Building the 80cm Shielded Loop Antenna

Building with Cellflex feeder is a rather tricky operation! Although it is flexible, it is also rather springy! The solid outer copper shield and the thick copper plated inner mean this is a job for the hacksaw and pipe-cutter, rather than the side cutters!

The 80cm diameter loop requires a Cellflex length of around 251cm. Ive tried to keep the cuts for the connections as small as possible, but it will still be approximate. This doesn't really matter though, so long as im within a couple of cm.

The pipe-cutter makes a good job of removing the outer sheath, but is not so good on the shield when near the ends of the cable. Here the hacksaw is preferred. I found that due to the springy nature of the cable, clamping it in the workbench was also a requirement!

The bare ends of the loop

 With the ends of the loop prepared, the next task was to create a 'bridging piece'. This section will bridge the gap in the shield at the feedpoint, making the shield continuous at this point. This was measured and cut, using a pipe-cutter, from an off-cut.

Removing a section of jacket and shield from an offcut

 Once the pipe-cutter was through, the section was carefully twisted off, over the dielectric foam, by hand so as not to risk crushing it.

Shield and jacket twisted off

 The hardest part of the build was cutting the section of shield lengthways. To do this, it was carefully clamped by its ends in the vice, and both jacket and shield carefully cut through with the hacksaw. Any swarf was then removed with needle-nose pliers and a fine needle file.

The laterally cut section of shield

 The photo below shows the loop ends and the 'bridging piece' ready to be soldered together. Before this is done, the connecting wires to the inner loop will be added, as access to the inner loop ends will be very restricted once the loop is joined.

Bridging section ready to be soldered on

Because of the huge thermal mass that this cable presents, even adding a few wires was an industrial job, requiring the use of my 150W soldering iron. A short length of red/black speaker cable was split and used for the loop connections, with a length of green hook-up wire used for the ground connection.

Fitting the 'bridging piece' was more akin to fitting domestic central heating pipe! The section was 'sweated' onto the loop, the close fit allowing the solder to 'wick' into the joint. This was a trade-off between using enough heat to properly flow the solder, and not melting the dielectric or the jacket too much!

Loop connection wires fitted and bridge piece soldered on one side

Feed-point wiring and bridge piece completed

The green ground connection was soldered into a trough in the corrugations of the shield, in order that a section of surplus jacket can later be added over the copper.
Physically, the loop is now almost complete! What remains is to make the break in the shield at a point 180° from the feed-point. To find this point i will use a fabric tape.

Once physically completed, the next stage will be to add an LNA.