Hi,
For a project I'm working on a device that will be installed inside a metal waste container, and will connect to the 3G/2G data network. The antenna I use right now is this:
http://www.taoglas.com/datasheets/PC104.07.0165C.pdf
and although it works, it doesn't work reliably enough - my device is set to only attempt connections for a limited time to avoid heavy battery drain, and in about 20% of the cases it is not able to establish this connection within 1 minute and a half.
Because of the nature of the project I'm looking for something compact and omni-directional. I don't really want something more directional as this device is going to be moved around a lot and I can't always be there to make sure it is fine-tuned for the right direction.
However, I'm unsure however what antenna properties to look into to find something that would send out a more powerful signal than the antenna I use.
Any tips?
Since you're rather looking for an omnidirectional antenna, your antenna should have a low directivity. The other most important parameters are the antenna matching and efficiency. The antenna polarization should also be the same as the recieving one's polarization. Concerning your communication failing, probably it's due to putting the antenna in a metal container. This container, if completly closed, will behave as a fraday cage and significantly attinuate your signal. Try to install your antenna in a way to use the container as a ground plane or a reflector and so that it can radiate directly into free spcae if possible.
Most of the time omni antenna is best suited and can be used as the donor antenna on your home or building. There are times when directional antennas are best , but omni antennas, overall, do the best job in typical situations.
To achieve a greater increase in this circular (or omnidirectional) pattern, you can stack multiple vertical dipole antennas above each other.This increases the vertical size of the antenna. Then, you feed power to the dipoles in such a way that they add together at a distant point – again, with transmission lines matching their radiation power limits for greatest efficiency.
1. Problem:
In simple terms, an antenna is a transducer intended to transfer the electromagnetic energy between air and cable. The antenna you have selected is not meant to be mounted to a metallic antenna (as per the specifications you referred to). Placing it close to metal may lead to 2 consequences:
a) nearby metal "loads" your antenna, which changes the antenna’s input impedance and so the antenna becomes poorly matched and cannot transfer the electromagnetic energy between the air and cable properly. Most of the energy just gets reflected back.
b) the metal behind the antenna also affects the radiation pattern. An antenna radiates part of the energy directly into the air and part of the energy towards the metal. The metal reflects this energy and sends it back to the air. However, if the antenna is closer than 1/4 wavelength to the container, the copy of the signal reflected by the metal starts to cancel out the direct signal due to destructive interference. As a result, whatever manages to overcome the impedance matching problem (a) and gets radiated out of the antenna, is effectively being cancelled by a copy of itself reflected from the metal.
To make matters worse, both effects affect all bands of interest, and in a different manner.
2. Possible solution.
A) A proper solution to address both parts of the problem is to find or design an antenna intended to be able operate when mounted on metal surface. This should really be fairly easy your bands of interest are around 1 GHz and above, so many designs will be compact and efficient.
A usual complication is to get all bands working. Assuming you need large quantify of inexpensive antennas, people like http://poynting.tech/ can do the design, if you cannot find a supplier.
B) If, for some reason, you have to use your specific already procured antenna PC104.07.0165C from Taoglas, you can try to improve the impedance matching and reduce or eliminate the radiation cancellation effects by playing with the distance between the antenna and the metal. You would be looking for at least 1/4 of the wavelength of interest.
Clarification: The “free space wavelength” is calculated as 3e8/frequency(Hz) for free space (air). If you will use some plastic (i.e. dielectric material) between the antenna and metal (which is perhaps a more practical approach for mechanical considerations), then you will need to compensate for the fact that plastics are thicker than air and so the formula will be (3e8/sqrt(eps_r))/frequency(Hz), where eps_r is the dielectric constant/permittivity of the plastic (usually between 2 and 8). You can look up some details at
http://maxwells-equations.com/materials/permittivity.php
and
permittivity values at for example
http://www.rfcafe.com/references/electrical/dielectric-constants-strengths.htm
http://members.tm.net/lapointe/Plastics.htm
or
https://en.wikipedia.org/wiki/Relative_permittivity
NB! This approach (B) will not give the same optimum impedance match as for the free space (you will still loose some dB), but will offer much better energy transfer as compared to not using the approach (B). Please however note that placing the antenna near metal (within around 1/8 to 1/4 of the wavelength of the metal will result in a stronger radiation perpendicular to the metal and very weak radiation in the plane of the metal.
I hope this helps. Good luck!
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