900 mhz antenna - spec / ideas needed

[ghb]

antenna 900 mhz vertical dipole

rules

vertical polarization
monopole style pattern
planar 2.5" x 3.5" max
tuning done on same plane as antenna

so far i have not been anywhere near this goal
i have no polarization purity
or monopole pattern for that matter

ive tried several loop styles
with different tuning methods

i realize ground plane size is a severe short coming

i'm looking for a fresh idea
or someone to tell me to give it up

any ideas?

thanx in advance

ghb

 

[flatulent]

must there be a ground plane

Are you required to have a ground plane on the other side? Will this antenna be placed against a conducting surface when used?

Here is a type that will work if the answers to the two above questions are no.

Basically, make a vertical dipole with capacitive hat loading. To describe this, imagine a vertical dipole fed in the center running vertically across your area and in the center (left-right) of the area. Then top load the dipole with perpendicular traces at the ends of the vertical dipole. They run equal lengts to the right and left of the ends. Trim the lengths for the driving impedance to be real with no reactive component. Then use an impedance transforming network to bring the real impedance to the value you need.

 

[tapa]

I don't know if it can help you but, there are a patch configuration called "0 Mode" that it's plane, you can control the size with substrate material and has a monolope like pathern. I have uses this some times with realy good results


TaPa

 

[ghb]

the unit has to be planar

this '0 mode' patch sounds interesting
do you have any references on that?

or perhaps some simulation files that i may take a look at

thanx

ghb

 

[multanova51]

Hi

I am using an antenna for 900 MHz : etched dipole on a printed board size about 80mm by 20mm...
It is on FR4 so you have SQRT(epsilon) electr. length reduction.This is not enough for normal resonance though.
Therefore it is loaded in the center by 2 pcs chipcoils in the order of about 27 nH.
It's a bit lossy though and quite sensitive for the loading tolerance and
has about 4 dB less gain as a normal vertical dipole.
Since it is used in a TX only , I make a bit more power to overcome the loss. For a RX or a trcvr , the loss is too much, I not recommend.

Rgds
Mike

 

[bmn123]

900 mhz antenna

thanks for the info

Added after 13 minutes:

great info .........thanks a lot

 

[Azulykit]

Re: 900 mhz antenna

Looking at your requirements the size restriction is severe (too small) and your lack of success with polarization purity is not surprising.

You might try a monopole buried in a block high dielectric material but I would anticipate gain on the order of -5 dBi at best. That is not particularly good news when you consider a dipole is at 2.1 dBi and a monopole is higher at around 5 dBi.

See if you can push back on the size restriction to get more room to build monopole and include a groundplane in your thinking. You may have an enclosure that you can take advantage of as well.