Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Ask for help: An electrically small antenna

Status
Not open for further replies.

Avia

Newbie level 5
Joined
Jul 20, 2006
Messages
10
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,346
electrically small structure

Recently, I engaged in designing an electrically small antenna with input impedance of 50Ω and the working frequency is 30MHz.

The length of the antenna is expected to be about 30cm, I tried TEM horn but found the S11 is too poor.

Is there anybody who can provide me some proper structure or give me some advice?


Thank you very much.
 

G4BCH

Full Member level 6
Joined
May 10, 2006
Messages
365
Helped
123
Reputation
246
Reaction score
99
Trophy points
1,308
Activity points
4,579
30 mhz small antenna

Hello Avia
Designing electrically small antennas is a difficult problem and there a a number of compromises that are going to have to be made.
You say you want a 50ohm input impedance. Over what bandwidth do you need to have a good match? If it narrow band then you may be able to come up with a matching network to match your small whip antenna, but it will be lossy, that will help increase the bandwidth.

If the antenna does not have to be a wire then a small loop may be worth considering. Again it will be narrowband and require large tuning components if any significant power is to be used.

I suggests looking at mobile CB antennas ( 27MHz )
or ' magnetic loop' anttennas.
A controversial antenna is the crossfield antenna. It is supposed to work by synthesising the EM wave and can be made very small. Some say they work, others are not so sure that they work the way they are supposed to.

Whatever antenna you choose it will be narrow band and affected by whatever surrounds it. If it is a whip will be affected very significantly by the ground.

If efficiency is not a problem and wide band instantanious match is important then load the antenna with 50 ohms and live with the loss.

Hope that tis helps a little, but as I say small antenna design is difficult, just look at what is squeezed into a cell phone and scale that to 30MHz.

Peter
 

jallem

Advanced Member level 1
Joined
Jul 23, 2005
Messages
452
Helped
72
Reputation
144
Reaction score
10
Trophy points
1,298
Activity points
4,993
Some design can be electrically small and still have
a 50 ohm impedance. Of course there is a limit and
I would estimate that you are way down the limit.
For 30 MHz the wavelength is 10 m and the dimensions
of you desired antenna is 30 cm, that is 33 times.
when the structure is too small then the impedance
also is small. And that is when the dificult resides,
to match 50 ohm to 0.5 ohm for example.
 

Avia

Newbie level 5
Joined
Jul 20, 2006
Messages
10
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,346
You helped me a lot, Peter, I really appreciated it.

The antenna is narrowband and expected to work at 30MHz, so the efficiency is more important.

I will looking for the antenna you provide me and consider which will be the better one.

Added after 7 minutes:

Some design can be electrically small and still have
a 50 ohm impedance.



Jallem, can you provide me some more detail information?(e.g. papers)
 

Avia

Newbie level 5
Joined
Jul 20, 2006
Messages
10
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,346
Another question,

Can I simulate an antenna by using HFSS when its working frequency is some lower.(e.g.30MHz)

I mean that when the work frequency is 30MHz, whether the simulation accuracy of HFSS still can be assured?


Thanks a lot
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top