A Sim Jim Antenna based on 300 ohm ribbon cable.
As a newly licenced amateur, those Chinese made handheld radios are a tempting way to get onto 2m FM cheaply and quickly. The radios themselves are, in my opinion, quite good but like most handhelds, they are let down by their small "rubber duck" type antennas.
A Slim Jim is relatively cheap and easy to make. It will provide a significant improvement over a "rubber duck". At the club meeting on November 28th 2011, two versions of the Slim Jim were demonstrated. One was built of 300 ohm twin wire feeder. The other from hookup wire on a PVC former.
There are many designs for Slim Jim antennas on the Internet. Almost all specify a length of 58" (147cm). We have not been able to get Slim Jim antennas of this length to operate on 2m. The length of the antenna will depend on the frequency of operation and the velocity factor of the cable. (Electricity travels at 300,000,000 metres per second in free space but travels through a cable at a slower speed).
Here is our experience:
For the 300 ohm version, you will need:
- a length of 300 ohm ribbon cable (e.g. Maplin XR31J)
- a length of RG58 coax (e.g. Maplin XS51F)
- a suitable plug for your radio (for RG58 coax)
- solder, tools etc.
RG58 or a similar small diameter coax is recommended as the thick coax is difficult to solder to the 300 ohm ribbon cable but don't make it too long. A maximum of 3 metres is recommended. If you need to go further than than, use a short length of RG58 at the antenna end and then convert it to thick coax, which has much lower losses.
Strip the wires at one end of the 300 ohm ribbon cable and solder the wires together.
Measure 127.5cm down the cable. Strip the wires back to this point and solder the wires together. You should now have a piece of 300 ohm cable, 127.5cm long with the two wires shorted together at both ends. Measure down the cable 48.5cm from one end and cut through ONE of the wires. Measure another 2.5cm from this point (51cm from the same end) and cut through THE SAME WIRE. Remove the 2.5cm strip between these wires. You should now have a length of 300 ohm cable, 127.5cm long with both ends shorted and a 2.5cm strip missing from one of the wires only.
Strip the insulation from both wires at a point 5cm from the soldered end of the 48.5cm wire and at the same and connect the screen of the coax to the 48.5cm wire. Connect the inner of the coax to the 127.5cm wire.
Put the plug or adaptor onto the other end of the coax.
Our completed Slim Jim gave an SWR of about 1.5:1 at 145.4MHz. Please remember to hang the antenna at least 50cm away from metal objects. The best way to hang the antenna is probably to cut a hole in the insulation between the two wires and to use a piece of string to hang the antenna from a hook. The top link of the antenna (away from the coax connection) and the ends of the cable by the gap could exhibit high voltages when the antenna is used to transmit, even with relatively low powers. Maximum input should be limited to 10 watts (unless you undertake further tests).
This antenna, as shown, is designed for indoor use or for outdoor use in dry conditions. Some designs specify the use of slotted black coloured 300 ohm cable for outside use but we have had reports, from club members, that the capacitance of this cable also changes during wet conditions and this adversely affects the performance. Conclusion: do not use 300 ohm cable for outdoor use unless you keep the cable dry. For example, you could mount the cable in a plastic tube.
This information is offered in good faith but we cannot guarantee that an antenna built to this design will work exactly as ours did.