This is my QRP antenna tuning unit for use with my ZachTek WSPR Desktop Transmitter. My plan is to set up a dedicated WSPR antenna and, using the ATU, ensure matching on various bands. The ATU can handle up to 5 Watts, making it ideal for the 200 mW output of the WSPR transmitter. Transmitting on 10 metres, I’ve been able to match the TX to my 30 metre end-fed wire with no problems. Matching was easy as the SWR LED on the ATU lights up with only 200mW of RF. Another way of matching is to watch the S meter on a communications receiver and tune for maximum signal. The easiest way is to watch the green TX light on the Zack Tek TX and tune for maximum brightness. Whichever method you use, the transmitter can withstand infinite SWR. The only problem with high SWR is the radiated power could be down to single digit milliwatts due to reflected power. The QRP ATU solves this problem.
The circuit diagram:
Tests so far are encouraging. Out of interest, I tried the ATU on my AR88D communications receiver. Using my 30 metre end-fed wire antenna, I tuned into several HF broadcast bands and found the ATU to match the antenna very well with good peaks on the S meter.
Using the ATU with the Zach Tek transmitter on 10 metres, I’m able to tune out the residual SWR reflected from the 5/8 vertical. The green TX LED shines brightly. The ATU and the Zach Tek set up is shown below.
Specifications:
Power: Nominal power of 5W. Intermittent power 10W.
Matching impedance range: 25 to 2500 ohm.
Frequency: 3-28 MHz.
Size: 88*80*38mm, excluding protruding parts
Connectors: BNC.
You can see my WSPR page here.