The
Slim Jim
Slim Jim
Since it’s publication in 1978, the Slim Jim aerial has been built by many UK amateurs. It was described by the late Fred Judd, G2BCX, in the UK magazine Practical Wireless and subsequently in other journals and books. In Simple terms it is an easy to build high gain vertical aerial that needs no ground plane or radials. Originally intended for use on the 2m Band but can be scaled for any frequency/band. E.g. I have successfully built one for 1090 MHz, for use to receive Aircraft data for use with FlightAware.
Dependent upon the materials used to make the Slim Jim, you will find that some adjustment of the feed point may be necessary. An important note is that the Coax screen is connected to the short stub and the Coax centre is connected to long side.
Made with care, the Slim Jim is capable of a performance superior to the J-pole and the radiation angle is reduced to about 8 degrees (note that there are many quotes as to the exact angle, but the real world performance is often enough for a couple of S points).
The original aerial for the ham band was made from 1/4 or 3/8 inch aluminum tube and designed to mount on a vertical mast. It’s design centre frequency was 145MHz (at the centre of the UK 2m band).
The Slim Jim is a vertically polarised omnidirectional end-fed antenna having considerable "gain" and this is concentrated almost parallel to ground toward the horizon rather than skyward making it more efficient than a ground plane type antenna by about 50 percent better. Due to it's SLIM design, there is very little wind loading and is fed with 50 ohm coax.
It uses a 'J ' type matching stub (J Integrated Matching = JIM), hence the name SLIM JIM. Since the vertical angle of radiation is so narrow, about 8 degrees toward the horizon, it usually out performs 5/8 wave or ground plane type construction due to their much higher angle of radiation. It is estimated that the Slim Jim appears to have about 6dB gain over a 5/8 wave antenna due to the extreme low angle of radiation.
Construction:
NOTE: NO PART OF THIS ANTENNA SHOULD BE GROUNDED!
It should be totally insulated from it's mount, mast, tower, etc with at least 1/4 wavelength of "free-space".
For the correct dimensions for the required frequency please refer to M0UKD's excellent calculator.
A very efficient aerial can be constructed from 15mm copper plumbing pipe using a suitable plastic box to create a water proof coax connection. The plastic box can also be used to create a mounting to a mast.
My preferred method of construction is to use White plastic pipe (as found in your local DIY Store) suitably sealed and made weather tight at the top. The aerial is formed using 2.5mm coper wire (normally stripped from mains twin & earth cable). This is then formed around a square wooden dowl and then slid up inside the plastic pipe. I normally create a friction fit inside the pipe using insulation tape wrapped around the assembly or foam.
This method realises a simple method to provide a water proof coax connection and by making the plastic pipe longer than the aerial it is easily connected to a mast.
You now have a nice looking (White Stick type) aerial that is light weight and can also be used for Portable operations.
NB: Never use Black plastic pipe because the pigmentation used may have a carbon content which will interfere with the aerials performance.
