modified: Sunday 24 April 2016
Selectable turns AM loop antenna
I've made a couple of AM loop antennas before, but I have never been sure of how many turns to wind and the results (when connected to my radio) have been lacklustre. Today's job was to make another but with a variable number of turns.
I started by cutting and marking out a sheet of plywood to mount and wind the antenna on. The circle was made using a stick with two holes, a nail and a pencil. The other lines were for centre and square-line finding:
I drilled out 16 holes around the perimeter of the circle and hammered in wooden pegs. Despite the saying, square pegs do fit into round holes (nice and snugly):
Nothing fancy on the backside:
I then struck ten nails in at the bottom of the loop and used these as taps for my windings. I wound each loop of the antenna with a seperate piece of wire and terminated each end on the nail farm:
In the picture above you can also see a few other features:
- a rotary switch to choose how many windings are connected
- a pair of brass clips to attach the antenna lead onto (going to my radio)
- an extra peg at the bottom to make routing the wires easier
It works wonderfully! Different parts of the AM band work better with a different number of turns selected on the antenna. At certain ranges I found that 7 turns picked up something nasty that sounded (by ear) a good 10 or 20 more dB louder than everything else. At other ranges it provided the loudest/clearest.
- All wires were twisted around their terminals and then soldered all together at the very end.
- I didn't have a matching wormscrew to put the knob onto the rotary switch, so I cut the head off an ordinary screw and gave it a flat slot using a dremel.
- The 'short' mode provides zero turns. Ie shorting the two wires coming from the radio.
- The 'open' mode leaves the loop floating. Ie one wire is not tapped onto any point/connected.