Slim-Jim antenna calculator

I have just finished my Slim Jim antenna calculator and will be using it to build a Slim Jim in my workshop. Photo’s and video coming soon.

Click on the continue button to see more.

Slim Jim antenna calculator

Antenna design

See below the Slim-Jim antenna design.

Calculator explained

Frequency (MHz): Input the frequency for calculations

Velocity factor:  The conductive material user for the antenna’s radiating element will affect how well RF propagates through it and certain materials can slow the RF.

Speed of light is used to determine the speed at which radio waves are travelling in free space. Conductors such as copper, aluminium and steel will slow RF down. Velocity factor (VF)  needs to be taken into account when calculating your antenna.

A common conductor used in antenna construction is copper. Copper slows RF by around 5% so the VF will be 0.95 (95% speed of light).

This calculator uses 0.95 as default but this can be changed depending on what material you will be using to make your radiator.


Wavelength (m): 300 / frequency in MHz

Actual wavelength (m): (300 * velocity factor) / frequency in MHz

#1 – Overall length(cm): (22500 * velocity factor) / frequency in MHz

In calculation #2 I have deducted 1/2 the length of the calculated gap (#5)

#2 – 1/2 wave radiator section (cm): (30000 * (0.5 * velocity factor / frequency in MHz)) – ((300/frequency in MHz)/2)

In calculation #3 I have deducted 1/2 the length of the calculated gap (#5)

#3 – 1/4 wave radiator section (cm): (30000 * (0.25 * velocity factory) / frequency in MHz)) – ((300/frequency in MHz)/2)

#4 – 50 ohm feed point (cm): (30000 * (0.025 * velocity factor)) / frequency in MHz

#5 – Gap (cm): 300 / frequency in MHz

#6 – Spacing (cm): (30000 * 0.02175) / frequency in MHz

Note: #2 + #3 + #5 = #1

Total radiator length (cm / m) = (((#1 + #2 + #3) + (#6* 2)) – #5)

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73 de M0NWK

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  1. Hi Adrian.

    Could you please tell me what would be the formula to calculate the feed point for 75 Ohm?


    Juan C.

  2. I’m building a 2 meter slim jim out of .5″ copper pipe. When using the calculator, do I need also to take into consideration the velocity factor of the feed line I will be using, LMR-400, for example, which, I believe, has a velocity factor of roughly .85, or should I just go with the velocity factor of the copper pipe, which is something like .95? Thank you.

  3. Hello.
    I have been digging around for suggestions and tips on building a j-pole or slim-jim antenna.

    I have noticed that you have the outer shield of your 50-ohm feed tied to the small 1/4 wave radiator and the center conductor to the longer 1/2 wave end.

    Does it matter which conductor goes to which side?

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