4
$\begingroup$

I'm working on some outdoor Meshtastic nodes, and am considering materials for a radome to enclose the antenna, transceiver/host, and (possibly) battery all in one tube. The most important considerations are:

  • Commonly available at any hardware store (PVC, PEX, LDPE, etc.)
  • Works outdoor (UV, waterproof)
  • Reasonably cheap (doesn't have to be the cheapest, but not Gore-Tex either)

For the 900 MHz band, what material would have the least effect on the signal and still meet these other requirements? I understand PVC may not be the best radome material in general, but how does it stack up against the other commonly available pipe?

Improve this question
$\endgroup$

2 Answers 2

Reset to default
6
$\begingroup$

At 900 MHz the radome losses will be quite small, whatever you choose.

Mechanical considerations will be much more important - price, availability of fittings and boxes, strength, glue, etc.

For the antenna part, if it's going to be in a pipe of some kind that sticks up, try to use the thinnest wall pipe you can find, and keep the antenna away from the plastic. The plastic will detune the antenna. This is not a problem if you're designing and tuning it yourself, but if you're using an off-the-shelf antenna it won't work properly in a thick plastic radome, taped to a sheet of glass, etc. In this regard, common plastics (PVC, ABS, etc, Er of 2.1) are better than things made mostly of glass (fishing pole, pultruded pipe, Er of 4). Carbon fibre (arrow shafts, tent poles) is right out!
You can evaluate the loss of the material by putting a piece in the microwave oven for 10 seconds, if it gets warm then it's lossy. PP is best, ABS OK, PVC and Nylon worse, but the actual loss is very small at 900 MHz.

Choose something nice and rigid, PEX might be too flexible. It should be glueable. You should probably paint any common plastic building material you install in direct sunlight. Make sure it's completely waterproof at the top, but be sure to leave a drain hole at the bottom, for condensation. Completely waterproofing an enclosure is not a good idea, and nearly impossible if there are cables.

Finally, if you're building a few and you want them to look neat,look at ready-made waterproof ABS boxes from the usual mail-order suppliers. I've been pleased to find smallish boxes for just a dollar or two, quite good quality with a soft o-ring sealing gasket, brass inserts, screw mounting holes etc. Clear lids even. At the hardware store everything is built to meet all sorts of plumbing / electrical standards, you'll be lucky to pay under \$20 for what you're describing, while a 6x6x3" waterproof box with absolutely no certification might cost \$10, shipped.

Improve this answer
$\endgroup$
4
  • $\begingroup$ Thank you for this comprehensive advice! Can you elaborate a bit on the physics of what's happening with an omnidirectional antenna right up against the wall of the pipe and the detuning? Generally, I understand wanting to get attenuating obstacles away from the antenna, just so there's less of an angle that's blocked. But for something completely enclosed, it seems like it wouldn't matter much. Or, is the plastic pipe acting as a part of a capacitor? Or, some other effect? Is there some frequency ratio of distance from the wall of the pipe I should shoot for? $\endgroup$
    – Brad
    Commented Aug 14 at 0:44
  • 3
    $\begingroup$ @Brad the dielectric effect — the same thing that causes the velocity factor in transmission lines. An antenna is trying to let the field escape to infinity instead of keeping it confined, so the effect is less. But the greater the thickness of the plastic and the closer it is to the antenna, the more the resonant frequency drops. $\endgroup$
    – hobbs - KC2G
    Commented Aug 14 at 1:55
  • 1
    $\begingroup$ @brad as Hobbs says, it's a frequency change, leading to poor VSWR at your chosen frequency. Eg. on 1.6 mm FR4 you need 80% length. Without retuning, you could easily lose 6 dB at your frequency. There's no magic distance, the effect is strongest when you're touching the plastic, half as much 1 cm away, quite small 2 or 3 cm away. So a 900 MHz dipole touching the side of a 1/2" schedule 80 PVC pipe will be noticibly less sensitive, while in the centre of a 2" lightweight drain pipe there will much less impact. If your node can measure signal level accurately, you can test this - off vs on. $\endgroup$
    – tomnexus
    Commented Aug 14 at 4:45
  • 1
    $\begingroup$ Regarding "what influence has plastic in the vicinity of an antenna", ham.stackexchange.com/questions/21667/… . In short, yeah, you'll detune, but it's quite a manageable effect (agreeing with tomnexus and hobbs) $\endgroup$
    – Marcus Müller
    Commented Aug 14 at 14:59
1
$\begingroup$

Honestly, a sturdy plastic box that you just put upside down over your transceiver system

Eurobox
source: https://www.hornbach.de/p/stapelbox-eurobox-20-l-400x220x300-mm-transparent-mit-deckel/10705737/

wouldn't be the worst choice. Don't get your antenna too close to the plastic (see the comments on the other answer), and you'll have cheap, removable, raintight, but not airtight (important for condensation reasons!) and easy-to-transport radome. Choose a material that's somewhat UV-resistant, if you're in a sunny area. With a power drill you can add cable inlets.

(Assuming your 900 MHz antenna is the size of typical ISM application antennas – a quarter wavelength or less, so less than 10 cm.)

Somewhat more professional: Junction boxes

Junction box
source: https://www.tme.eu/de/en/details/epn-2750-00/multipurpose-enclosures/elektro-plast-nasielsk/2750-00/

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .