Reflectometer for VHF UHF high power – Larcan Splitter Combiner Scrap – Up to 1kW

Reflectometers are an important component in a VHF / UHF solid state power amplifier. As well as providing a visual monitor of output power and VSWR, the reflectometer provides inputs to SWR protection circuits. As the power output level and the operating frequency increases, a homebrewing design which is reliable and repeatable is not easy.

During some projects with surplus xTV solid state finals, I recognized the original splitter combiners were being discarded. These boards were designed to handle up to kilowatt RF levels. Some included reflectometers for monitoring in the original application.
Tests of some of the reflectometers cut from splitter combiner PCBs showed low SWR, good balance (reflected voltage close to or equal 0 on dummy load terminations), and acceptable output voltages at power levels as low as 100W.

The photos show a section of a Larcan combiner PCB. The wide tracks are 50 ohm impedance. These tracks when reinforced with a copper shim will easy handle 1kW power levels.

When terminated by 100 ohm, the thinnest tracks provide a balanced coupling loop. At higher frequencies (23cm) a small brass shim flag improves the sensitivity and balance.
In order to accommodate a range of power levels, operating frequencies, and output voltages, a technique was developed where the splitter combiner was cut into sections and then joined by a copper plate sweated onto the ground plane side of the PCB.

The maximum length of through 50 ohm stripline is determined by the lengths available in the original PCB. The sensitivity of the coupling loop is set by the spacing from the through transmission line and the length of the coupling sections. At lower frequencies / lower powers a coupling track can be placed down each side of the transmission line section.
The following annotated photos show the construction of a 400W 23cm reflectometer to drive a dual SWR meter.

The maximum length of through 50 ohm stripline is determined by the lengths available in the original PCB. The sensitivity of the coupling loop is set by the spacing from the through transmission line and the length of the coupling sections. At lower frequencies / lower powers a coupling track can be placed down each side of the transmission line section.
The following annotated photos show the construction of a 400W 23cm reflectometer to drive a dual SWR meter.

VK4AMG Jan 2016