ATU-100 EXT Automatic Antenna Tuner
The ATU-100 EXT is the popular “Extended” version of David Fainitski’s (N7DDC) open-source automatic antenna tuner. It is widely used by homebrewers because it’s affordable, effective, and handles up to 100W—making it a perfect companion for a 100W HF linear amplifier. Here is a technical breakdown of how the ATU-100 EXT fits into a homebrew station.
Core Specifications
- Max Power: 100W (SSB/CW). For digital modes like FT8, it is safer to keep it under 50W to avoid overheating the small toroids.
- Tuning Range: Typically handles SWR from 1:5 down to 1:1.1.
- Minimum Power to Start Tuning: 1–5 Watts (Adjustable in firmware).
- Inductance/Capacitance: Uses a 7×7 network (7 capacitors and 7 inductors) switched by relays.
Integration with Homebrew Projects
If you are integrating this into a 100W linear amplifier build, keep these points in mind:
- Placement in the Chain:
Rig -> Linear Amp -> LPF -> ATU-100 -> Antenna.- Note: Always ensure your Low Pass Filter (LPF) is before the ATU if you’re using a harmonic-rich homebrew amp.
- Firmware Customization: * The “EXT” version usually comes with an OLED display. You can tweak the
cellvalues in the PIC16F1938 firmware if you want to change the “Auto-tune” threshold. - Tandem Match (SWR Bridge): * The ATU-100 uses a Tandem Match on a small binocular core. If you find the power readings are inaccurate compared to your external meters, you may need to adjust the turns ratio or the calibration offset in the software.
Common Modifications for Homebrewers
- Relay Power: Ensure your power supply can handle the sudden current spike when all 14 relays click simultaneously during a tuning cycle.
- External Control: Many users add a “Tune” button to the front panel or a switch to toggle between Auto and Manual modes.
- Protection: It has been mentioned that you are interested in linear amplifier protection circuitry, you can actually tap the SWR voltage from the ATU’s bridge to trigger a PTT-cut on your amplifier if SWR exceeds 3:1. That seems to be very nice, though I am not very sure on how to implement it for my VU2EVQ linear amplifier!
Quick Comparison for some of my Antennas
| Antenna Type | ATU-100 Performance |
| 80m Inverted V | Excellent for fine-tuning the band edges. |
| 40m Full Loop | Works great if the loop is slightly off-resonance. |
| EFHW (49:1) | Useful for expanding the bandwidth, but the ATU may struggle if the wire length is a “worst-case” impedance for a specific frequency. |