Re: Antenna matching 433MHz

From: John Woodgate (jmw_at_jmwa.demon.contraspam.yuk)
Date: 09/04/04 

Date: Sat, 4 Sep 2004 13:58:25 +0100

I read in sci.electronics.design that Andreas <somewhere@else.com> wrote
(in <chc8vu$li0$04$1@news.t-online.com>) about 'Antenna matching
433MHz', on Sat, 4 Sep 2004:
>I have problems to match an pcb antenna of a small embedded μC
>controlled RF-system. The frequency is 433MHz, matching target is 50
>Ohm, antenna is very small, about lambda/10. Therefore the antenna
>radiation resistance is very low, about 1 Ohm, and it behaves
>capacitive. Let's say we have a complex Z=1-j60 Ohm. Now I can calculate
>or use a smith chart to get a matching network. Simulation with RFSIM
>showed that calculation is correct, but if I measure the pcb with
>antenna and matching network with an HP spectrum analyzer, I don't get
>50 Ohm. What goes wrong?

It's quite normal.

>Do I have to consider a more complex physical
>model of the used inductors and capacitances with its resonant
>frequencies, Q etc.? Any hints?

There are probably just one or two 'stray' components that are causing
enough difference to be noticed. What impedance do you measure with your
SA? And how do you measure it? An SA is not the best instrument,
normally, for that job.

You can tune out any residual reactance by tweaking the inductor value
in your matching network. If the resistive part is wrong, you need to
re-scale the network values. This is usually a lot easier than trying to
track down strays, which you often have to live with anyway.

What bandwidth do you need? Antennas and matching networks are often
made broader in bandwidth than the signal requires, because they may be
detuned by nearby objects. 

-- 
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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