Is there an issue with adding a properly tuned matching network?

Dan - Owner
http://www.Hi-TecDesigns.com

In simulations??

Monopolies are typically installed vertically over a ground plane, exactly like a dipole isn't.

If the ground plane is flat, then the feedpoint Z would be about 35 ohms.

Bending the ground plane down will raise the feedpoint Z, until the extreme case where the ground plane is formed (conceptually) into the missing half of the dipole and the Z approaches the dipole Z of 70-odd ohms.

I'll see if I can find a link to a more complete explanation.

Edited to correct spillchucker.

Here's a nice picture from Wiki that shows an example of a ground plane (the radicals) angled downward. This would not only improve the match to 50 ohm cable, but may also put slightly more signal on the horizon.

Here's the best link I can find with a quick search. It touches on the angle of the ground radicals (the point I mentioned), plus another interesting concept (folded driven element).

http://www.radio-electronics.com/info/antennas/ver...

Thanks a lot for the link and your reply

For tha simulation of helical antenna connected to a pcb

https://www.google.fr/search?q=helical+antenna&...:

How we can adjust the imput impedance??

Try adjusting the design of the surrounding counterpoise (the ground plane equivalent).

Also, don't set your expectations too high. The system will still function even if there's a mismatch.

After that, then add series/parallel LC components to 'tune' the feedline. The normal Smith chart calculations.

You could also experiment by building a standalone dipole using two of the little helical monopoles. Thus eliminating the ground plane question. Just to better understand what you're starting with. Separating the variables.

Another point. In your simulations and measurements, keep the frequency sweep very wide. If there's a good match anywhere, then you can sometimes scale it onto the desired frequency range. While rechecking the other performance criteria.

The ground plane is fixed by the PCB shape, I can't modify it

If the antenna element is a bought-in part and can't be changed, and the counterpoise is also fixed, then that means that the entire antenna is what it is.

So another approach is to flatten the line by means of an antenna tuner, i.e. LC components to move the feedpoint Z around the Smith chart.

Or do what many designers of inexpensive consumer electronics do, and essentially ignore the entire issue.

It is just an example and the antenna dimensions are not fixed.
My aim is to learn how to simulation a fixed pcb+this kind of antenna but I don't know how to ameliorate the S11 because I can't add a matching components in simlation

Well, if this is a learning exercise, then consider my previous advice about "...building a standalone dipole [or a simulation of same] using two of the little helical monopoles..."

The point is to separate the variables. Learn what the helical monopole 'looks like' (on the Smith chart or equivalent), without being presented with a mixed-up combination view that you'll have a difficult time unpicking.

Just for laughs, build a dipole with two of your PCB counterpoise. That exercise will hopefully help to make clear that the counterpoise cannot be a complete afterthought.

If the counterpoise isn't the ideal ground plane (to mirror the monopole), then it's the other half of a badly mangled dipole.

Regarding tuning the helical monopole, what can you do? Some adjustments to the coil parameters would be equivalent to changing frequency, thus aren't very useful except as the final tuning step.

Axial mode helical antennas, where the CP RF is fired out the end, can be tuned by adding capacitance to the feedpoint by bending the first part of the coil closer to the reflector, or welding a plate onto the start of the coil.

Once you get into that sort of approach, then there are a zillion options. A sim would need to include bulk LC components, or be some fairly advanced software to model such details.