Highfields Amateur Radio Club
Constructors Corner.

The Stealth Antenna using RF shielding paint may work, anybody tried it?

Original article:

Here's a simple project that almost anyone can try, all that is required is a few special products and a head for heights.

The theory on how this directional antenna works is based on the work that Augustin Jean Fresnel carried out with light and optics.

This project utilises a Fresnel "lens" to "focus" the radio waves into a tight beam. The effect was first documented by American Alf Sprooli II in 1942, but his research was lost due to his death on 31 March 1943 and the war effort that was in place at the time.

Bjorn L. Eyars, NE1CIT, recently re-discovered Alfs' papers while clearing out an out-building on land he had purchased. In an article he wrote for a local newspaper Bjorn says;

"Alf had theorised that a curved wall would focus the beam better than a flat wall, although he never had time to test the theory before his death." Bjorn continues that when time allows he will carry on with the experimentation where Alf left off.

This project uses the flat wall of a neighbours' house as the reflective focus. Coupled with the, now available, RF shielding paint it gives excellent results as a stealth antenna. Of course, there is no reason why you can't use a normal vertical antenna for this project if you wish.

You will be "stuck" with the direction of the antenna so choose well. This project also gives ultimate satisfaction if the neighbour you choose as the reflector is the one that complains most about your antennas!

Look at your chosen reflective wall from the location (to be) of your antenna. That direction and to the left and right will suffer some loss of signal. Now turn around to face directly away from the chosen reflective wall, that is the direction where your signals will be strongest. The signal actually radiates in a "conical" shape (truncated at the reflecting wall), with a further "cone" formed by the interaction as the reflected signal passes the antenna. See the diagram below for a visualisation from above. Reception is also enhanced in that main direction and degraded in the other directions.

As you can see the RF is reflected at a different angle to the incoming wave, this is due to the interaction with the reflective circles/oblongs that you painted on the wall.
The red lines show the radiation from your antenna. The green lines show the reflected RF. The blue lines show the radiation of the cones.
NOTE: at certain angles of incidence the reflected wave appears to "gather" the transmitted wave and refract it along the reflected path, This phenomenon has yet to be fully explored and explained. Gain has been reported between 10dB and 50dB depending on configuration!

Parts / Materials required:

• House or other large structure to paint the antenna on, with a neighbours' wall facing it (preferably 1/4 wavelength away for best results, but can be any distance up to a full wavelength).
  
  
• Ladder to reach the height required.
  
  
• Hammer drill and masonary bit(s).
  
  
• Wall bolt (a large screw, "penny" washer and wallplug would work as well)
  
  
• RF Paint:
  • YSHIELD HSF55 (Carbon fibre based, € 249.90 for 5 litres).
  • YSHIELD HSF54 (Graphite based, € 349.90 for 5 litres).
  • YSHIELD HSF91 (Silver based, € 1499.90 for 5 litres, pricey!).
  • XSHIELD 1X79 (Gold based, € 2490.99 for 5 litres, astronomical so we'll avoid that one, although it is the favourite for the "Protection from Alien Abduction" (see below)!)
  or similar (prices & links valid as of March 2009) for the antenna and reflector lines.
  The reflector lines can be painted with any metallic colour (or colours!) as long as the paint is conductive / reflective at radio frequencies. See the bottom of the page for some tested options.
  
  
• Paint brush or roller.
  
  
• Sufficient coax to reach from shack to antenna.
  
  
• Earth Rod and cable clamp.
  
  
• A sunny day (to entice the neighbours down the beach - not required if assisting to paint the wall).
  
  
• 1 can spray paint (Ford Moondust Silver is popular) - not required if assisting to paint the wall.
  
  
• Digital camera - not required if assisting to paint the wall.

Method:

If not assisting in the wall painting:

1. Before starting this project, take photographs of the tags of local graffiti artists (preferably those on high structures eg. railway/motorway bridges etc). You will need this later.
   
   
2. With the neighbours down the beach, use the RF or metallic paint to paint their wall with a large bulls' eye and 3 or 4 concentric circles, squares or rectangles. The more bands, the narrower the radiation angle - squares and rectangles will work just as well, but do not have the aesthetics of a well painted circle. Using the spray paint, copy the graffiti tag onto your artwork. Below are a couple of photos of before and after application to a house.
   
   
   
   
   
   
3. On your wall, facing your recent artwork, paint a line 40m high by 100mm wide to form the 1/4 wave vertical for Top Band or whatever height for a 1/4 wave at the band you want to work. Take care to leave a 100mm gap between the ground and the bottom of your line, or it could short to ground. (If your house is not tall enough you can extend the height with some supported wire affixed to the painted line the same way as for the feeder, see step 5, you will need extra fixings though, or a ground mounted loading coil between coax and he paint or even by painting in a zig zag fasion, with each zig (or zag) being no less than 1/16 of a wavelength).

   For the absolute last word in stealth antennas, paint the whole wall with normal exterior grade paint after completing the antenna, this will also help protect it from the weather. Can you spot the antenna on this house (right)? No? It's the line down the middle!
   
   

4. While the paint is drying, sink the earth rod into the ground adjacent to the painted antenna. You can also run the coax feeder at this time.
   
   
5. Once the paint has dried, drill the wall at the base of the "antenna" and fit the wall bolt, or wall plug and screw/washer. The coax centre (or loading coil, if used) will attach to this, and the braid to the earth rod. Once you have connected the coax centre to the wall bolt, paint over it with the RF paint, this will ensure a good connection and seal the cable from moisture ingress. Do not get any RF paint on the shield though, it will cause a short!

If assisting in the wall painting:

1. Offer to do the undercoat yourself or persuade the neighbour that the circles you want to paint are the latest thing in (select your neighbours' "fear" of the moment):
   • Heat insulation.
   • Mobile phone radiation protection.
   • Wi-Fi security shield (stops people "hacking" into your wireless broadband connection).
   • Improves reception for satelite & freeview TV.
   • Protection from Alien Abduction (in the US this is a favourite!).
   
   
   
2. Proceed from 2 above, (no need to wait for them to go to the beach though) but omit the graffiti tag.

Results:

Head to the shack with a nice hot cup of tea or coffee, tune up the antenna and wait for the rare DX to come in.

If you find difficulty tuning and need to shorten the antenna, just get the ladder out and, using a wire brush, brush off an inch or so at a time from the top until the SWR is acceptable.

Options Related to Stealth:

This design can be easily scaled for other bands / antenna types (1/2, full wave etc,). The spacing of the reflectors is not critical, however they should be spaced with at least the width of the paint line between them.

If you don't want a directional antenna you could just paint the antenna on your wall, like the block of flats (right), painted at night when scaffolding had been erected for building maintanence (the black stripe in the middle), for the ultimate in vertical stealth antennas!

Another option for a stealth antenna is if you have PVC guttering & down pipe you could paint them as an inverted "L" (ideal for flat dwellers that are not allowed to put antennas up) using a large "Jubliee" clip or similar for the coax connection.

Tested (useable) metallic paints for the reflector:

• Fiat Raspberry Metallic.
• Fiat Vibrant Lime Green Metallic (who tried that one?).
• Ford Bronze Metallic.
• Ford Champagne Metallic.
• GM (Vauxhall) Charcoal Metallic.
• GM (Vauxhall) Copper Metallic.
• GM (Vauxhall) Electric Daytona Blue Metallic.
• Toyota Copper Metallic.
• Toyota Ice Blue Metallic.
• Toyota Titanium Metallic.
  Other colours may be suitable, the above are known to work. The information comes from an American site so some colours may not be available here in the UK.