patch antenna tuning using varactor diode

varactor diode antenna

Hi, I was recently tasked to come up with a circuit diagram on how I would connect varactor diodes to my patch antenna so that I can tune the antenna electronically. I currently have 2 designs.

One is for a normal patch antenna in which the varactor diode is connected across the patch and the ground plane.

The 2nd one is a small patch antenna where the patch is shorted to the ground plane. Which is a bit trickier because I have to make sure the varactor diode is not shorted as well. However, I was told by my final year project supervisor that my designs have a very obvious error in them and the 2nd patch antenna design has no chance of working. I have no idea and I need help.

The circuit diagrams are below:

varactor,antenna

quarter patch varactors have to be reversed cos, varactors adjust capacitance with reverse dc voltage. Also shorting patch toground causes varactor malfunction cos, there ll be 0 valtage applied to varactor.

Good luck

In second case you short to ground the fringing fields from the edge of the patch element.

In a Patch Antenna the surface conductor element does not form the radiating element, as in other antennas (dipole, monopole, etc). The radiation occurs from along edges (Length and Width), and which edge depends by the operating electromagnetic mode.

Greater the fringing fields from the edges of the antenna will increase the efficiency and the radiated power of the patch antenna.

Thanks for replying guys, but let me explain why the patch was shorted to the ground plane.

The quarter patch as its name suggests is smaller than the full patch, but by shorting it to the ground plane, it becomes electrically wider while maintaining a small physical dimension. At least that's in theory.

Is there some way we can install the varactor diodes while still keeping that property? Maybe add series resistors across the vias to the ground plane? Would that still work?

Resistor may work(as capacitor. i m not sure about patch ll work or not) on varactor reverse voltage polarization(series to varactors) But you have to calculate structure parameters(S parameters, gain etc.)

Good luck

Hmm looks a little like what I had in mind, maybe it really could work. Kudos.

Having resistor will lead in to poorer control over the tuning range and also effect on the BW.
I suggest having big capacitor in series with inductor will solve the problem in fullpatch.
A similar technique can be applied to other patch too.

I would not do any of that. I would:
1) I would stick to the full patch. If I had to use the quarter wave patch, I would try to get rid of the short on the one side. If I could not, due to restrictions on physical size, I would replace the short with maybe five 1000 pF capacitors, equally spaced along that "shorted" edge.
2) to change the resonant frequency, you need to vary the electrical "length" AND the electrical "width" of the patch. So I would place varactors to the right side of the patch, and along the bottom edge (or for the quarterwave patch, where the "feed point" is drawn). That way, one tuning voltage will simultaneously change the effective length and width.

I see, thanks for the help!

Hi Biff44,
I am bit confused. are you saying that there4 is no need of big capacitor for full path and yet we can get the tunability?
May I ask you to explain with drawing?
Thanks in advance.

1) I am not an "antenna guy", so take anything I say with a grain of salt

2) As I understand patch antennas, the radiation is from the 4 edges, where the Efield is strong from the patch to the underlying ground plane. To get the Efield to be a maximum, you need the width and length of the patch to "resonate". You can lower the resonant frequency of the patch by capacitively loading the edges (makes the width and length appear physically longer). There is no need to put the capacitors on one edge and the opposite edge, as you can get the same electrical length with the capacitors on only one edge. So I would pick one edge, and one of its adjacent edges, and load them with variable capacitors. You might be able to do it with one varactor right in the middle of the edge, but I suspect it would work better (more uniform radiation pattern) if you had multiple varactors spread along each of the 2 edges.

3) I do not understand what the inductor's purpose is in the "full patch" drawing above, so I can not effectively comment on it. If the inductor is used to tune the patch, then yes, a variable capacitor across the inductor will vary the frequency of resonance also. I am not sure if you can get away with only having variable tuning on one edge of the patch. It seems that if it did work, you would not be able to tune very far.

Hello,

See an example posted here...




---manju---

hello,
now i working on design tunable patch antenna for my final year project.what the main characteristic/parameter I have to focus during doing this design because I didn't have any experience in tuning an antenna.

Just for curiosity, Suppose we use small Overlay for tuning, will it help?.
Over lay will also help in varying fringing fields.


Added after 1 minutes:

& no power supply is needed!!