# Pi/L filter versus impedance matching network the same?

#### pgib8

Jul 26, 2015
107
Hi everyone.

I'm familiar with impedance matching of antennas via a series element and a shunt element but i'm not familiar with filters. When I look at the two side by side they look exactly the same. For instance I used from the source to a 3.3pF series capacitor followed by a 12nH shunt inductor, to the antenna, for 900MHz. With those I modify the antennas inherent impedance to something that the source likes. Well my question is, what I did here is also at the same time a filter I guess but not intentionally. I'm missing the connection of how two identical circuits can be a filter on the one hand and an impedance matching network on the other hand. Are those two the same thing?

I did a quick calculation at http://sim.okawa-denshi.jp/en/RLCtool.php
using:
0.000000012H
0.0000000000033F
0.1Ω

The output showed a peak at about 800MHz, which I feel is really close. Is that just a coincidence?

In a similar way, suppose I have a 50 ohm transmission line, source and load all 50 ohms, but I want to insert a low-pass filter. Seeing how the impedance is already good to go, I would think that by inserting this filter I'm changing the impedance. Perhaps I would have the filter in the center and then re-match the impedance on either side again to 50. Or can one design a filter at basically any desired frequency that also keeps the impedance the same?
I'm sure you can see my disconnect in my thought process. Can somebody help me out and explain this to me?

#### Arouse1973

Dec 18, 2013
5,178
The filters do both. They filter and match impedances, well if you get the values right . If you want to change the impedance you scale the L and C making sure you keep the frequency response the same. if you want to change the frequency of the filter you change the values of L or C or both.

Cheers

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