Radome for 2.4G & 5.8G Radio Antennas 1

You haven't been specific, but this sounds like an antenna housing for something like a WiFi or ISM band application (which is what I will assume). In these cases, you should be choosing your plastic almost like you would choose any other plastic for a harsh marine enviroment exposed to the elements and UV that is protecting electronics.

The material is only important when enclosing a highly directional antenna, or a high power antenna as found in a marine radar where the antenna and it's pattern are of high importance. For lower power use where you are already working fairly omnidirectional, then only how the dielectric of the plastic affects the antenna tuning is of interest. Your primary interest is still in protecting the antenna. Even some slightly conductive ESD plastics are almost completely transparent to RF, unless they are heavly loaded with carbon or conductive additives.

I once analyzed several antennas sold by big name antenna "distributors" for mobil and fixed applications only to find that the plastics used were of a variety of inexpensive types. The antennas all worked well, but testing a few in weather proved that simple UV additives were not present, the plastics had no impact resistance, and shattered at freezing temperatures with no external stress applied! (due to manfacturing assembly stresses) and they all failed after a few weeks. A subsequent tear-down showed that all must have been built in china, acid core solder was used internally in some!, there was no sealing, coaxes filled with water, One used audio cable, not coax and the internal assembly quality was non-existant.

Thanks for the info.

Yes, this is an ISM type application.

We are pushing the envelope on range. Trying to squeeze out as much performance as we can. Pushing the antenna height as high as we can get, using directional antennas, keeping coax lengths as short as possible, etc.

Regards
Larry

If the total coaxial loss is significant, check out the LMR-series of cables. Some of the larger diameter versions have surprisingly low loss compared to some of the more commonly used cables.

The presence of the plastic housing may de-tune your antenna slightly. Select an antenna type based upon the pattern, operating bandwidth, and your return loss requirements. Experiment with tuning your antenna using a network analyzer to gain some experience. Once you have a prototype housing, play again with the antenna tuning to make sure you still have a good match over your operating frequencies.

A couple years ago on a similar project I found the enclosure had only a small effect on the return loss, probably because in the device there was about a inch clearance on all sides from the antenna to the housing. In fact, a SLA (stereolithography) prototype ended up behaving much like the final plastic. Later, the plastic was changed by having a conductive additive added for anti-static properties (for intrinsic safety applications) and the change in range for the device was very slight.

Comco,

What did you use for a conductive additive? I'm working on an intrinsically safe device, and I've seen carbon impregnated Teflon. Are there other materials to reduce the surface resistivity of Teflon?

I don't mean to hijack this thread; if this discussion goes more than a couple responses I'll start another.

John D

Zappedagain
I don't recall what the additive was. I was the design EE on a product line and the plastic issues were being handled by the ME. Neither of us work for that employer now for the same reasons that are currently being discussed in thread732-263643

I do have a business card for PolyOne Corporation, so they made the plastic that was being used by an line of instrinsically safe wireless devices. I do not recall which particular plastic it was. When IS regulations changed to require the wipedown/antistatic we were working out a change in the existing plastic with a new additive to our prior plastic suggested by PolyOne engineers. Prototypes were inhand and proved to have a high resistance of several hundred K-ohm/sq - adequate for the surface area of the products.

The devices were 915 MHz ISM and 2.4 GHz 802.15.4 and some GSM Cellular (non IS due to cellular power requirements). The signal attenuation from the additive was very low and could only be seen because of years of prior testing data of the non-additive plastic products.

what size is your antenna sets? at 2" wavelength, you can use some fairly thick material (0.1") if you want to just try any type of plastic.
For fine tuning a radome/antenna interaction, you can change the spacing from the Radome to optimize your results.It'll get you probably a dB or better signal by changing the distance.

alternatively;
I purchases a Raytheon Radome for it's 9.4 ghz boat radar.
It's a honeycomb with skins. I think it was $300 ish. That'd work very well for your frequencies and it's made to be on the water. Approx. 20" tall and 16" diam. Hemisphere shaped. That with a bottom plate will work.



kch

There's costs from $450 to $1125 for what physically appears to be the same Raytheon Marine Radome. Maybe it's different, or maybe it's just the mounting plate.

$450,
kch