N7DDC ATU-100 Antenna Tuner
An antenna tuner is commonly used in the HF radio bands to match the impedance of the antenna and typically the feedline as well to the transceiver. If the antenna tuner is located at or very near the antenna feedpoint, losses and feedline radiation can be greatly reduced. For practicality most applications place the antenna tuner near the transceiver, which may require grounding the antenna tuner and/or putting a coaxial choke in the feedline to reduce common mode currents that can cause interference to one’s own transmission.
The N7DDC ATU-100 is a compact antenna tuner that can be purchased as a kit or assembled. It is designed to handle up to 100 watts of power and is typically built to cover the HF bands from 160m to 6m.
A popular implementation is the “7x7” configuration that uses 7 inductors and 7 capacitors to provide a wide range of matching options. The “7x7” setup is comparable to the “8x8” configuration used in more expensive commercial tuners. An example maximum capacitance of 1.822 nF and maximum inductance of 8.53 uH is typical for the “7x7” configuration.
| Tuner model | Maximum Capacitance (nF) | Maximum Inductance (uH) |
|---|---|---|
| N7DDC ATU-100 | 1.82 | 8.53 |
| MFJ-994 | 2.95 | 17.0 |
| Icom AH-4 | 2.4 | 19.0 |
| Elecraft T1 | 1.3 | 7.5 |
By putting the ATU-100 into Test Mode by holding the Auto and Bypass buttons on power up, the user can flip the high / low tuning by a short press of the Tune button. A long press of the Tune button switches between tuning “L” inductance and “C” capacitance. Thus by writing down the auto-tuned settings for a particular frequency, the user can manually tune the ATU-100 for that frequency in the future without needing to go through a full tuning cycle. This can be useful to get a more efficient match or handle a tricky matching scenario.
Be sure to ground the radio and antenna tuner to a common ground point, and if possible, use a coaxial choke in the feedline to prevent common mode currents. Note that the RF voltages on any antenna even at “low” power levels can reach thousands of volts, particularly where baluns and ununs are used to match high antenna impedances (e.g. 4:1, 9:1, 49:1, 64:1).
The efficiency of the antenna is a limiting factor of the effectiveness of all antenna matching schemes, whether manual or automatic. The tuning impedance range of the relatively inexpensive ATU-100 is remarkable, as observed in this video testing a 102" whip antenna from 160m to 10m bands:
An important feature lacking in the current N7DDC ATU-100 is tuning memories. The tuner simply remembers the last tuning setting, but any significant frequency change will require a complete tuning cycles. This can be surprising as compared to the near instant retune common in commercial antenna tuners that typically remember thousands of frequency tuning settings even after power is removed and restored. The ATU-100 price point is about 1/3 most commercial antenna tuners, so this lack of tuning memories is a compromise one might be willing to accept.