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From: markp on 22 Jul 2010 05:57

"Viktor" <vkesler(a)gmail.com> wrote in message
news:d18bc5dd-04c7-422f-bd30-4d9a0fbf0054(a)r27g2000yqb.googlegroups.com...
>>
>> Well surely the antenna should be in resonance at the desired frequency,
>> ie.
>> real not complex? The beauty of the Si4421 is that it contains circuitry
>> to
>> change its capacitance internally so that any detuning of the external
>> resonant antenna, for example by hand effects or component/mechanical
>> tolerances, are compensated for. This would I presume change the complex
>> output impedance to match that of the detuned circuit, and it does so
>> dynamically.
>
>PCB loop antennas at resonance have a relatively high, real impedance
>but the output transistor of the TX IC has a small output capacitance,
>which at 433Mhz, comes to -j50... -j1k (depending on the IC). That's
>why you need to anull it with an inductive load. The Si4421 would need
>to lower its own output capacitance to zero to be able to adapt itself
>to a resonant PCB loop antenna without any matching components. (One
>would still need impedance matching to adapt to the loop's high
>resonant impedance, though.)
>I didn't find much about this automatic impedance matching feature,
>though it seems fantastic and they should be making a great fuss
>about it. How does it work? What's the range of antenna impedances it
>can cope with?
>Will it still work when the 50Ohm whip-antenna matching circuit is
>present (as on the modules)?
>
>>
>> If you really want to take all the hassle out of it, use
>> this:http://www.antennafactor.com/documents/ANT-433-SP_Data_Sheet.pdf
>>
>> I've used that Splatch antennae and it is very good. It is less immune to
>> hand effects than others. It requires a ground plane (I used a PCB 70mm x
>> 35mm with the Splatch at the top and the lower part of the PCB flooded
>> copper pour for the ground)
>
>This looks great. You can use the matching circuit for a regular whip
>antenna.
>Unfortunately it's too big for this application, especially with the
>ground plane.
>
>Viktor

OK, I don't really know how the antenna matching part works.

My undestanding though is that the matching network they use for the RFM12B
(i.e. Si4421) is such that it is designed so that you connect a 50ohm
antenna directly to it, such as a 1/4 wave whip. The hassle of calculating
the impedance matching components for the driver for that is done for you.
This I assume means that if you can arrange your PCB loop antenna to be
electrically equivalent (by placing inductors or capacitors or by physical
layout) then you're done. Whether this is the most efficient way of doing it
I don't know, it may be that a different matching network specifically to
match the driver output impedance directly to the PCB loop would be.
However, I feel this may impact on the ability of the driver circuitry to
actively match because it may well be designed to use an external matching
network that interfaces to a 50ohm antenna. Given that the matching networks
they suggest must be designed to be efficient at matching the 50ohm load,
your best bet might be to use a balun type network to interface with your
PCB antenna.

Mark.


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