Balanced Antenna Tuner

QRP tuner for 10-40 meters by OZ1BXM

My homemade antenna tuner has three functions: The tuner is kept in-line all the time.


Figure 1. Front view.


 Knob  Function
TUNER/DUMMY LOAD  Input selector switch.
Tuner: input  signal goes to tuner via Stockton Power meter 
Dummy load: input goes to dummy load via Stockton Power meter 
METER 5W: Measure input power. Max. meter deflection is 5 W.
20W: Measure input power. Max. meter deflection is 20 W.
FORW: Measure forward power (input power)
REV: Measure reverse power (reflected power)
FORW SET Adjust to maximum meter deflection when the METER switch
is in the FORW position.
Cin Antenna tuner adjustment
L Antenna tuner adjustment
Cout Antenna tuner adjustment

The different blocks are seen in fig. 2. Please compare with the photo in fig. 3.
 

Figure 2. Left: Physical block diagram. Right: How the blocks are connected


Figure 3. Top view.

Circuit diagram

The tuner's input circuit is unbalanced and it consists of Cin and Lin. The balanced output circuit consists of L and Cout. L is an inductor with 2 x 6 taps. The output terminals can be connected to any pair of taps (symmetrically) using S1. Furthermore, Cout can be connected to any of the taps making tuning at the higher frequencies easier. This is because a smaller part of L's inductance is used if connectiong Cout to the two taps labelled 3, for example. Cout should always be connected symmetrically to L.


Figure 4. Circuit diagram.


 Component   Remarks
Cin 500 pF variable capacitor 
Lin 5 turns 0.6 mm (22 AWG) on center of L 
L  26 turns 0.6 mm (22 AWG). Length of winding is 
28 mm (about 1 inch); make 6 taps every 2 turns beginning 
from one side; then make 6 more beginning from the other side. 
S1 2 x 6 position rotary switch
Coil form Diameter is 26 mm (1 inch); length is 60 mm (2½ inch). 
Material: Plastic container originally used for coins. 
I asked for one in a local bank and got it for free. 
Cout 200 pF variable capacitor 

Examples of use

The balanced antenna tuner has many applications. I have used it for the following antenna types:

Pictures

Here are more pictures showing the construction of L and its taps.


Figure 5. Cout is connected to tap 6 (used at lower frequencies).


Figure 6. Cout is connected to tap 2 (used at high frequencies).


Figure 7. Rear view.

Latest revision July 2010 by Lars Petersen, OZ1BXM.

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