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The use of stubs.

Updated: May 28

Every impedance can be matched with the use of two components. Capacitor or inductor. Dependent of the place you are on the smith diagram.

Those components are easy to find (or to make). This situation changes when high powers are used.

Some amateur radio stations are using HF powers of 1.000 watts and more. In this situation it becomes difficult to find (make) this coils/capacitors because they have to be able to work at high voltages.

Therefor the use of tubs is very convenient in the high power world. Stubs are not expensive at all and very easy to make.

Just cut al coax line on the exact length and your finished.


I have made a dipole antenna of 2 x 3 meters. This antenna resonates at 25 MHZ. I want to use this antenna at 14.000 MHZ. The value measured with a VNA was 342 ohm -j328 ohm.

Antenna is too short at this frequencies so it behaves capacitive (-J).

I have used the software:

https://www.fritz.dellsperger.net/

This is a very good program to see and understand how antenna matching can be done.

To go from DP1 to TP2 on the diagram we need an electrical length of 4.2 m. When we use coax cable with a Vf of 0.66, we need to cut our cable after 2.8 m.




To go from TP2 to TP3 on the diagram we need an electrical length of 0.991 m. When we use coax cable with a Vf of 0.66, we need to cut our cable after 0.654 m and make a short circuit at the end.

Now we are at the centre of the circle. This means we are on the right impedance.

Left side = to the antenna, down = to the stub and right is any length to the transmitter.

Practical stub in the garden.


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