Quote from: ulli on February 09, 2015, 05:51:38 AM
Does anybody have experience with that helical antennas?
How is the range? Looks like a cheap alternativ for outdoor modules...
It depends...
If you take a look at the antenna specs then PCB mount helicals look pretty good. From Linx data on
peak gain:
Quarter wave dipole (rubber ducky): 3.3dBi
Quarter wave SMA helical: -5.6dBi
PCB mount helical: 1.9 dBi
So, looking at this, you would say that the PCB mounted helical is significantly better than the SMA mounted helical and only -1.4dBi worse than a rubber ducky. However, this would be wrong (in MOST cases).
If you read the very EXCELLENT antenna article in this forum, you know that the power going into an antenna is radiated around the axis of the radiator and that an Omnidirectional antenna, like the rubber ducky, radiates in a pretty uniform manner ( you get the same signal no matter where you are positioned around the axis). However, ignoring reflections, you don't get ANY of the signal radiated in a direction away from you. And this is a bad thing, right? In most cases it's not a bad thing because it means that you get the SAME signal no matter where you're positioned around the antenna.
The quarter wave SMA helical is also pretty omni directional so basically you get roughly 9dBm less signal from the SMA helical than you would from the full quarter wave antenna, but, again, it's uniform.
So, now, looking at PCB Mount helicals, you see that the PEAK gain is only 1.4dB worse than the rubber ducky. What you don't see, but should now be able to infer, is that the power from that antenna is NOT uniformly radiated, but is 'lobe'y. There are very high peaks (eg, 1.9dBi) because some of the radiated power is reflected off of the PCB, etc, and is combined with signals travelling in the opposite direction, but there are also incredibly low valleys, ie, no signal at all because those signals are being reflected elsewhere. If you are fortunate, your endpoint is in the direction of one of those peaks, if not, too bad... 'Mother Nature' says that the
average gain very unlikely to be better than an SMA mounted Helical.
Now, given the output power and receive sensitivity of RFM69s, this might not be a problem, just bear in mind that the PCB mounted antenna will only work well in some directions - which directions are WELL beyond the scope of this post.
Now to the good news: Talking about cost, if you take a hunk of wire, which is quarter wave length long, and wind it around a plastic straw about 5mm in diameter, space the windings uniformly over the length of your antenna, and mount that on a PCB, you'll have the same performance as a commercial antenna...
Tom
thanks, for your good support.
I triedto built my own helical antenna and I used a 8.6 cm wire and winded it with a screw.
I just have one question, based on the wire length I just get 4 turns and the antenna is very short.
is that right to do so? or should the antenna be 8.6cm when it is winded?
Something like this might help (http://www.ebay.com/itm/321404959228) (see the images). These are for 868 MHz, I think you can quickly find ones for the other 2 main frequencies if you need to.
Quote from: kobuki on February 22, 2015, 05:52:47 AM
Something like this might help (http://www.ebay.com/itm/321404959228) (see the images). These are for 868 MHz, I think you can quickly find ones for the other 2 main frequencies if you need to.
Thanks for the link. Rough calculations I'd 'guess' that these are 1/2wave length, which, apparently, is ok. So, 173mm as the length of wire to be wound in this case. It would be interesting to see what the VSWR is at 433MHz for these antenna...
Tom
Here (http://www.ebay.com/itm/370673175379) and here (http://www.ebay.com/itm/370673175379) it says: V.S.W.R：<=1.5@433M - also the precise dimensions are specified.
Quote from: kobuki on February 22, 2015, 10:05:38 AM
Here (http://www.ebay.com/itm/370673175379) and here (http://www.ebay.com/itm/370673175379) it says: V.S.W.R：<=1.5@433M - also the precise dimensions are specified.
Thanks again for the links. However, I was wondering what the VSWR was at 433MHz for the 868MHz antenna. It wouldn't surprise me if they were resonant at 433.
Tom
Oh, I see. I don't know about that, but when I used the 868 MHz "default" Moteino monopole, it provided me about 40-45 dB of RSSI through a door and about 5 m distance. With a 915 MHz Radio. I forgot to change the frequency before testing :)