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433Mhz RF Transmitter Antenna

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I got pilight working yesterday and was able to control a (very close) Etekcity RF outlet, but I discovered that the transmitter power is too low to work on an outlet in the same room behind a couch. By comparison, the remote that came with the outlets works on outlets through 2 floors (first floor to attic).

So how can I get performance similar to the remote? I was looking forward to all the outlets I’d be able to control. How can I get a stronger signal? Is there a particular transmitter chip with higher power? Can I double or triple up the cheap transmitter chips I bought? What’s the important parameter to compare if I buy a different transmitter? Volts? Amps? Are there other solutions?

  • 1
    The remote says it’s 12V 23A. Maybe that’s what I would need. But is that even possible with RPi3? The highest voltage in the GPIO is 5V. Is that the max or is there some way to combine multiple 5V connections? Or maybe there’s an RF amp I can buy to boost/relay RF SIGNALS? – hepcat72 Mar 26 ’19 at 13:30
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    I found a 433Mhz RF repeater on eBay. It learns specific signals to amplify so that it’s not amplifying noise. Sounds like it would work, but it seems like overkill. It extends range over long distances – like 500 meters. I don’t want to blow out my outlet receivers… – hepcat72 Mar 26 ’19 at 13:40
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    Maybe I should go to radio shack. Does that still exist? – hepcat72 Mar 26 ’19 at 13:41
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    Is there a chip with higher power than what? What transmitter are you using at the moment? – Mark Smith Mar 26 ’19 at 15:46

2 Answers

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I use cheap Ebay transmitters but add an aerial, this covers the whole of my house transmitting through brick walls with ease

 

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    My transmitters are cheap as well. I saw a blog post earlier today that showed you can solder a wire to a spot on the chip marked “ant” for antenna. Is that what you did? samkear.com/wp-content/uploads/2014/12/IMG_3548-600×577.jpg – hepcat72 Mar 26 ’19 at 17:07
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    Exactly just a matter of the right length of wire and ensuring it’s not running next to anything metal – Bra1n Mar 26 ’19 at 20:06
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    I use a 17.3 cm antenna, ordinary insulated cable (type is unimportant) – Bra1n Mar 26 ’19 at 20:19
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    Does the angle or straightness matter? Does the length of the path through the circuitry on the chip matter? You said type doesn’t matter, so I assume you mean thickness and type of metal doesn’t matter? Why not? I know nothing about harmonics and I’m just curious. You’ve piqued mild interest. – hepcat72 Mar 26 ’19 at 21:48
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    Lots of questions there and I’m not an antenna expert I would suggest you do some Googling on antenna design and theory – Bra1n Mar 27 ’19 at 7:32
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    Yeah. I googled before asking. There’s nothing out there that concisely answers these questions that I could find in 20 minutes. Anyway, if you don’t know, nbd. I just have no understanding how dividing a speed (meters/second) at which light travels by megahertz (million events/second) to get a length in meters that light travels which contains a million events, relates to antenna length. I have no intuition of why photons emitted from certain lengths of wire are stronger at specific lengths. I would have thought that the more wire emitting photons, the more photos, the stronger the signal. – hepcat72 Mar 27 ’19 at 13:11
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    photons are light not electricity ! – Bra1n Mar 27 ’19 at 17:33
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    Correct. And RF is light/radiation, i.e. photons. All I know about emission of photons is that excited electrons returning to stable orbitals causes an emission of energy in the form of photons. I don’t know if that’s how a transmitter operates to emit radio frequency “light” and how that relates to the antenna. Anyway. Never mind. It’s just idle curiosity based on my chemistry background. I know nothing about electronics or antennas or transmitters specifically. – hepcat72 Mar 27 ’19 at 17:46

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The transmitters have a soldering spot/hole for an antenna. I discovered this via a blogpost I read this morning. It will extend the range significantly with the given voltage/amps, so no need to get a repeater or mess with the voltage.

In that example, they used a “12 inch piece of wire from the inside of a cat 5 ethernet cable”, but there are specific lengths that are optimal for RF antennas at certain frequencies. The length needs to be a harmonic of the wavelength in order to optimize harmonic resonance. To get the best signal, the length of the antenna should be a multiple or 1/4 or 1/2 of the wavelength of the frequency. There are antenna length calculators online to determine these lengths. Make sure you use one that calculates monopole lengths (not dipole). However, here’s a quick cheat-sheet:

  • Full wavelength antenna length: 27.2″
  • Half wavelength antenna length: 13.6″
  • Quarter wavelength antenna length: 6.8″

I went with the 27.2″ length (I rounded it to 27 & 1/8″) and it covered my whole house.

I have not yet soldered the antenna. I ran it though the hole in the transmitter board and twisted the end back and around in a loop just to test it out (like how you twist a twist-tie).

Note, on my transmitter, the antenna hole was not precisely marked. It wasn’t the closest hole to the “ANT” label. It was a hole on the corner of the transmitter board, similar to this:

Transmitter chip with antenna label

The wire I used was an old copper speaker wire I had lying around. Note any wire should do. Diameter is unimportant.

 

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