ANTENTOP- 01- 2014 # 018

Simple HF ATU on Lengths

of Coaxial Cable

Igor Grigorov, va3znw

The ATU was made by me in far 80s. It was may be a simplest ATU what I made ever. It contains only one rotary switch and rolls of a coaxial cable. But the ATU works very well. The ATU has only one lack- sizes. Sizes of the ATU are not small. Below there are several words to the theoretical base of the ATU.

For those who know the Smith Chart the principle of operation of the ATU is not a secret. Using Smith Chart we can find how impedance of antenna system is changed along a transmission line. Based on this we can find optimal length of the transmission line and place(s) on transmission line where we can install stub(s) to eliminate the reactance in the line. Of course, the description is too simple and GOOGLE helps those who want to know more about Smith Chart and how we could match antenna impedance with our transmitter using only transmission line of definitely length with stubs.

For described here matching unit it means: let's turn on our antenna through line with variable length and find when SWR at our transmitter would be best. It is easy. It is simple. It works in most cases.

Yes, at some cases the ATU does not work or work not good but only in some cases... Figure 1 shows schematic of the ATU.

The ATU contains 11 lengths of 50-Ohm coaxial cable. First length is 1 meter long, second one is 2 meter long, third one is 3 meter long, and so on, next one has length in 1 meter longer the previously one. The lengths of coaxial cable are connected to a four-pole 11-position rotary switch S1. So with help of the S1 you may choose the length of the transmission line from transmitter to antenna system. And if you are lucky (you will be lucky!) match the transmitter with existing antenna system. The matching device works fine at HF- Range 3.5- 30.0- MHz.

If you wish use the device only at 7.0- 30.0- MHz the lengths of the coaxial cable should have step 0.5- m. First one should have 0.5- m length, the second 1.0- meter length, the third one 1.5- meter length and so on. Figure 2 shows the design of the ATU.

Figure 1 Simple HF ATU on lengths of Coaxial Cable

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