Antenna impedance matching question

[liletian]

Hi Guys
I need to match an antenna impedance of 200 to a 50 Ohms at 2GHz. Can I use capacitor and inductor on circuit board to do the matching or I have to use transmission line to do the matching? I am not sure if it is justified using capacitor and inductors since the wavelength is 15cm.
Please comment on the macthing method.
thanks

 

[E Kafeman]

Yes, you can use discrete components but at 2 GHz do things start to be sensitive so it can easily also go very wrong. I would not do a any design in this frequency range without verifying result by measurements. Unnecessary losses from the antenna is very hard to compensate for, and these losses can disturb other components due to ground current or the other way and ruin performance in a else good receiver.
Result due to losses depends a lot of what type of matching components that is used, if PCB is good designed from RF view and what these 200 Ohm is (reactive?). If it is important to have low losses, must the matching network be designed to avoid requirements of high network Q. I am currently working on a matching network calculator that do just that kind of calculations, a bit more complex then just a smith diagram matching. As result do it reduce losses and component values becomes less critical but more discrete components are needed. It can be downloaded for free, but not until next month as I am working on another antenna software project right now. If it is a purely resistive 200 Ohm load can a simple balun transformer probably do the best job both regarding wide bandwidth and minimum losses. Also transmission lines can be just as lossy as any discrete component if bad designed.

 

[liletian]

it is a real 200 ohms. So it will be preferred to do the matching through transmission line?

Yes, you can use discrete components but at 2 GHz do things start to be sensitive so it can easily also go very wrong. I would not do a any design in this frequency range without verifying result by measurements. Unnecessary losses from the antenna is very hard to compensate for, and these losses can disturb other components due to ground current or the other way and ruin performance in a else good receiver.
Result due to losses depends a lot of what type of matching components that is used, if PCB is good designed from RF view and what these 200 Ohm is (reactive?). If it is important to have low losses, must the matching network be designed to avoid requirements of high network Q. I am currently working on a matching network calculator that do just that kind of calculations, a bit more complex then just a smith diagram matching. As result do it reduce losses and component values becomes less critical but more discrete components are needed. It can be downloaded for free, but not until next month as I am working on another antenna software project right now. If it is a purely resistive 200 Ohm load can a simple balun transformer probably do the best job both regarding wide bandwidth and minimum losses. Also transmission lines can be just as lossy as any discrete component if bad designed.

 

[E Kafeman]

I had personally winded a ferrite balun. Or, as most ports that is called 50 Ohm in radios, seems to have an tolerance for at least 50%, I had measured first and if I had found a complex load, had I used discrete components. Main reason for not using transmission line or stubs, is for my part, that in most cases is it not enough space on PCB and components are a bit more flexible choice.

 

[varuniyengar]

striplines or lumped ports are available for imp matching