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Antennas inside fabric-covered structure?

Sep 7th 2016, 22:18

jazzop

Joined: Aug 23rd 2016, 22:57
Total Topics: 0
Total Posts: 0
I'm looking for a clever way to run a long wire or loop antenna (20m or more), and I want to explore this possibility:

I'll likely be building an aircraft hangar, and one option is a fabric-covered aluminum frame. This manufacturer supplies the military with these hangars out in the sandbox, so we aren't talking cheapo materials. Let's assume for now that the fabric is RF-transparent (I'm still awaiting technical data). The structure will be about 50'x75', with a peak of about 22'. Obviously this gives rise to the clever notion of installing antennas inside the roof of this thing.

I could suspend antennas from the upper structural crossmembers. Or I could run a wire antenna along the peak, between the frame and fabric. Or I could insulate the junctions between various frame components (they are bolted, not welded) that happen to be the right length for a particular band, and turn those segments into antennas themselves.

I'm not very knowledgeable about antenna design for any band other than 2m, but when I build this place I want to get into MF, HF, LF, etc. Tell me if I'm identifying the right concerns here:

1. Effect of the structural frame network on antennas inside (Faraday caging)?

2. Reflectivity and conductivity of the structural frame on antennas mounted on or near it?

3. Positional angle of the frame members realtive to the desired radiation pattern, if it's feasible to actually turn them into antennas themselves?

4. Impact of cross-sectional shape of the frame members (e.g., box-section, I-beam, asymmetrical extruded), if they are to be used as antennas?

5. Effect, if any, of the exact aluminum alloy? Anodization? Coatings?

6. Of course, the fabric's RF transparency must be determined.

You might wonder why I don't just build a tower; well, the aircraft hangar should give away the fact that this is a private airfield, so towers aren't exactly kosher. Max height allowed of any structure or tree is 50'. I could put a short mast on top of the hangar and run the long wire antennas back to the ground. But I like the idea of keeping all my antennas inside the shelter for easy maintenance, concealment, and protection from the elements. So unless there is a clear performance penalty for doing so, I want to explore it further.
Sep 8th 2016, 13:57

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
It is usually time consuming, but method of moments computer analysis is typically used to design such antennas. You can come up with possible antenna designs and the computer can calculate the relative performance of each one. AC6LA.com sells an optimizer that may assist in your design. There are also 3D finite element analysis programs that can deal with the fabric and questions about modeling simplifications, but most hams don't have access to these expensive programs.

VOACAP can use antenna patterns to predict the performance of antennas for specific propagation paths.

Anodizing aluminum forms a good insulator--I once fixed a cheap TV antenna by removing the anodizing to allow good electrical connections.

Zack Lau W1VT
ARRL Senior Lab Engineer

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