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What is the best antenna shape for Virtual Wire?

I purchased 3 lots of wireless RX/TX modules from eBay and being a noob I opted for 2 sets (433MHz) with a presoldered coiled antenna and 1, 315MHz set without.  I thought that because they came with the antenna they would be the best antenna for the job and would save time.  Well, it's taken me a whole day to work those built in antenna are crap and I'm now using the 315MHz RX/TX modules with my own DIY antenna; some stiff 30CM single core bell wire on the TX and a 17cm on the RX.  Even when I remove the receiving antenna and run my transmitter at 3.3V I still get a signal with 315MHz modules, which is much better performance than the rest: I could barely get 15 foot with the 433MHz modules on 5V!

Is there a particular shape antenna that will give me the best range or is my straight wire ample?   Should I try unravelling my 433 antennas and see if that helps?  If a straight wire is the best solution, it doesn't look very professional; should I buy a little doggie's tail antenna instead?  Any pointers are appreciated, Thanks! smiley-wink
« Last Edit: January 02, 2012, 01:01:41 pm by supercrab » Logged

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17cm sounds a bit short for 315MHz, more like 24cm for a quarter wavelength.
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@supercrab

Well, I look into "fractal antenna", the cell phone companies and telecom companies are using fractal design for their antenna. It is smaller and maybe ?  better ?  - I am building a DIY fractal antanna to receive the local HD signal. Here a link
 http://www.ruckman.net/home/news-archives  It show the calculation of a fractal antenna, so you can use it for 433 Mhz, calculated, and make a DIY fractal and see what happen.  Just don't go over FCC / Industry Canada regulations...
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@AWOL The 17cm wire is on the receiver at the mo, but the weird thing is it doesn't seem to make any difference if I remove it!  I shall go for your suggested length as it's better ratio to the wavelength smiley 

@Techone Thanks for that, I shall take a nosey now smiley
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@techone the fractal approach is very intriguing.  I'll let you know the results!
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Update:  I'm not sure my issues with the 433MHz is related to antenna setup.  I've just unplugged both anntenas on the 315MHz setup and it still worked fine.  I'm starting to think that perhaps the 433 MHz are suffering from interference from my wireless routers and that's why they're behaving so poor!
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You do realize those keyphobs are only good to 100ft clear line of sight. Claiming 10mW ERP would be quite a boast for power.
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Hi,  I'm not using key fobs here, I'm using the separate transmitter and receiver modules.  100ft is way more than enough range as this project a temperature sensor and remote control for my boiler. 

Where did you get 10mW for ERP?

I can't understand why I can't get any good range (15 foot) with my 433MHz devices but the 315MHz works like a beast without any anntenas, going through a floor and a Victorian brick wall?!  (40 foot)  smiley-yell  Doesn't make any sense to me! 

The modules look like this:

http://www.glacialwanderer.com/_blog/blog2009/08_August/hr_RF_circuit.jpg
« Last Edit: January 02, 2012, 07:09:26 pm by supercrab » Logged

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10 mW is pretty much the standard FCC limit for those bands, and a keyphob is the term for a multi function keychain sized transmitter, there is always a seperate reciever. Remember to set your unit addresses, they all ship unaddressed, you don't want your neighbor lighting off your boiler with his remote car start kit.

http://www.ebay.com/itm/200683881664?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

keyphob.... a very loose term.
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I have a simple piece of 17 cm 30 guage wirewrap wire on my 434 MHz Tx & Rx modules.
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@supercrap

I did go to the website in one of the reply in this tread, Calculated and I have this : 1/4 wavelenght of 433 MHz is 6.812580081 inches or 173.0395 mm. 
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I shall go for AWOL's suggestion of 24cm for a quarter wavelength for my 315MHz devices.

Any ideas why it works fine without any antennas?

@ajofscott: how do I set my address?  I can't see anything in code and my module do not have a coil to change the frequency like my other 433MHZ devices do.  Hopefully it shouldn't be a problem, as I'm sending a string to the boiler which has to be understood.
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For the internet; here's a list of various online calculators

http://www.dxzone.com/catalog/Antennas/Antenna_Calculators/
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I shall go for AWOL's suggestion of 24cm for a quarter wavelength for my 315MHz devices.

Any ideas why it works fine without any antennas?

@ajofscott: how do I set my address?  I can't see anything in code and my module do not have a coil to change the frequency like my other 433MHZ devices do.  Hopefully it shouldn't be a problem, as I'm sending a string to the boiler which has to be understood.

The simplest high-performance antennas are 1/4 wave against a ground plane, or 1/2 wave dipole.  A 1/4 wave antenna pointing away from the power supply wires approximates a 1/2 wave dipole.

For best performance you need to experiment tuning the antenna for max transmit range (changing the length by 1mm increments or so).  Sometimes people go for less resonant antennas (which don't need such careful tuning) but they have more losses.  I suspect a fractal antenna is one version of this.

The reason you don't need an antenna is that some small proportion of the RF signal leaks on the power and ground wires.  1% leak means 20dB lower power (which is 1/10 the range).  If you are at the limit of range then this isn't "working fine" !!
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@supercrab

Thank for the link. It will be usefull.
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