What is the SMALLEST antenna can be used

[Dderijken]

Dear All,
Can some Guru out there pls tell me:
What is the SMALLEST antenna can be used ?
lambda/8 ?
lamdba/4 ?
Thanks
Derijken

 

[eirp]

Hi, Dderijken
It depends on efficiency you need. Efficiency fall down rapidly if antenna is lower than lambda/4 (holds for classic monopole with ground plane) because small monopole/dipole antennas has small radiation resistance which makes it difficult to match (+losses of matcher). It's clear when you see simple equation for efficiency:

Rradating/(Rradiating+Rloss)

Another fact is the directivity of antenna which is connected with volume occuped by antenna. There exists rigorous equation which connects maximum available directivity of antenna and its occupied volume.
Eirp

 

[StoppTidigare]

reply to smallest antenna

For a metallic structure to be in resonance its length has to be n*lambda/2. This is a general resonance requirement in physics.

However if you use a metallic structure that is lambda/4 or less, you can use the ground(conductive) to provide for "rest" of the metallic structure.

However to a metallic structure does not need to be in resonanace to pick up signals compare with the simple radio you have at home.
That antenna is simply a bar, not a resonant structure.
You can pick up anything with a bar, the question is if the signal that you get is strong enough. The same reassoning must be agree for a transmitting antenna.
Does this sound reasonable to you ?
Kind regards,
StoppTidigare

 

[eirp]

For info how is connected directivity with occupied volume see attached file. :lol:
Regards,
Eirp

 

[pkn11425]

Types of antenna

WHat do you mean by asking the smallest antenna?!?
Halfwave dipole is just one type of realizable antenna
I think you should also say in which frequency range you are planning to work.
For example at very low frequency (up to kHz) you can use hertzian dipoles (very poor efficiency),and ferrite rods.
Magnetic loops and Yagi-Uder array antennas are good solutions in the VHF band. At microwaves you have planar structures such as lambda/2 * lambda/4 (parabolic)
Let me know more precisely what you are doing.
Bye, pkn11425

 

[Dderijken]

Dear All, :roll:

Thanks a lot to all of you .... :wink:
for all the explanation ....
I am appreciated that very much ....

With smallest antenna ... I means ,
for a fixed frequency (therefore wavelength also fixed)...
Due to the requirement of minimum dimension ...my question is:
what is the smallest antenna (dimension) I can take ?

I have read al your replies ... It seems that 1/4 lambda is the smallest
structure that I can take .... smaller than that ..it doesn't work .! right ? :?:

You said, an antenna radiate only when it is resonant ...
So, the antenna is reonant when it diension is n*lambda/2 ...

My next question (pls understand that I am not twisting you) is:
I understood that this rule hold for wire antenna ...
Is this RULE valid also for patch antenna ? f.e antena in mobile antenna ?


Thanks again for all the efort to open my mind.

If you have any other idea ... please let me know..


Derijken.

 

[Dderijken]

Hi :roll:

Can some Guru tell me how to derive the gain function

Gain =4*Pi*effective Area/ (lambda)^2

I have tried to fied in some books ..but failed to find the answer...
there must a simple way to derived or prove this ...

Thank you for time and help.

 

[pkn11425]

If you talk of patch antennas, I say you are right. I think your problem is another. You are asking if you can build an antenna smaller than lamda/4.
In general you cannot say this, because you should demnstrate that a solution for the Maxwell equation exists for any geometric structure.
In case of patch antenna or resonant cavity a solution exists according to certain dimensions (m/n * lambda). But this is just one geometric structure. It strongly depends on tghe structure. That's why I asked you of frequency. You cannot use ferrite rods antennas at uwaves.
TO convince you of the fact that lambda rule has notihng to do with a general principle you are asserting try following :


h**p://www.ee.surrey.ac.uk/Personal/D.Jefferies/aperture.html

It's about aperture antennas...

 

[Nobody]

Most of the handy phone use the invert-F antenna. U can layout it directly
in the PCB . The maximum lobe is tilt from the central direction.
Erricson use 2 back to back Invert-F to adjust the main lobe direction to
central direction. I have designed one and test in chamber . It's good both in H or V plant. U can use some of the commercial em simulator such as sonnet or HFSS to try ur detailed dimension out .
The mechnism of this antenna is doubt. It's soming like cross-field antenna (E X H) to radiate. I have tried one structre to shrink the length to near to its feed point ( ie. almost no metal extend ). But the simulation result in the significant real part of input impedance. It's still a unresolved secrect for me since my last project several years ago in designing PACS handset.

 

[eirp]

I found in some book how to derive equation for G (more exactly D
as directivity, because gain includes antenna effectivity) asked
by Dderijken. Is it so simple?..
Regards, Eirp

 

[zorro]

Hi Dderijken,

As stated above, size is limited in practice by efficiency and bandwidth.
I recommend this very interesting paper:

Hansen, R. C., "Fundamental Limitations in Antennas", Proceedings of the IEEE, vol. 69 N. 2, pp. 170-182, Feb. 1981.

Electrically small antennas, superdirectivity and other subjects are discussed.
Unfortunately, I have not the file but just a quite bad photocopy.
Perhaps you or some of our friends can get the file and share it.

