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Radome

I found a used book on composites. It indicated that C-glass was the preferred fiberglass for use in radomes. Googling around, it looks like the Chinese have flooded the market with C-glass, so I guess that statement isn't operational anymore.

I found out the AWACS aircraft uses S-2 glass in its radome. Now that cloth is easily purchased at Tap Plastics (well if you are on the left coast). The claim is the material is good to 10GHz.

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They stock the Hexel version. S-2 glass is not very rigid, so you can make just about any shape you want. Now for the AWACS, they used epoxy resin. This might be a function of dielectric constant, which doesn't seem to be listed for any of the resins carried at Tap. I've only used the marine grade vinyl ester, since it is compatible with all the cloth they sell. Possibly in manufacturing the radome for the AWACS, they didn't want to vacuum bag it, so epoxy resin would get around that need.

For DIY construction (i.e. without vacuum bag gear), you just build the composite so it cures between mylar sheets. The vinyl ester doesn't really get hard in the presence of air. Given the cost of the resin, you might as well get the full strength. You don't need a vacuum, but you do need to restrict the air a bit, hence the mylar.

If anyone gets serious about radoming home brew antennas, I can write up a bit more on the fabrication. PVC pipe certainly effects the antenna.

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Reply to
miso
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On Tue, 29 Nov 2011 00:59:12 -0800, miso wrote: (...) Very good info if you're going to build a radome from standard extruded shapes, which are great for wire antennas. Not so good for flat antennas, that lend themselves to vacuum forming:

Works with ABS, PVC, Styrene, Acrylic, Lexan, HDPE, PET-G, etc. Acrylic is nice for making see through radomes.

Reply to
Jeff Liebermann

The vacuum forming links were interesting, but it isn't the same as vacuum bagging.

I think the goal is to get the dielectric constant as close to free space as possible. That was the reasoning behind using C-glass. I suspect S-2 glass was for strength more than RF properties.

I do my composites over a form that can be sacrificed. Foam board for instance. As long as the surface is reasonably rigid and can be covered with mylar, it will work. I've also used cardboard tubes. Ideally you want the form strong enough that you can put pressure on the outside layer of mylar to compress the layers of cloth and mat.

If you don't need RF transparency, graphite/mat/S-2 sandwich is the way to go. Graphite is so much stronger than S-2 that it greatly reduces the number of layers required. Tap also sells graphite tubes that you can use in your project to act as a spine (maybe rib is a better word), also reducing the number of layers required. The trick is to get a strong matrix by whatever means necessary with the minimum number of layers. I have a feedhorn done using this scheme. Predicting the strength of the matrix is for the true gurus. For DIY, you can always add another layer or sandwich if need be. The graphite doesn't absorb much resin, and who knows where to buy prepreg, so you just add layers of mat on the graphite. Probably the outer layer too if you worry about abrasion. The marine vinyl resin has the advantage that it doesn't have to be sanded to get the next coat to stick, while epoxy needs sanding. I rough it up anyway just to be sure.

PVC detunes the antennas a bit. It is barely detectable at a gHz, based on the 1090MHz j-pole I have. The guy who built it did before and after sweeps.

Reply to
miso

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