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I am using a 315MHz RF receiver and transmitter pair, which are connected with Arduino UNO board. The objective is to copy 315MHz RF signal, and replay it back. FYI, the design is very similar with this.

The overall design works already. But, the problem is: the RF receiver must very close to RF signal source when copying (e.g. less than 10cm).

When the distance is more than 15cm, the copied signal is always unstable (e.g. the value of LOW and HIGH vary). When the distance is more than 25cm, the receiver always cannot receive the signal.

How to solve this? Can this solve by software? Maybe add antenna? I do not have such knowledge about antenna.

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  • \$\begingroup\$ It sounds like the receiver isn't receiving enough power, the easiest fix is to increase the transmitter power (if you can) or buy a higher gain antenna. \$\endgroup\$ – jramsay42 Oct 12 '17 at 3:31
  • \$\begingroup\$ @jramsay42: I connect receiver to 3.3V. The problem is when I connect it to 5V, a lot of noise will be received. 3.3V almost does not receive noise. \$\endgroup\$ – Li Dong Oct 12 '17 at 3:41
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Maybe add antenna?

Yes!

Yesterday I started with my 433 MHz wireless transmitter and I noticed as well that it only worked within 10 cm. Then I looked around on the interwebz to find a solution, just like you, and I found out that adding an antenna with a length equal to a quarter of the wavelength makes wonders. So now it works for a couple of meters.

So you need to hook up two wires that is roughly \$\frac{c}{4f}=\frac{3×10^6}{4×315×10^6}=23.8\$ cm long. One for the sender and one for the receiver.

The antenna will radiate in a donut shape around the wire. So the orientation of the antenna makes a huge difference.

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  • \$\begingroup\$ I find this online: solidremote.com/blog/rf-module-external-antenna-design. and can I make a easy antenna using any wire? do you know where I should install the antenna on the receiver? \$\endgroup\$ – Li Dong Oct 12 '17 at 9:24
  • \$\begingroup\$ @LiDong You're really great at finding stuff and not absorbing information. But I shall absorb it for you, in your link under the "Basic Antenna - Whip" it says this: An antenna can be defined as any wire , or conductor, that carries a pulsing or alternating current. And regarding the placement, well there should be a hole for you to solder it to. For me it was one of the the corners for the sender and one of the corners for the receiver. \$\endgroup\$ – Harry Svensson Oct 12 '17 at 13:31
  • \$\begingroup\$ @LiDong And if you continue reading under the same chapter you shall encounter this: A simple formula for the quarter-wave (in cm) is 7500 divided by the freq. (in MHz), or for inches 2952 / freq. (in MHz). This formula is only a starting point since the length may actually be shorter if the whip is overly thick or wide, has any kind of coating, or is not fed close to ground. It may need to be longer if the ground plane is too small. \$\endgroup\$ – Harry Svensson Oct 12 '17 at 13:37

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