Regards

Z

 

[Dderijken]

Dear All,

Thanks alot to all of you for sharing the kownledge.
I begin to understand the problem clearly now ...(thank to you) ..
:roll:

Regards,
D. Derijken,

 

[eirp]

I've found Hansen's paper mentioned by Zorro "Fundamental limitations in antennas" !
Here you have,
Eirp

 

[zorro]

Thanks a lot, Eirp!

This is (attached) a paper on the subject:

Harold A. Wheeler: "Small antennas", IEEE Trans. Antennas Propagat., vol. 23, pp. 462 - 469, July 1975

Regards

Z

 

[eirp]

Thanks too, Zorro!!!

You saved my time, because I was looking for this paper :lol:
It's a classical one and very interesting for our discussion!
Best regards,
Eirp

PS: I'm looking for
Wheeler, H.A. ,"Fundamental Limitations on Small Antennas"
Proc. IRE, 1947 (December), p. 1479-1488
also and I need complete info (vol./nr.) to get it.
Can you help?

 

[sick_man]

small antenna 15 years in radio tech me heeps

the smallest antenna is one that works........

i transmit on 10 mhz or so at 5 pm on a sat in november as per sked {known ham radio bands} but at 100 mhz it is flat as per atmospheric but by ground wave {by hirchy there must also be an inversion to gnd wave of this flux to carry asbouve 30 mhz by inverssion of e layer and above to below to down to gnd and strartospheric layer and lets all be honest the brain cavity of the averagfe adult resonates at 400 female to 650 mhz for a male 100 -200 for childern growing to 200 -400 - 500 -600 in 3 year steps 100 megs per year this ha sbeen known to me all l my life i have dreamed of a spititual direct connection to rf brilient no pieces when i sleeep anti grav amchines being zapping my thoughts like a sewing machine of life on our ohmic planet ffs wherte does it stop
so for hf ground wave is best for long distance above this it is line of sight

30 mhz bandwidth to divide up so 30mhz to blocks of 10


to make it easy 3 - 1 becouse 30 - 10 will be then 3 - 1 ratio
if you work it out by pi a reduction of 3.142537451933332779 too 1 will be best or concentric multiples of it to give the next resonant circle relative to the earths permiable flux desitity at each level of flux tare.....

as some area s but there is great research into

300,000,000 / by speed of light 300,000,000 for vhf and above

so ~ 1


therefore 1 meter long is optimal for hf as a small antenna and theory of yagi to apply read thory of yagi {japan direct translation} i think bigger is best but i know {by hirachy } that small {but big } is best
and apply this thory to higher freq then adjust resonace {by diameter and thickness of media} increase permiability using silver gold paltinun etc etc } to increase skin multiple metals plated one on top of the other to make resonant layers of resonace...{you heard it from me first} and xtal antenna may be a good choice{ones grown in space custom or prehaps very regular shape makes a good skin more complex research needed}}

also the feeder is very stupid
one thing i learn is place a unit near antenna and then drive {phones are good saturation becouse the antenna direct drive}

feeders are bad becouse they cost food to feed them also


so direct drive and amp at mast

etc etc


for shf then xtal of metal but very regular xtal geometry {not dispearison of shit grown xtal}
this slows down transmition radiatior reflex to the wave lenght that it is resononating {like a tare in a guitar string

i see someone asked about fratil antenns i send a bad reply but an informative one some moths ago
i think xtal fractiality to layersd resonators {like a pymarid {they used to be covered in polished granite slabe till they were stiped tell us a lesson inresonator the great pymarid is in antichamber a magnetics free zone relative to earth flux{harmonic 288 695 by bruce crathy {miss directed to a fiction book {but dead on acurate he sellls software to work stuff out called grid works}}}


hope this heps you but where you are in earth {referance } flux is important as the positions of the currents in a cake or the way tea leaves sit
each touch a subject that is real as any

antenna perfect swr is 1 .1 1 traveling wave to i standing wave {but why a perfect antenna has gain relative to 1 1 traveling wave = 300,000,000 transmited waves
same a light

hope my views stimulate minds

 

[plasma]

Hi All

I think that the simple way to reduce the
antenna dimentions is to use dialectric material
with high dialectric constant (Er) e.g. Er=10
that can help you to reduce the size of L/4 antenna.
like dialectrics in patch antennas
RGZ

 

[flatulent]

Another factor to consider is which government regulations you are designing to. Small remote control devices are limited to microwatts in many countries. In this case antenna inefficiency is sometimes an advantage.

 

[clk]

Where are the uploaded papers?

- Hansen, R. C., "Fundamental Limitations in Antennas", Proceedings of the IEEE, vol. 69 N. 2, pp. 170-182, Feb. 1981.
- Harold A. Wheeler: "Small antennas", IEEE Trans. Antennas Propagat., vol. 23, pp. 462 - 469, July 1975
-and Wheeler, H.A. ,"Fundamental Limitations on Small Antennas"
Proc. IRE, 1947 (December), p. 1479-1488

Thanks

 

[eirp]

Hi, Guys!

I haven't found exactly the requested papers (mess in my files), but I have some very similar which covers discussed topic.
Please see attached files.
Best regards,
Eirp