Highfields Amateur Radio Club
Constructors Corner.

2 Metre Slim Jim Antenna Project.

A good starter project is this 'Slim Jim' antenna for 2 metres, although the antenna can be scaled for use on any frequency with a little number crunching, see the Slim Jim page for details.

What is a Slim Jim?

A Slim Jim is a vertically polarised omnidirectional end-fed antenna having some "gain" over a quarter wave ground plane type antenna, up to about 50 percent better is claimed. It is very narrow in profile (depending on what you make it from) and exhibits low wind resistance. It uses a 'J ' type matching stub (J Integrated Matching = JIM), so put the two features together and you get the name SLIM JIM. The vertical angle of radiation is narrow, about 8 degrees toward the horizon, so 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. When correctly matched for lowest swr, it has wide bandwidth.

Credit for the original design goes to F.C. Judd, G2BCX.

Construction Details:

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 "freespace" distance.

The measurements above are not extremely critical but may have to be lengthened or shortened for your particular operating frequency.

You take a piece of 300Ω twinlead (See: 450Ω ladder feeder Slim Jim page for a 450Ω ladder feeder version) about 2.5 cm (one inch) longer than in the picture (right), trim both of the ends down to bare wires, twist and solder both ends. Apply tape or sealant to prevent moisture from seeping inside twinlead if using outdoors.
Your final length for the total antenna length will be about 150 cms (59 inches).

On one side of the twinlead, measure up 49 cms (19 inches) from the bottom of the antenna and cut out a complete section of the wire (ON ONE SIDE ONLY) of about 2.5 cms (1 inch) at that 49 cms (19 inch) point (again, sealing the cut ends if using outside).
This creates an air gap and also technically creates some capacitance at this point.

Assembly of the antenna is now complete except for attachment of coax and final SWR adjustment. Once you get the hang of it, you can make this antenna extremly fast. Makes a great emergency antenna for attic installation out of the weather!

SWR Adjustment and Installation.

Now make some sort of temporary, non-conductive, support with the antenna hanging straight down (or you can lay it out on the ground, table, work bench, etc) so that you can attach the coax (50 ohm small type like RG58) to the feed points, center conductor of the coax to longest side, shield to the cut side. There is a very good reason to use small coax at the antenna - your fingers will let you know why if you have large hands! Those tiny wires inside the twinlead are very difficult to work with using large coax like RG8!

The usual connection points will be around 10 cm (4 inches) from the bottom of the antenna (as viewed in the picture), however due to various construction methods, brand and type of 300 ohm twinlead used, these points may change, so bare the wire for about 13mm (half inch) on both sides of the 10 cm (4 inch) points to allow for adjustment of the SWR.

Attach the coax in a temporary fashion (a tight twist should suffice), center conductor of coax to longest side, shield to the cut side, by twisting each end tightly onto the bare conductors of the 300 ohm twinlead. One piece of electrical tape wrapped on one side will keep them from shorting out.
Now get the antenna up and away from surrounding metal objects at least 50 cm (20 inches) or more and as high as possible for SWR adjustment. The final location is the best place to test it as moving it will cause the reflections from near by objects to alter the final reading.
Note: The coax should hang straight down from bottom of antenna during tuning and at final installation.

Adjustment of SWR: Using the lowest amount of power that will give full scale on the set portion of your Metre, move both coax connection points equally up or down for best (lowest) SWR reading. After lowest SWR is obtained, (you may not be able to achieve a 1:1 so just trim for lowest SWR), solder coax to antenna seal end of coax from weather and let it hang down from bottom of antenna.

This Slim Jim antenna can be used inside by supporting from string, nylon cord, etc from a high point in the room or used outside by hanging or supported from a suitable length of PVC pipe, wood or other non-conductive material.
In this method of construction it is not capable of handling high powers but should be able to handle the output power from most production radios on the market.
If long runs of coax are needed to connect to your rig, then it is suggested that a short length of RG58 type coax be used at the antenna connection, then add a barrel type connector and convert the coax to lower loss type such as RG8 going to your rig, Trying to use the large type coax at the antenna is very difficult due to sizes of wire and limited space between conductors of twinlead.

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