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ATU for SW

‘‘Six Permutation ATU - peak up your SW signals for general RX’’

N. S. Harisankar / VU3NSH
Tel : +91 491 2576102, 9895741932

I hope this project will give flexibility to construct and design "more" on ATU's. For my General Coverage Receiver, I wish to connect an Antenna matcher or Antenna Tuning unit for getting the peak signals without images, intermode or other phantom signals. For homebrewing ATU I had searched it in ARRL. And I also asked to VU2ARA - Arasu and VU2HRS - Siraj for giving some ATU Circuits from their collections. VU2HRS has provided some miniature designs and tips of ATU design for reference . These designs are having toroid transformer and two gangs. VU2ARA has provided a fantastic design which was used by him when he was an SWL and it is taken from ETI Top Project. The design is quite simple and having its permutations for different matching!! I hope this article is very much useful for HAMs and SWLs.

Everybody knows about ATU. It is only having two components, an inductor and capacitor. Fgure.1., illustrates SIX combinations of matching circuits and they are very much useful for different occasions. These six combinations are made out from a single circuit.

Permutations of ATU
Figure. 1. Permutations of ATU

The maximum transfer of RF signals from the antenna to the receiver occurs when the end impedance of the antenna properly matches the input impedance of the receiver. In 1.7 MHz to 30 MHz an end-connected wire antenna is popular among HAMs and SWLs. This may be anything from a few feet of insulated wire at indoors or a long outdoor antenna. Such antenna provide good long distance reception, but the matching of the antenna and receiver input impedance is often totally disregarded.

The end impedance of the antenna depends upon the wavelegth. When half wave length long or multiple half wave length then its impedance is high. In other hand, if it is a quarter wavelegth long or an odd multiple of quarter waves, it's end impedance is low. I have selected the mode in figure V for my use. For making air core inductor I selected PVC tube. Don't use black PVC tube for this project. I have made pitch on the surface of the PVC tube, of 0.3 mm depth, for the entire windings as a stepped winding form. After winding, apply some adhesive (araldite) at tappings to prevent turns' moving. Never apply adhesive on the whole winding. This will reduce Q of the inductor.

For variable capacitor I am using 500 PF air gang capacitor. You can raise this gang from junk box. The one pole ten way switch is available from ELCOM. The switch is fully sealed type and it is a PCB mounting type.

Circuit of Six Permutation ATU - 1.7 MHz to 30 MHz RX
Figure. 2. Circuit of Six Permutation ATU - 1.7 MHz to 30 MHz RX

Any circuit in figure 1 can use for your antenna matching which gives good results. For the sharp signal peaking, you can try short length of antennas. If your receiver is highly sensitive, then this circuit may not gives much significant result. You can try different types of receivers, and in some cases, remove AVC line to get significant peak in levels. So do experiments more on the permutations. Enjoy this project.

Coil winding data
Figure. 3. Coil winding data.
All turns on 1½" dia, 3½" length PVC, 24 SWG Total 60 turns (2+2+3+4+5+6+7+10+21=60T)

ATU Coil and Rotary Switch
Figure. 4. ATU Coil and Rotary Switch

Amateur Radio Band Spectrun and Component List
Table. 1. Amateur Radio Band Spectrum and Component List

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