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KG7GTE 2 Meter Slim Jim Antenna Project from Arizona

Arizona's most famous nickname is "The Grand Canyon State".
It also has another nickname,
The "
Copper State" (The state of copper Antennas!)

KG7GTE Shares A First Time Experience Building the Slim Jim Antenna for 2 Meters



Parts, and cost:

10

1/2" copper pipe

foot

$1.03

$10.25

2

1/2" copper end caps

each

$0.74

$1.48

4

1/2" copper 90 elbow

each

$0.74

$2.96

sub total

$14.69

sales tax

$1.51

Safford, AZ Home Depot

 

total

$16.20 
 (2013 prices)


Slim Jim Metric Formulas used in the construction of the Slim Jim:
(For results in meters)
(For results in Centimeters, multiply results by 100)

213.74 / fmhz    = 3/4 wave overall length
142.496 / fmhz  = 1/2 wave length
71.248 / fmhz    = 1/4 wave length
Feed point = About 10 to 20% of 1/4 wavelength (+ - tuning)
75 / fmhz = 1/4 wave "freespace" in Meters
Note: These formulas are believed to be accurate. Some trimming
or tweaking of lengths may be needed with YOUR construction!
See the "Slim Jim Antenna Project" on Hamunivere.com here!

I put the formulas above into excel, and cut the individual pieces as close as I could get with the exception of the 1/4 wave section which I cut 1" short.

Construction was straight forward, and very easy, however I did learn that you do not want to use a welding torch for soldering..  WAY TO MUCH HEAT as is evidenced by the photographs.  The center feed is a piece of #10 electrical wire cut to length, and soldered to the center of the SO239, and again I learned that you DO NOT want to do this with the antenna in a vertical position ,see the evidence in the pictures.  I put a small bend in the wire, lined up the torch where I wanted it, grabbed the feed with a pair of pliers in one hand and the solder in the other..  it worked, but holding that sucker in place while waiting for the solder to set was a pain.  Then I soldered the SO239 to the other side.

For testing, and pictures, the bottom edge of the antenna is located 525mm (23 inches) above ground level.

I used an old MFJ-249 Antenna Analyzer and connected it to the feed point with a 1' section of LMR400, 
VERY HAPPY CAMPER HERE. The swr is nearly flat across the entire 2meter band. As evidenced by the accompanying photographs. See photos below for swr readings across the 2 meter band.

   
 
The SWR readings above were taken using an MFJ-249 Antenna Analyzer and the antenna was fastened to a piece of 1 1/4" Schedule 40 PVC with the bottom of the antenna about 20" off the ground. The coax is a 1' section of LMR400 with a PL259 on each end. See photos below for testing setup, air gap and SO239 connection.


Test setup

 
Covered Air Gap on left------Feed point connection on right
 
The antenna is 1500mm (59.1 inches) overall {SEE IMPORTANT NOTE BELOW}, and center to center of the vertical elements is 50.8mm (2 inches). The air gap is 36mm (1.4 inches) and the feed point is 252mm (9.9 inches) above the bottom edge of the antenna.
I used a few pieces of old 1 inch board cut to fit between the verticals, which are held in place by black zip-ties.  To cover the air gap I used a piece of old 3/4" heater hose. The major connection points were sealed with an aerosol spray to protect it from weather especially at the SO-239/PL-259 feedpoint.
Important note....don't forget that the 90 degree elbows are included in the overall length of the vertical section lengths so you should use 58.1 inches total length from top to bottom instead of 59.1 inches like I did.


 
Slim Jim up and mounted in left picture --- Slim Jim and "UFO Tracking antenna" in right picture!

~Additional notes about construction and results~

I had planned on making a change, by threading one of the end caps and putting an adjusting bolt so that the air gap could be modified in hopes of making it easier to fine tune the antenna, however, my taps were no longer in the shop, so I ended up with a MUCH wider air gap than was calculated.  However it did not seem to hurt though it did raise the feed point drastically.

I stripped the insulation from a section of #10 Electrical wire and soldered it into the SO239 connector.  What I will do next time is NOT try to solder this connecter onto the antenna with it in a vertical position.  You will really laugh when you look at the pictures, I had solder dripping all over the place. 

I did do one other thing, I cut a piece of 2x4 down until it just fits inside the 1 1/4" PVC in order to stiffen it.
This should prevent movement from the wind, which often reachs 50-60 miles per hour here.
 


Closeup of mounted antenna

On the air testing!
I ran a check with the analyzer from inside now that the antenna is up and the swr is just below 1:1.2 at 144 and just below 1:1.5 at 148.  The swr did go up some vs testing on the ground, but still within limits.  I was a bit disappointed in the lower limits being at the bottom of the band, was hoping that the center would be the lowest point.   Hey, for $16 I can always build another to "perfection"!!!


Got it up in the air, 5.8 meters (18.8 feet) above ground, and connected to the hand held.  Had nice clear
reception of a conversation I could not even see was going on using the hand held rubber duck when the scan stopped, but there was not a signal strength indication so I switched to the Slim Jim..  Signal was full bars on the hand held and everything was clear.  Found an open channel and worked a party in Tucson, air miles from my QTH to the repeater is approximately 45 miles.
He reported full quieting of my signal from the repeater....good enough for me! Now more on the air fun starts! 
Have fun building yours,

73 KG7GTE - James 

Credit for the original design goes to F.C. Judd, G2BCX.
Plans for this project taken from the original article on Hamuniverse.com
here!



 


  

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