Sorry, I'm not sure how this makes the bandwidth limit irrelevant. It would still be a very real limitation that must be accounted for and designed around.

 

[Baluncore]

"There is a fundamental theoretical limit to the bandwidth and radiation efficiency of electrically small antennas. Attempting to circumvent these theoretical limits can divert resources in an unproductive manner to tackle a problem which is insurmountable"

That is quite true. I think that after more than 100 years, electro-magneticians would have built smaller antennas if there was any advantage. You must compromise because the dimensions of the components needed to match a small antenna to a generator or feedline get bigger and more lossy as the antenna element gets smaller. An antenna can only capture energy incident on it's aperture area. If you halve the dimension of an aperture, you only receive one quarter of the power. That minimisation path is as rational and as terminal as is anorexia.

Can the limit of antenna efficiency be overcome by using zero resistance conductor if it really exists ?

No. The dielectric losses associated with the antenna are greater than the resistive losses. Perfect conductors are not much better than silver plated copper. But silver tarnishes and absorbs water, and then RF energy, so you might do better plating with gold instead of silver. But bare copper works well so why bother? If resistance was a problem we would use thicker copper wires.

It seems that the limit of bandwidth is irrelevant since we can still use an extremely small antenna to transmit a narrow band signal (or low bit rate in digital) according to the related formula which quantitatively describes the relation between the minimum Q and physical dimensions of the antenna, namely it is still useful for many applications even this limit of bandwidth is insurmountable.

The bandwidth problem rises from the need to match the abominable impedance of a small antenna. The requirement for data transmission is now for extremely wide bandwidth. The time it takes to transmit a bit through a 1 Hz wide channel is about one second. Would you be prepared to run 10 mega-channels and 10 million antennas to get 10 MBPS, still with a latency of 1 second. I could ping the next continent over an optic fibre and transmit a megabit either way in the time you took to synchronise your serial data.

The only way to get an advantage from smaller antennas is to use many elements as a directional phased array or to use a shorter wavelength. It is not surprising that we use optic fibres, each is extremely wide band and very small because the wavelength of light is small. Each fibre is in effect it's own universe, with the same incredibly wide bandwidth being used again for more data in every parallel strand of fibre.

 

[Windadct]

I'm not in any way an antenna man - BUT - an antenna is just a part of a communication system - and then it is only part of the Transmission ( or receiving) subsystem. Looking at an element without considering the impact or adaptation needed in other parts of the system - is practically irrelevant.

The fundamental principals that make an antenna work are pretty well understood having been a major field of study for over 100 years. Making a tiny antenna relative to its operating frequency makes IT inefficient, how can we make up for this? Is it worth the effort? Why do we need it smaller ? Yes - some significant technology jump could make a difference - but without some valid tech we are just dreaming about what could be.

As a side note - the link is an EE site, right, but today I am mostly in sales, and have done a good bit of marketing. In EE we frequently work with analogies (Like Berkman's bell) and this is one case where our technical definition of communication has proven to be vitally important to me professionally...(Input Message-transmission-medium-receiver-Output Message - THAT is the objective - right?) It can easily be seen to add feedback to this model - and again, the exact basic feedback model has proven to be very useful to me in many roles - But to close this wired LOOP - think about how feedback could HELP or allow you to make an antenna smaller ( this is already done by the way).