Matching Network Design
Matching Network Design
(OP)
Hi,
I am new to antennas, so this is a fairly basic question. I am trying to design a matching network for a 2.4GHz chip antenna. I use a vector network analyzer to measure the antenna's impedance with no matching network. I then use software to design the matching network, and I lay out that circuit on the board. However, the impedance I measure with the matching network is nowhere near what the simulation says it should be.
We were reading some on this and saw that we should account for the electrical delay. Could somebody explain why this is? Also, if you have any other suggestions or hints on how to get accurate measurements?
Thanks for any help.
I am new to antennas, so this is a fairly basic question. I am trying to design a matching network for a 2.4GHz chip antenna. I use a vector network analyzer to measure the antenna's impedance with no matching network. I then use software to design the matching network, and I lay out that circuit on the board. However, the impedance I measure with the matching network is nowhere near what the simulation says it should be.
We were reading some on this and saw that we should account for the electrical delay. Could somebody explain why this is? Also, if you have any other suggestions or hints on how to get accurate measurements?
Thanks for any help.





RE: Matching Network Design
RE: Matching Network Design
RE: Matching Network Design
RE: Matching Network Design
One problem I run into with antennas, especially the ones below 1 GHz, is that the mass of the network analyzer, and its cable, make a really big ground plane for the antenna. This will change the antenna characteristics somewhat. So the antenna impedance you measure will be a little off from what it really would be if you disconnected it from the network analyzer. Is your matching network a "little" off, or a lot off?
Also, if you are doing a very narrowband antenna, it might just need to be re-resonated slightly. Is it well matched just a little off-frequency?
www.MaguffinMicrowave.com
Microwave and wireless design consulting
RE: Matching Network Design
When do I need to account for the electrical delay? For example, I am getting demo boards of some antennas from the antenna makers and would like to measure them to see how well they are matched. Do I need to do the procedure (short to ground, then adjust electrical delay) for this case? Or is this something I only need to do when designing the matching network?
Thanks.
RE: Matching Network Design
If you add a length of 50 ohm transmission line between port 1 and the antenna, the answer you get for reflection coefficient magnitude will be the same, but the phase answer will be different. This is because the signal travelling towards the load has to go thru this additional length, and the reflected signal has to go thru the additional length once again. You have to compensate for that additional length to get an accurate measurement.
One way is to use a short circuit at the exact location that the antenna would be, and vary the "time delay" on the network analyzer front panel. This just dials in a phase offset to the reflection coefficient displayed.
This is important, since the amplitude AND phase of the reflection coefficient are used to calculate the measured load impedance. If you get the phase wrong, your measured impedance will be wrong. And any matching network you design with that wrong measured data will not work well!
www.MaguffinMicrowave.com
Microwave and wireless design consulting
RE: Matching Network Design
RE: Matching Network Design
The board has an antenna, with the matching network pads nearby. There are pads for 2 parallel and 1 series connection. Does it matter which of the 2 parallel I short?
After the matching network is a transmission line that is about 50mm in length, then a connector. Do I want to short to ground near the connector instead?
Thanks again for all of your help!
RE: Matching Network Design
For other reasons (bandwidth), you usually want the matching network as close as possible to the load you are trying to match, so I would do it close to the actual antenna.
There are other, better means for doing all this. You can make a number of different standards (trl standards if you like agilent products) to do this measurement more accurately. The higher the frequency, the less likely you are actually making a "good" temporary short circuit, and therefore the less likely you are going to get accurate data. Certainly above 2.5 GHz, I would use a more accurate method than just dropping in a temporary short.
Also, a short cicuit must be physically small! No long lead lengths, use multiple vias to ground, etc. If it is not a short (ie. has inductance) you will get erroneous data too.
www.MaguffinMicrowave.com
Microwave and wireless design consulting