antenna enclosure
what i can/cannot do
this is a zigbee application
will plastic have a great effect on my gain?directivity?
front: lcd display+microcontroller (digital)
back: power ckts+heat sink (digital)
top: input + other ckts (analog)
bottom: conditioning ckts (analog)
sides: no pcb. just plastic
i am thinking i need to put my RF circuitry at the sides. but this would be problematic. (production purposes)
i was told that the easiest way is to either put it at the top or the front. is this acceptable?
thanks!
vpm
I think plastic will not affect gain, the only things that affects are
1) human inteference such as hand
2) near metallic objects.
try simulation in EM simulators such as HFSS, CST etc, put plastic ( polypropylene ) as a enclosure & study effect.
Not all the plastics are good to place near an embedded antenna.
Also some paintings are more conductive than others. For example some red colors (or red derivates) contain iron oxides.
Antennas work better when outside. Placing them inside introduces many factors that degrade their performance.
The old story is that it is really unfortunate that airplanes still need to fly after one mounts the antennas. The same situation occurs with many "practical" applications. Find a way to put it outside or else plan on degraded performance that will have to be made up elsewhere in the system. More transmitter power, diversity receive, reduced data rates, fancy error correction codes, expensive LNA's and the like can compensate.
i have made some simulations in HFSS. plastic does indeed have little effect.
metal is another story.
i think i have found a perfect place for my antenna. though it is still inside. sadly, i can't put it outside.
i have another question though, the simulations i made were just plastic and metal. but the enclosure will have circuits in them. i want to know how much all these currents and voltages will affect my antenna. i don't know how to do it in hffs though.
question: can i put current/voltages in simulations in hfss?
You can with lumped ports. If I were to try this I would be inclined to load the lines or just let them float rather than trying to drive them.
my experience is that you can put antennas inside a plastic enclosure, but you have to design to minimize the effects.
If you can have an air gap between the antenna element itself and the plastic, that helps a lot. You want to make sure that the antenna stays "tuned" to your operating frequency, and adding a plastic dielectric nearby can lower the resonant frequency.
Small ceramic chip antennas, due to their very high dielectric constant, are less prone to detuning from the dielectric of the housing.
If you are using a wire, coil, or helical antenna, and it is fiting into a specific spot in the plastic body, then you want to make sure it goes into the spot THE SAME WAY each and every time. If the metal part of the antenna can randomly flop around inside of the plastic body, you are going to have random tuning effects. You can put a small reactive tuning element to in series with some antennas, and trim that element until the antenna+housing are tuned up. You can design the plastic housing with pins, slots, etc, to capture the metal antenna so it does not move around.
The metal and components on the overall PCB will tend to change the directional pattern of the antenna. LCD screens, with metal bottoms, screw things up too. Instead of a nice omnidirectional pattern, you will end up with an "egg-shaped" pattern, due to the non symmetry of the RF ground plane in a standard RF board layout. Empirically it seems that loop type antennas are less effected by ground plane shape than monopole type antennnas.
Rich
Can you plz tell where can i find polypropylene in HFSS, i cannot find this material in HFSS libriary
You can add a custom material if you do not find what you want in the pre-assigned HFSS material library.