Sunday, May 17, 2015

Shield stabilization

The 15mm copper pipes that shield the H-field antenna against E-field noise are not mounted to the center post in any rigid way. The are just pushed through 15mm holes in the center post and held in place by the blue wire running inside of them and some bare copper wires through small holes in the copper pipes.

The copper pipes can not be pulled out of the center post, but they can still wiggle around. To stiffen the whole antenna assembly I build a cross piece from aluminium and plexiglas.

According to the manual the shield must not cover the complete loop, otherwise the antenna will not work. That is the reason why I build the shield from two semicircles that are connected at the bottom and not connected at the top. The plexiglas acts as an insulator to avoid connecting the semicircles at the top.

Friday, May 1, 2015

Building a shielded H-field loop antenna

The next step after soldering the controller and amplifier boards is building the antennas. I'll start with the H-field antenna. The manual describes two different kinds of H-field antennas that consists of two orthogonal coils:
  • Loop antennas have no core, a big diameter and only a few turns.
  • Ferrite rod antennas have a ferrite core, a small diameter and many turns.
I decided to build a shielded loop antenna with a diameter of 92cm and 8 turns in each loop. The main reason for a loop antenna was that the manual advised against winding my own ferrite rod antenna and suggested buying ready made ones. But that's boring.

Bill of materials:
  • 6m of 15mm soft copper pipe acts as shield against E-field noise
  • 50m of 1.5mm² insulated single-core copper wire (H07 V-U) to form 8 turns in the copper pipe
  • 1m of 75mm plastic pipe acts as center post (not shown)

I build a simple jig out of some scrap wood to help me bending the soft copper pipe into nice and even semicircles with 92cm diameter:

The four copper semicircles are done:

I use a 75mm plastic pipe as center post with 15mm holes 92cm apart to hold all the semicircles. The image shows the top part of the plastic pipe and a semicircle with a wire soldered to it that will be used to ground the shield:

I split the 50m of copper wire into two 25m sections, one for each coil, and prepare it for winding into the copper pipes:

Here is the first loop done with 8 turns in the copper pipe:

The second loop is almost done. I take the prepare copper wire and wind it into the copper pipes basically the same way as you would wind a typical key ring onto a key. My key ring just has 8 turns instead of 2.

Both loops are done. The image shows the antenna upside down. The next steps include testing the antenna with the H-field amplifier and building some kind of stand for it, because it is really unstable and easy to tip right now.