# Antenna mounting question

Here is the datasheet and drawing of the antenna on this link

I need to stick out the antenna from a thick metal enclosure, how much of the antenna should stick out? Is it 19 mm (the black rubber part), 24mm (till connector) or another length?

In case it is 19 mm, how poor would it be if 15 mm sticks out?

I use it for a 2.45 GHz application.

It looks like a quarter wave monopole to me. At 2.45GHz a theoretical quarter wave monopole will be 31mm long because: -

Wavelength of 2.45GHz = $\dfrac{velocity\space of\space propagation}{frequency}$ = $\dfrac{3 \times 10^8}{2.45\times 10^9}$ = 12.25 cm

A quarter wavelength is therefore 3.06 cm or approximately 31mm

But, it is a bit shorter than this so maybe it is helically wound and this is a way of shortening antenna length a little bit. The data sheet doesn't say so it's a little bit of guesswork.

I would estimate that it needs to stick out at least 19mm above the metal enclosure to be totally effective. Lowering it inside the metal enclosure is going to affect the antenna gain and the field pattern but it may still work effectively over shorter distances.

I'll also add that the data sheet is very poor in specifying this antenna. It talks about antenna gain but doesn't clearly state whether this is dBi (relative to the theoretical isotropic antenna) or some other reference.

• I got an answer from Microchip that it has a helical antenna under black part (19 mm) and is enough to stick out only 19 mm. It is difficult to make a judgment based on a poor datasheet but your assumption was correct. Thank you for the detailed explanation as well. – Angs Mar 27 '15 at 11:00

The whole of the antenna should be outside, if it is to radiate properly. This is usually achieved by mounting the socket on the enclosure.

• While that's true as a general guideline, it isn't always strictly necessary and the connector shell is grounded so recessing it has a minimal impact on performance. – DrFriedParts Mar 27 '15 at 11:16
• A quarter-wave antenna like that has to work against a counterpoise of some sort for proper operation. This is usually the enclosure. – Leon Heller Mar 27 '15 at 11:33
• Agreed. I think I wasn't clear. I'm saying that the connector shell itself may be recessed without a real impact on performance. The antenna element proper should remain above the enclosure profile. – DrFriedParts Mar 28 '15 at 2:51