Electronics > RF, Microwave, Ham Radio

ISM band antenna selection

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HSPalm:
Hi,
I'm making a product with a radio-on-chip device from TI and was hoping to discuss some antenna alternatives. Which ISM band is not chosen yet. It wall have an enclosure of maybe 5x5cm. Antenna must be omni-directional.

Direct soldering/pcb solutions:
PCB antenna
Chip antenna
Spiral/spring type http://www.banggood.com/10-Pcs-433MHZ-Spiral-Spring-Helical-Antenna-5MM-p-932781.html
Solder stripped coax or wire (monopole) and curl it up inside the box.

Connector solutions:
SMA connector and cutout in enclosure for whip antenna
SMA or U.FL connector and cable gland or wire protrusion for stripped coax monopole

Cost is a factor, but firstly I'd like an idea of what is the best/worst solution in terms of coverange/range. Of course the ideal solution is something soldered directly to the pcb and it's done when you close the lid, but not sure how much it affects range compared to the best solution.

Any input is appreciated, and I gladly accept even more solutions that I haven't thought of already.

T3sl4co1l:
There is a fundamental trade-off between coverage and gain.  The most and least (respectively) you can get is a dipole or loop, which do not receive signals along the rotational axis.  (A 1/4 wave whip is equivalent, it just has a ground plane replacing one element.)

There is no such thing as a true isotropic antenna, because EM waves are transverse and will always cancel out along some axis (which comes from what's called the Hairy Ball theorem).

If by "omnidirectional", you mean all directions within a plane, that's okay: place the dipole vertically (perpendicular to the plane of 'sight') and you will cover all compass directions.

You can even get some gain back, by using a vertically taller (e.g., colinear) antenna, which restricts reception to be closer to the plane.  (The more precise you want the radiation pattern in a given direction -- in this case, closer to parallel in the plane -- the longer the antenna must be in the same direction.  Again, trade-offs all over the place.)

ISM bands are not very wide (low bandwidth), so you will not need a large (wide bandwidth) antenna.  An overly small antenna will cost you in terms of precision (needing to be tuned) and efficiency, so don't expect too much.

Tim

HSPalm:
Thanks for replying. Yes, I meant omnidirectional as in all directions in the compass plane, but of course I'm limited in regards to directing the antenna precisely if I loop it inside a junction box :)

What I did was just to attach an U.FL connector so I can experiment with different types of antennas. U.FL -> SMA are quite common, and I can also make different types of antennas with an U.FL pigtail. 

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