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Questions about covering an antenna with plastic - RF Cafe Forums

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Below are all of the forum threads, including all the responses to the original posts.


 Post subject: Questions about covering an antenna with plastic
Posted: Tue Sep 13, 2005 10:39 am 
Good day.

We have a microstrip patch antenna that is operating at 1 GHz. Everything works very well when the antennna is in free space. But due to environmental considerations we need to cover the antenna. We are looking at a couple of different methods and I would like some input (articles, books, websites, personal experience, etc.) on these methods.

Method 1) Totally encapsulate the antenna with potting material. The dielectric constant for the potting would be 3 to 5. The thickness of the material is still under discussion.

Method 2) Use a plastic cover over the antenna. The plastic would also have a dielectric constant of 3 to 5. There will be an air gap between the antenna and the plastic cover. The gap could be anywhere from 0 to 5 mm (For a gap of 0 mm, the plastic cover would actually be touching the antenna). The thickness of the plastic has not been decided.

Thank you for your time and help.
Michael


 
  
 
 Post subject: Covering your antenna
Posted: Wed Sep 14, 2005 11:09 am 
Can't recommend any books but..............

1. Your epsilon R is an issue but this is not the major concern (see 2 below). With epsilon R greater than 1 your antenna will be de-tuned to some degree depending on how near to the actual high Z (high E field) bits the higher Er material is. but you can account for this by offsetting the tuned frequency when in free space OR you have some method of tuning once the cover is on. In any case its not a great issue.

2. The thing you need to watch for is the Tan delta or dielectric loss of the materia you put on or near the radiating elements. Materials like PTFE are nice as they have very low tan delta. If your not careful you can waste quite a lot of power in a poor dielectric. Note that some coloured plastics have fillers that are carbon based (definately not good).

Hope thats somthing to go on with


 
  
 
 Post subject:
Posted: Wed Sep 14, 2005 2:42 pm 
 
Colonel
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Joined: Mon Jul 18, 2005 4:31 pm
Posts: 32
Location: Baltimore, MD
Remember that the antenna was (I assume) tuned in air. The wavelength in plastic will be smaller. So, for the potted option, since the plastic is right on the antenna, the ant dimensions might need to be changed slightly to maintain tuning.


 
   
 
 Post subject: Re: Questions about covering an antenna with plastic
Posted: Wed Sep 14, 2005 7:57 pm 
As mentioned previously by the other responders, dielectric is less of an issue than loss tangent or tan delta (or loss of the material).
GE Plastics has posted on their website different material properties. Generally, Lexan141 or the like (a type of poycarbonate plastic) is ok to use.
A lot will depend on your application. Is the antenna meant to be a stand alone base station type or is it meant to go into something like a cell phone or laptop computer, etc.
Antenna efficiency or gain will go down with whatever material is covering the antenna. How much degradation is dependent on the material properties of the material being used.


 
  
 
 Post subject:
Posted: Wed Sep 28, 2005 7:43 am 
 
Colonel
User avatar

Joined: Fri Sep 02, 2005 7:25 pm
Posts: 34
Location: Hampshire UK
Additional to all the good stuff so far, glass reinforced plastic radomes use polyester rather than epoxy.

Polystyrene is also low loss, but may not be robust enough. You need to consider sunlight degradation. The right exterior paint is the solution. I once stuck a piece of white PVC (push-on system) 40mm wastepipe into a microwave oven, along with a glass of water. It stayed cold, so I used it for a mandrel support for a 2.4GHz helical antenna. Whatever the loss tangent, it seemed not to matter.


 
   
 
 Post subject: antenna encapsulant
Posted: Thu Oct 06, 2005 3:29 pm 
Hi,
This may sound pretty hokey, but I had a small printed loop (~2"x2.5") that had to be ruggedized. I tried the liquid electric tape route (Wal-Mart). I just painted it on the entire PCB. The VSWR actually improved!


 
  
 
 Post subject:
Posted: Tue Oct 11, 2005 7:15 am 
 
Colonel
User avatar

Joined: Fri Sep 02, 2005 7:25 pm
Posts: 34
Location: Hampshire UK
Oh Gosh. That may not be a good sign.
Consider that VSWR is *not* the "goodness factor" that says it is radiating better. It actually tells you that less is being reflected back down the feed.

A very lossy antenna, say one that has a dummy load connected across it, or maybe laying on the ground in a pile of wet grass, might only be warming up the leaves, and has an excellent VSWR!

Try an unpainted in a test. Then paint it, but touch nothing else.
Actually, I am thinking that the paint will not make any difference. There may be other factors that made the VSWR measure better second time around.



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