The Homebrew Slim Jim Antenna Gain

I’ve been searching for information regarding the radiation angle and the gain of slim jim antenna, but found none of it. So I decided to build the model of the antenna inside the ever useful MMANA-GAL software to get the information I needed myself.

From what I can surmise, the slim jim has a slightly lower radiation angle than vertical dipole antenna and is easier to mount, thus it is suitable to use for point-to-point communication VHF-UHF operation. It has a gain of about 2.8dBi-3.2dBi in free space and 4.8-5.8dBi when mounted at about 30feet from the ground. Here’s the result of MMANA-GAL :

Slim Jim 9w2wtf

Slim Jim mounted 30 feet above ground
Slim Jim Antenna gain

Slim Jim in free space

Howto – 2 Meter Slim Jim antenna from Ordinary Wires

Slim Jim (J Integrated Match J-Pole) is probably the most easiest and powerful 2 meter antenna to build provided you have the exact measurement and material to build it.

This how to will show you how to build a 2 meter slim jim antenna from ordinary insulated copper wire commonly used for carrying AC (alternate current) electricity in your household.

Slim Jim construction basic
I am not only going show you the measurement of slim jim antenna for specific frequency, but I’m going to show you how to calculate slim jim antenna by your own using the basic formula below.

Basic Slim Jim Idea

The figure above shows that the longest side of slim jim is 3/4 wavelength long and the shorter side of the slim jim consist of 1/2 wavelength and 1/4 wavelength long seperated by a gap.

The feedline (coax cable) is normally connected 1/20 wavelength from the bottom of the slim jim antenna with the center conductor connected to the longest side and the shield/braid is connected to the shorter side.

Building the Slim Jim antenna
This guide assume you want to build a slim jim antenna that centered on 146MHz.

Calculation
The formula for calculating wavelength in metric system is 300/(freq MHz)

Using the formula from the figure, we have :

300/146 = 2.055 M
Wavelength = 205.5 cm

Wavelength x copper wire velocity factor = 205.5 cm x 0.94
= 193.17 cm

3/4 wavelength = 193.17 x 0.75
= 144.88 cm (57″)

1/2 wavelength = 193.17 x 0.5
= 96.585 cm (38″)

1/4 wavelength minus gap = 193.17 x 0.25 – 2.6 cm
= 45.69 cm (18″)

Coax tap = 193.17 x 1/20
= 9.6 cm (3 3/4″)

Building Materials

  • 3/4″ diameter PVC (20mm) – 6 feet (180 cm)
  • ordinary insulated copper wire for carrying altenate current (AC) – 11 feet (3.40 meter)
  • Cable ties

Tools

  • Soldering iron
  • Glue gun
  • Somthing to make a hole on PVC pipe

Wire Slim Jim Building Steps

  • First take the PVC pile and measure it according to the 3/4 wavelength formula above (144.88 cm).
  • Make two holes at the opposite side of the pipe. This hole is used for putting the copper wire through the pipe. Repeat this step 144.88 cm away from the top hole. Both of these holes will hold the copper wire.
  • Insert the wire through the hole until both end reaches each other on one side of the PVC pipe. Then measure the length of the wire and cut the wire on that side so the setup resembles the figure above.
  • Cut the wire insulation (but leave the wire uncut) 1/20 wavelength away (9.6 cm) from the bottom of the PVC pipe, again refer the figure above.
  • Solder the center of the coax cable at the longest side of the slim jim (3/4 wavelength part) and the braid/shield at the shorted part of the antenna.
  • Test the antenna using SWR meter to ensure that its SWR is at minimum or within acceptable level.
  • There you go, you’ve build yourself your own 2 meter Omnidirectional Slim Jim antenna for less than USD2 (RM 6.00)

2 Meter Wire Slim Jim Antenna in action

wire_slimjim.jpg

mypapit homebrew 2 meter slim jim

Portable 2 Meter Slim Jim antenna using 300 Ohm Twinlead

300Ohm slim jim antenna diagram plan

Due to popular searches, I’ve included a diagram to make a flexible slim jim antenna. This antenna is useful to increase the range of your portable radio or as a simple QRP mobile antenna for emergency situation.

Basic Slim Jim conceptual plan

In order to proceed with this project you need

  • about 165 cm (64 inch) 300 Ohm twinlead cable
  • RG-58 coax cable (RG-8, RG-213 might be too big for soldering), any length but keep it shorter than 7 meters for portable radio
  • Soldering iron
  • Wire cutter
  • Connector to your Rig (usually BNC type or UHF Male)
  • insulating tape

This is an ideal antenna for first timers to build. It is powerful yet simple to construct, once you get the hang of it you would certainly have no problem to construct other variants of Slim Jim antenna using different material.

This exercise would also prepare you in the world of Amateur Radio where real hams homebrew their own antennas.

Important Notes

  • Make sure you solder the center conductor to the longest part of the antenna, and the outer conductor (braid) at the shorter side of the antenna
  • If you use this on mobile rig, keep transmission power lower than 50watt to avoid the antenna from being burned away.
  • Make sure the antenna is held straight for best transmission and reception. Best way to make sure of that is to hang the antenna at a higher place or strap it to PVC pipe or other non-conductive pole

Advantages of 300ohm Twinlead Slim Jim Antenna

  1. Easy to construct
  2. Has radiation angle almost parallel to the ground which makes your transmission goes farther than 5/8 or ground plane antennas
  3. Wideband
  4. Portable; Easy to carry, store and deployed
  5. Can be use during emergency situation
  6. Light and flexible

Completed Antenna
Here is a screenshot of my own completed slim jim antenna which I use with my Icom V8 portable radio.

Completed slim jim 300ohm coax

Upon connecting this antenna to your portable radio, you can feel its differences when compared to using the stock rubber duck or telescoping antenna.