Monday, August 30, 2010

Build a roll up portable 2m (144MHz) slim jim antenna

The "slim jim" antenna has a number of advantages over a helical or quarter wave:

  • It is quite broadband because of its folded element.
  • It does not require any ground plane radials because it is an end fed half wave.
  • It has more gain.

To make a portable one you will need:

  • a 1.7m length of 450 ohm twin feeder (sometimes sold as "ladder line").
  • some string.
  • two cable ties.
  • RG58 50 ohm coaxial cable or similar. Any length to suit.
  • coaxial plug to match your transmitter.

Here is how to build the antenna:

  1. Bare a section of each side of the twin feeder, fold over at 90 degrees and solder the two wires together.
  2. Measure down 151 cm, leaving a bit extra for the folding over and do the same so that you have a 151cm length of feeder shorted at each end.
  3. Measure up from the bottom 49.6cm
  4. Cut the wire on one side of the feeder here.
  5. Measure up 2cm and make another cut on the same side.
  6. Remove this 2cm piece of wire.
  7. Measure up from the bottom 10.3cm and bare the wires on each side of the feeder.
  8. Tin this wire with solder.
  9. Prepare end of your coaxial  cable so you have the inner and outer shield separated.
  10. Solder the coaxial to the feedpoint previously prepared. It does not matter which side of the antenna the inner and outer go to.
  11. Drill four holes either side of the coaxial just below the feed point through a solid part of the cable and secure the cable to the feeder with two cable ties. This makes it very sturdy.
  12. Fit coaxial plug.

Now you have a roll up 2m antenna. You can use the string to hang it from a curtain rail. You could even put it inside a plastic pipe and put it outside if you wanted to. It will roll up to put in your luggage.

To tune the antenna you would move the feed point up or down the open wire feeder, but I have made a few of these and found these dimensions give a good match. Another way to vary the impedance of the antenna is to change the size of the gap. I remember seeing a commercial slim jim with a moveable section there to allow tuning. Starting with a gap of 15mm and extending it till the match is perfect may be worth trying.