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Topic: "tuning" the antenna for packet radios like RFM69 and RFM95 (Read 371 times) previous topic - next topic

arusr

Hi,
I've been reading up on how antennas work and Pi circuits.  Basically, in order to optimize range, you want many things, but the two things you can easily effect without changing the application totally:

Quote
1)  Antenna length:  ensure the right length for the operating frequency (1/4 wave length for wire monopole)

2)  Impedance matching
I see a lot of people just put a 3" long wire antenna on the 915MHz packet radios, and that's it.  What about impedance matching the feedline to the antenna?  Is it just "close enough" most of the time that it doesn't matter?  These radios don't really have a place for a "Pi Circuit" to add components like capacitors and inductors.

Looking at the RFM95 module, the datasheet states something about 50 ohm load, does that mean the optimal antenna would have a 50 ohm impedance?  I see monopole SMA antennas with a rating of 50 ohms, but I don't understand how they get 50 from what is basically a wire?  Are there some components inside those SMA antennas that adds impedance to make them 50 ohm at 915MHz?

I admit I'm pretty new to this, so I'm just trying to understand.

jremington

Antenna theory is an extremely involved and important topic, worthy of years of study.

The better the antenna and the better the impedance match, the longer the range. However, 1 inch of straight wire might work well enough at short range. Dimensions are critical for long range.

The "50 Ohm SMA antenna" is most likely NOT "basically a wire". It is probably a balanced dipole sleeve antenna made from coaxial cable similar to this design, or as described in this thread. Plenty of other designs and ideas if you search the web.

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