A friend is asking if I can fabricate a low-cost SWR/power meter with accurate performance up to 144MHz. One of the designs I have attempted is the modified Bruene directional coupler from Roy Lewallen, W7EL.
The above circuit represents a portion of W7EL's original article. I have extracted only the directional coupler circuit, as we intend to employ an analog 200uA meter for indication.
The toroidal transformers were made from FT37-43 cores, using 10 turns in the secondary winding and a single turn in the primary winding. While wire thickness is not critical, I have opted for #24awg enamel wire for the secondary winding. The two diodes employed are a closely matched pair of 1N4148, and the two resistors are rated at 47 ohms (1/2 watt). Measures have been taken to maintain minimal lead lengths, and the compact PCB housing the circuit is conveniently positioned adjacent to the SO239 connectors.
The meter circuit incorporates toggle switches and calibration trimmers essential for precise meter calibration.
Toggle switch S1 serves the purpose of selecting between forward and reflected power, whereas toggle switch S2 facilitates the choice between power and SWR modes for measurement.
During the calibration process, I am using my Diamond SX200 meter as a reference standard, and my ICOM IC-2200H is employed to verify the power readings for both power meters. Additionally, I have prepared three distinct dummy loads – each with impedance values of 50 ohms, 75 ohms, and 100 ohms respectively. These dummy loads are necessary components for SWR calibration.
Meter calibration using SX200 as reference standard.
Actual dummy load used in the swr calibration.
Testing power response at 88MHz to 108MHz.
Testing the power response from 26MHz to 27MHz.
Testing the power response from 136MHz to 150MHz,
Assembling this project is relatively straightforward for me; however, the calibration process presents challenges. Notably, around 150MHz, there is a noticeable decline in power response. It appears that this might already be pushing the upper limits of my power meter's capabilities. Moreover, I've observed toroid transformer heating, particularly during continuous 60W testing.
I extend my heartfelt gratitude to my friend, Sir Leandro, for placing trust in and providing unwavering support for this undertaking. --- 73 de DU1VSS
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