Highfields Amateur Radio Club
Constructors Corner.

Slim Jim Antenna Project.

A good starter project is the 'Slim Jim' antenna for 2 Metres (See: 2 Metre Slim Jim page for 300Ω twin lead version or 450Ω Ladder feeder Slim Jim if you wish to use that), although the antenna can be scaled for use on any frequency with a little number crunching.

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.

Formulas are provided below for all the measurements including the freespace distance.

The Slim Jim can be constructed from 1/2" copper pipe, old tv antenna elements or aluminum tubing, heavy gauge wire supported inside PVC tubing or attached to insulated material such as wood could also be tried and would probably be successful. 300 ohm twinlead versions also work great!

Using copper pipe, bends are made with soldered 90 degree copper elbows. An adjustable slip sleave made from copper can be added to the element on top above the gap for tuning purposes or possibly some sort of nut & bolt arrangement soldered into the upper end to adjust spacing if required.

Depending on the frequency or band, the average length of the gap and spacing between the elements is 3" at 70MHz and 1 1/2 to 2 inches for 145Mhz.
Some experimenters report about 1 inch or less works well. Experiment with the adjustment for best results. The recommended mount is the use of PVC pipe and PVC pipe fittings.

Testing and tuneup:

Support the antenna as high as possible from the ground and as far away as possible from other nearby objects especially metal, and fit the coaxial cable to the antenna with some crocodile (alligator) clips. It is suggested that the centre conductor be attached to the longest element, shield to the shortest.
Attach about 2 to 4 inches up from the bottom and check the VSWR at the design frequency. USING LOW POWER!
Adjust the clips up or down to get the best match, mark where they are to be finally installed, remove the clips, and solder the coax directly or use clamps, screws, etc.
Waterproof or seal all connections and the end of the coax, this is important, coax will soak up water by capillary action!
Use the copper sleeve or nut bolt arrangement, if added, for any fine tuning.

FORMULAE
Slim Jim in Metric (meters)
(For Centimeters, multiply results by 100)

213.74 / fmhz = Meters : 3/4 wave
(overall length)

142.496 / fmhz = Meters : 1/2 wave section

71.248 / fmhz = Meters : 1/4 wave section

75 / fmhz = 1/4 wave freespace*
  Slim Jim in Imperial (inches)
(Divide results by 12 for feet)

8415 / fMHz = inches : 3/4 wave
(overall length)

5610 / fMHz = inches : 1/2 wave section

2805 / fMHz = inches : 1/4 wave section

2953 / fMHz = inches : 1/4 wave freespace*

Feed point = 10 to 20% of 1/4 wavelength (+ - tuning).
Gap spacing = 10 to 20% of 1/4 wave.


* This is the distance that antenna should be from mounting boom, mast or tower.
These formulas are believed to be accurate. Some trimming or adjustment of lengths may be needed with your construction.

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 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 a small coax at the antenna - your fingers will let you know why if you have large hands!

The usual connection points will be around 10 to 20% of 1/4 wavelength from the bottom of the antenna , however due to various construction methods etc these points may change, so experimentation is the answer.

Attach the coax in a temporary fashion (croc clips are ideal), center conductor of coax to longest side, shield to the cut side.
Now get the antenna up and away from surrounding metal objects at least the 1/4 wave freespace 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 meter, 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 or otherwise affix the coax to antenna, seal the end of coax from weather and let it hang down from bottom of antenna.

The Slim Jim antenna can be used by supporting with a suitable length of PVC pipe, wood or other non-conductive material.
This method of construction (copper water piping) it is capable of handling higher powers than the 300 ohm twin lead version, but I give no guarantee that it will handle that 1KW linear!
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 can be very difficult due to the stiffness of wire and the extra weight (which should be supported after the coupler).

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