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Impedance matching to heavily increase the centre frequency

 
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Yam
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Joined: 25 Apr 2015
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PostPosted: Sat Apr 25, 2015 2:20 am    Post subject: Impedance matching to heavily increase the centre frequency Reply with quote

Hello,

    I am trying to create an efficient inverted-F 868Mhz monopole antenna for an amateur potted object. Through lots of tests of various antennas, requiring long preparations, I have been lucky to find one that once impedance matched yields a nice VSWR and bandwidth around 868Mhz. But the antenna naturally resonates best around 420Mhz. Is this cause for concern ? Is there such a thing as being too far from the required frequency ?

    Shortening this antenna has been tried, a lot, but the results never were as good.

Thank you !
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helix
Antenna Theory Regular


Joined: 29 Jan 2015
Posts: 64

PostPosted: Sat May 02, 2015 10:12 pm    Post subject: Reply with quote

in general, the more you pull an antenna off of it's natural response in either frequency or impedance, the more "work" the matching network has to do. this is not a problem except that matching components have loss. i am not aware of general solutions/formulas for the insertion loss of a matching network, e.g., based on the Q of the matching network components. hansen in his book on electrically small antennas cites a result for transmission line transformers from moreno, where the insertion loss of the network is approximately the un-matched antenna VSWR times the matched loss of the transmission line (i.e., when that line is matched with its characteristic impedance on both ends).

if you can, in your matching network try to use transmission lines/stubs FIRST, then lumped chip capacitors, and finally lumped inductors as a last resort. even large size inductors can have significant loss. small ones (0402, 0603, 0805) are awful if your matching network is doing much "work". still, it's not hard to have 1dB of matching network losses, even when using only high quality capacitors (NPO/C0G dielectric). the loss you can tolerate is up to you and your application.

maxwell's equations are scale invariant, so a version of your 420MHz model a scaled down to 868MHz _should_ work just as well. make sure you change _all_ the dimensions, not just the height.
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helix
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Joined: 29 Jan 2015
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PostPosted: Sat May 23, 2015 6:51 pm    Post subject: Reply with quote

PS Glen Smith has a nice paper from 1977 "Efficiency of Electrically Small Antennas Combined with Matching Networks"... with some caveats, the efficiency of a matching network is ~ 1/(1+Qa/Qm) where Qa and Qm are the Q of the antenna and matching network. He goes into more details in his paper, but this is what I was sort of getting at when I said the more "work" (movement in frequency or impedance) the matching network has to do, the more loss you will have in the matching network.
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