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Measurements to determine practical losses in an ATU (Antenna Tuning Unit).

Updated: Jun 19



I built two identical tuners to go from 50 Ohms to a complex impedance and then back to 50 Ohms. This makes it easy to measure the Insertion Loss in a 50 Ohm system.


Schematic representation of the ATU. Thank you Eric ON1BEW for turning my sketch into a nice scheme.



I made three simulations. One pure resistor and two complex impedances.

 

First case: 220 Ohms pure resistance.

1)   Simulation on Smith diagram:



We do this exercise with two tuners and then place the tuners with the 220 Ohm side facing each other. Then we start and end with 50 Ohms each time.




This is the 50 Ohms to 220 Ohms and back to 50 Ohms.




2)     Practical measurements:

Resistor = 220 Ohms Start measurement: tuner bypassed


Tuner 1 and 2 tuning for 7.1 MHz


2 Tuners connected with the 220 Ohms side together.

A loss of 0,2 dB is measured for the 2 devices.

0,1 dB for 1 tuner means a power loss of 2,3 %


3) Simulation in Elsie:

Schematic:

Result in Elsie: Theoretical approach:

A loss of 0,14 dB is calculated for the 2 devices.

0,07 dB for 1 tuner means a power loss of 1,6 %


Second case: 40 Ohms -J150 Ohms

A different approach is needed for complex impedances.


1)     Simulation on Smith diagram:



If we want to go from 50 to 40 -j150 ohms, a different tuner configuration is needed.



If you use the network as a mirror, you end up in the opposite impedance (40-J150 --> 40 + J150). A different approach is therefore needed.


2)     Practical measurements:

Resistor = 2 X 22 Ohms Capacitor = 150 pF

Start measurement: tuner bypassed


Tuner 1 tuning for 7.1 MHz



After adjusting the first tuner, the second tuner is connected and adjusted to get back to 50 Ohms.



A loss of 1 dB is measured for the 2 devices.

0,5 dB for 1 tuner means a power loss of 10,9 %


Third case: 33 Ohms -J80 Ohms


1)     Simulation on Smith diagram:



2)     Practical measurements:

Resistor = 33 Ohms Capacitor = 270 pF

Start measurement: tuner bypassed


Tuner 1 tuning for 7.1 MHz


After adjusting the first tuner, the second tuner is connected and adjusted to get back to 50 Ohms.

A loss of 1 dB is measured for the 2 devices.

0,5 dB for 1 tuner means a power loss of 10,9 %

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