Monday, September 28, 2009

Slim Jim (J Integrated Match J-Pole) is probably the most easiest and powerful 2 meter antenna to build


This antenna is specifically built to resonates within 2 meter amateur radio band (144-148MHz). This antenna has been build with the help of my friend, Amet who is a trained electrician. 

Slim Jim Plan that we use
Please refer to 2 Meter Amateur Radio Slim Jim Plan post for more information

Among the items used to construct the antenna are :
Aluminium tube 8mm diameter, sold in 10 feet (can be obtained from hardware store, 10feet, RM10) 
U PVC Pipe (15mm-20mm) for holding with T junction, RM4
SO-239 connector (for mobile rig), RM10 
Coax Cable (either RG-8 RM7/m or RG-58 RM2/m, quality varies

Tools that we used are (included with cost if you want to buy them):
Bosch hand drill for drilling pipes and aluminium tube ( RM100) 
Rivet / Riveter for attaching coax cable to antenna (RM 20)
Bender (RM 30)
some wire cutter 
Hack saw for cutting pipes and aluminum (RM 3)
and oh yes, we *dont use* any torch welder for this one

As you already might guess, the cost for constructing the antenna alone cost slightly less than RM30 (~USD 10, except cable and tool cost), the Slim Jim is mounted on the roof of my house with ordinary TV antenna bracket, with the height of close to 40 feet from the ground. 

Why build your own antenna? It is a way for me to better understand about radio wave resonance and to learn more about how to build a radio antenna to tune in to specific frequency, something that commercial antenna lacks, because it is built to tune to a wider range of frequency due to market demand.

This antenna is said to have a gain of 3.2dBi, though I don’t have confirmation about this information.

SWR Reading

Here’s the reading I’ve obtained from Diamond SX-200 SWR meter, using 20 watt output power : 
144 Mhz - 1.2
146 Mhz - 1.3
148 Mhz - 1.5

The reading is kinda bit off from the center of frequency :p, I know. this is due to my poor handling with aluminium bender. I’ve made it almost an inch longer, so that kinda explains the SWR reading is kinda centered on the lower frequency.

However this is way cheaper than buying a commercial antenna which cost me a lot of bucks, and I surely have more fun!

What can be improved?

Supposed if you want the antenna to resonate better, you could replace aluminum tubing with copper or brass tube and replace RG-58 coax cable with a low loss cable such as RG-213 or RG-8 which was used in this experiment.

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.

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.

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

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)

1 comment:

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