magnetic antenna

[antenna21]

Hello all,

I am looking for magnetic antenna for UHF. I am trying loop but with loop antenna I am facing tuning capacitors problems like at UHF we need very low pico farad capacitors.

Do you guys have any suggestions?

thanks

 

[Comcokid]

I don't have any references at hand but I believe slot, notch, and loop antennas are all H-field type antennas.

You might try a slot antenna.

 

[antenna21]

Thanks for the reply,

I am trying to design UHF magnetic antenna. do you have any suggestions at that frequency? by which antenna I can get the best performance possible?

thanks

 

[Comcokid]

Lets start back at the beginning. How about a little more information on your application and what you hope to achieve.

If your application needs a fixed omni-directional antenna, then you choose the antenna type depending if you are tring to communicate with vehicles or communicate with satellites. Issues like radiation angle and polarization might be important.

If you antenna need is for a small portable (i.e. it's orientation maybe somewhat random), then other antenna types may need to be considered for size, cost, or loading from nearby dielectrics (users head or body).

It doesn't appear that you are looking for directional types of antenna, but these are a whole other category with their own advantanges and limitations.

 

[Higgler]

More info is always better.
It seems very common to ask for something without giving details.

Maybe we can encourage an antenna template for this site, it'll cut thru the standard response of "please give more info" followed by a silent "grrr....";

1) Frequency start;
2) Frequency stop;
3) Approximate Size;
4) Approximate Gain; in dB
5) Polarization; Linear or Circular
5) Environmental; (temp, vibe, wind speed, etc.)
6) RF Power Handling; watts
7) Application; where and how is it used?


Seems like that covers most of the keys? Can anyone suggest changes/additions to this small subset (without getting too detailed).
Thanks,

kch

 

[antenna21]

Hi,

Sorry about the that.

Antenna details:
frequency: 900-950 MHZ approximately
size of the antenna: doesnot matter, as long as it is not in some meters range
approximate gain: I want as much gain as possible.
Polarization: linear is fine but circular is better.
environment: room temepareture, no vibration or fast moving.
RF power: 1-4 watts
application: portable application where I am trying to transmit and receive information upto 2 feet maximum

Once again sorry about that.

thaks

 

[Higgler]

Sounds like a monopole antenna is all that you require.
If you unit has a ground plane size in the 3" range, then all you need is a wire(small tube) above your ground plane that's just over 3 inches long (1/4 wavelength).
Only mechanical reasons would push you away from a simple monopole.

Sometimes a thin wire monopole doesn't provide enough bandwidth to cover your frequency range. Increased bandwidth is obtained by changing thin wire to thicker wire (tube), or a printed circuit with a conducting triangle shape standing upright off your groundplane (can achieve 10:1 bandwidth). With the thicker wire option, usually you don't need to inductively tune anything, just trimming the length is adequate.

You'll need at most a thick wire or a tube.


kch

 

[antenna21]

Hello,

Thanks for reply.

Can we use ordinary monopole as magnetic antenna? We already have small loop antennas at the other end, so I need to design some magnetic antenna which can comunicate with those small loop antennas. One more thing we are using magnetic antennas beacuse we might need to communicate through water.

thanks
sheshi

 

[Higgler]

If you already have one antenna, why don't you use the same thing on the other end?

It appears you believe that magnetic antennas are needed when in water? Why? and Where is this info from?

In what I have read about using antennas to communicate thru water, you should insulate the antenna from the water. Make a monopole encased in plastic. Your ground plane should probably not touch the water directly either if possible.

kch

 

[antenna21]

Hi,

The antennas we have are receiving antennas, I need to design Tx antenna. I think if I use small loop antenna for transmitting it is not going give me enough read range. May be I am wrong about that.

We are using magnetic antennas beacuse water can obserb some of the electric field if not the total field. So we thought using magnetic field going to help us in reducing this effect atleast for some extnet if not full. May be I am wrong here? if I am making a mistake, I really appreciate if you can explain little more about it.

thanks

 

[Higgler]

In a recent article in the Antennas and Propagation Society, they point out that loss is two parts when in water. A natural loss thru the water and antenna loading of the water.
Most antennas are resonant, current bounces back and forth on the wire. If you have water contacting that, it removes the resonance. Moving the water away from the antenna by insulating it with low loss material (plastic, castor oil, foam, air, etc.) can enable the antenna to be resonant again, a resonant antenna is efficient. Part of this explanation is my interpretation of the article, but I think it's correct.

Hence if you place a monopole in the center of a square ground plane and added a plastic rectangular or cube shaped housing around the circuit card and antenna, that would be nearly optimum. A monopole won't resonate when water touches it.
The reflection from air to the water is a problem, but at least your antenna isn't contacting the water. A wider bandwidth antenna would be the triangular shape circuit replacing the monopole.
One step better would be to use high dielectric from TransTech, (somewhat expensive) dielectric of 80 and create a dielectrically loaded antenna, probably with the triangle shaped or maybe a cone shaped antenna. This would eliminate the VSWR mismatch with the water.

It depends on just how much antenna performance you need. i.e. is two feet a difficult distance to send the signal to your receiver?

If this is in salt water, you could make a liquid/fresh water tunnel to talk between your transmitter and receiver. i.e just a bag filled with lower loss material approximately connecting the transmit and receive antennas. That would cut the loss down and easily transmit from one antenna to another.

What two items are connected by the rf energy.

kch

 

[Comcokid]

RF signals, or EM radiation is Electro-Magnetic with both a H field and E field regardless of what kind of antenna you use to launch in from your circuit into free space.

Signals at 900 MHz or even 2.4 GHz can penetrate a few inches of water. How much water depends upon your TX power / receiver sensitivity, and salinity of the water.

There are many types of antenna available for 900 to 925 MHz ISM band and many of these antenna will work very well outside of the ISM band as well.

The best single article I know about for antennas that may fit you need can be found at the RFM website or at the following link:

http://www.web-ee.com/primers/files/antenna.pdf

 

[antenna21]

Hello

Thanks for the information. I learn lot of new things by reading this board. Thanks again for your help.

For my antenna problem, those reaceving antennas (small loop antennas) we are using, I cant change those antennas. I dont have that freedom, I need to design TX antenna to send information to those. RX antenna we are using are really small loop ( magnetic loop antennas). I think if I use monopole antenna, I guess I maynot be able to reach 2feet range we are looking for. May be I am wrong here.

I asked about magnetic antennas because I thought the receving antenna is magnetic loop so it better to use magnetic TX antenna so that I can get maximum range possible.

Once again thanks for the help.

 

[BrianR]

I think you are on the wrong track here. The received power is a product of electric and magnetic fields. Submarines use electric field antennas at VLF as microwave has very poor penetration of water.

At 900MHz the electric field will fall off very rapidly from a loop and it will be useless for communications. You need gain and I suggest you consider a cubical quad which is a closed loop antenna and provides gain.

 

[RF0]

I think you are confusing everybody, you are talking about 2 feet range which is far field at 915 MHz. By reading carefully what I understand, you are looking for an antenna which has H-field dominant in near field and has high gain in the far field.

In the far field E/H is always constant. If you take dipole which is electric antenna in the near field so it has has high E and low H in the near field as you move towards far-field elctric field decreases and magnetic field increases to reach E/H equal to constant. magnetic antennas have opposite, H is high and E is low in the near field as you move towards far field E/H becomes constant again.

you need magnetic antenna with high gain in the far field, I think you can use slot antenna.

Probably some other guys can help you out with more options.

 

[antenna21]

Hi RF0,

What you said is right, I am looking for a magnetic antenna which has good gain.

thanks