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I was considering purchasing these small transceiver modules but am unsure about sizing the antenna. I recall having read somewhere that the antenna must be in proportion (wavelength/4) to the transmission frequency or reception is impossible.

From what I recall, transmission at 915MHz would approximately require a 8cm antenna or so. What are the benefits other types of antenna over basic wire types? And what do you typically consider when sizing an antenna? Can you literally solder a wire onto the device and expect to receive/transmit properly?

I am looking to use these devices no more than 50 feet or so. I have read conflicting data about the dependence on distance.

enter image description here

Not sure where to start with this.

Thank you.

https://www.sparkfun.com/products/12031

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  • \$\begingroup\$ The specification states that it has 'automatic antenna tunning'. Just connect a wire to it. \$\endgroup\$ – david Feb 19 '14 at 5:09
  • \$\begingroup\$ What is being automatically tuned with respect to the antenna? \$\endgroup\$ – sherrellbc Feb 19 '14 at 5:17
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    \$\begingroup\$ A perfect antenna is only resistive, and the resistance is exactly what you want, and all power going into it is radiated. Any real antenna, and any real antenna of the wrong length, will be reactive and also have a particular resistance. To get the best out of it, you need to tune your transmitter to it. Your device does that automatically \$\endgroup\$ – david Feb 20 '14 at 10:28
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Shortening an antenna from its ideal length is not a problem providing you accept and possibly counter the limitations that shortening brings. Here are some words from this site that tell you the story: -

A shortened dipole is simply a dipole antenna that has been shortened. Since it is shorter than its resonant length, it will not be resonant and will exhibit both resistance and reactance at the feed point. Shortened antennas tend to have a capacitive reactance and therefore need an inductance to cancel the capacitance and bring the antenna back to resonance. Normally the resistive impedance also drops as the antenna is shortened, so additional impedance transformation will be needed to effectively match the antenna.

A shortened dipole will act similar to a full sized resonant dipole in many ways. The effect of ground will still be important, current will be maximum in the center and very close to zero at the ends with maximum voltage at the ends and minimum in the center. If the antenna is center fed, it will still be balanced, with equal voltage and current distributions on both legs.

From the principle of conservation of energy, we know that if we can feed energy into an antenna, it will radiate. We also know that with a suitable matching network, we can feed power into nearly anything, including a shortened dipole.

The article goes on to demonstrate how the impedance of the antenna becomes reactive and how the idealized "50 ohm" input resistance becomes significantly less when shortening is done excessively.

Here is another excellent site that explains you can make an antenna from any length.

In short, any length works providing you can get the power into it that you require for transmission. Reciprocity means that a badly shortened antenna works just as "inefficiently" as a transmitter and receiver. There is no silver bullet of length - optimum is quarter, half and full wave antennas but don't let this stop you running the antenna at significantly shorter lengths.

One caution - if you are wanting to transmit several hundred milli-watts or above, not having an ideal antenna (proper length or compensated by matching networks) you may damage your transmitter output stage.

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