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From Sunny North Lincolnshire uk

 

 

 

lat 53.596 Long -0.725 Grid IO93po WAB SE81: Operator Martin.

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Building a G5RV antenna My Early Projects   A Simple lightening arrester   

For Beginners

Basic Antenna Tuner Project  Getting Computerized  CI-V Interface Controller Receiver Workshop
Slim Jim Antenna Project A VOX PTT Circuit 6 Meter Centre Fed Dipole Transmitter Workshop
Taking the RAE
      

 More Projects

Slim Jim Antenna Build

As already mentioned elsewhere on the website, A while ago now I built a new Slim Jim antenna for the  2 Meter band, I last built one of these antenna's back in the mid 80's that version was made of coat hanger wire and very soon corroded and came off its mounting in a gale, but while it was new it was a very good antenna and I was well pleased with it.       

While I was browsing the internet for antenna projects that looked interesting I came across the old slim jim design, but this time made out of 15mm copper tubing rather than the coat hanger version, it looked interesting enough to make me try it out, but could you really get the SWR down on 15mm copper tubing.

   Well only one way to find out so off to the local plumbers merchants to buy the bits I needed, copper piping, elbows, and clips, that's it all you need, I chose to build mine for 2m but they will work on other bands, they are only a single band antenna though so you have to build one cut to size for each band you want to use it on.

Well now my antenna is now complete and in use, and guess what the SWR is below 1.5 to 1 almost all of the way across the band.

Left is a photo of the antenna mounted on a piece of 3inch diam UPVC down pipe, along with a dual band 2/70 antenna I use for 70cm working.

this antenna works so well I thought I would share the design with you, so set out below are all the details you need to get started, including the calculation for length.

FORMULAS
(For results in inches)

NOTE: Air gap and element spacing may have to be determined by some experimentation for various frequencies.
See new info about gap spacing below.


(Divide results by 12 for feet)
3/4 wave (longest section = 8415 / fMHz = inches
1/2 wave section   = 5610 / fMHz = inches
1/4 wave section   = 2805 / fMHz = inches
* 1/4 wave free space = 2953 / fMHz = inches         
* This is the distance that antenna should be
from mounting  boom, mast or tower.

Note: These formulas are believed to be accurate.
Some trimming or tweaking of lengths may be needed with YOUR construction!

Slim Jim Metric Formulas:

(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!
============================================
Some Examples

2 Meters 146.00mhz
3/4 wave section  8415 divided by 146 =
57.63 inches
1/2 wave section  5610 divided by 146.00 =
38.42 inches
1/4 wave section   2805 divided by 146.00 =
19.21 inches
1/4 wave freespace  2953 divided by 146.00 =
20.22 inches
Feed point about 10 to 20% of 1/4 wave =    1.9 to
3.84 inches (+ - tuning)
The gap would be a guestimate at about 1 1/2 to 2 inches  (+ - tuning)
Remember, the 1/4 wave free space is the distance from the mount as a minimum.

============================================================
6 Meters 50.150mhz
8415 / 50.150mhz =
167.79 inches
5610 / 50.150mhz =
111.8 inches
2805 / 50.150 =
55.93 inches
Gap spacing 10 to 20% of 1/4 wave =
8 inches (15%)
Free space mounting distance
58.8 inches

============================================

If you have not got any experience of soldering 15mm copper pipe don't panic, you will need a blow lamp (gas is essential), rub the ends of the copper pipe with wire wool or pan scrubber, apply a small amount of flux to the clean pipe ends, assemble with a 15mm bend on each corner and apply heat to each bend until a small amount of solder starts to appear out of the joint then stop immediately and move to another corner, at the gap clean 1/4 wave and 1/2 wave ends and place the two 15mm blank ends to make the inner water proof and solder as above.

 Note:

The Calculations on the left are example frequencies you must add the frequency you want the antenna to resonate at.

For example take the 2 meter band, I did my calculation for 145.000 and not 146.00 and my antenna is built as below.

3/4 wave (long section)  58.034 inches
1/2 wave leg                       38.68   inches
1/4 wave leg                       20.36   inches

these figures work fine and as I say my antenna has an SWR of less than 1.5 to 1 across the band.

Tuning the antenna is easy enough, connect a good SWR Meter and connect two temporary croc clips to the inner and outer conductors of the feeder coax (RG58) or similar, clip these to the connection points shown inner to long leg screen to 1/4 wave section, move the clips up and down equally on both sides until an SWR of 1.5 to one or less is found, then remove the clips and solder the coax to the 15mm tube seal coax end with silicon sealer and recheck SWR before lifting into situe.

If your looking for an interesting project for the summer month's try this out it really does work well.

Cheers and beers

Martin (G8NQN)

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