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RE: coupler
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by W8NF on November 27, 2003
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There are two approaches.
The one requiring some calculation, but ends up easy to build is to simply design a 1/4 wave coupler using edge-coupled microstrip, and then it's merely an etching exercise. Peak voltage is the risk you need to watch for. If you design it for "weak" coupling, say 40dB, then the two microstrips will have a relatively large amount of spacing between them. If you get the board commercially etched, you can have a solder mask applied, making sure to cover the entire area where the microstrips are close together. This will improve the voltage withstand, while contributing about a 2% reduction to the characteristic impedance - which you can compensate for by actually designing for 51 ohms, not 50. Also, the required spacing will be farther apart if you design it to work on 1/8" material, instead of the more standard 1/16".
If, by VHF, you mean 6 meters and 2 meters, you can design a broadband coupler quite capable of this power level using the traditional two-transformer approach. The fundamental technique is covered nicely in W7ZOI's book "Introduction to RF Circuit Design", available from the league for $40 or so. It uses two broadband transformers to sample voltage and current. The voltage transformer's output connects to the center tap of the current transformer's output, and then the two output leads of the current transformer provide samples that can be correlated with forward and reflected power. By selecting the magnetic cores, insulation thickness of wires used for winding, and so forth, this sort of coupler can be designed for literally any power level. And they can be made extremely broad; a variation of this design is used as the directivity sampler in network analyzers with an effective bandwidth of 45 MHz to 50GHz.
A popular variation of the two-transformer design subsitutes a capacitive voltage divider for one of the transformers. This is known as the Bruene Bridge (named after the developer, who was, if I recall correctly, a Collins employee in the 1940s), and you can find many links to it via a search engine.
73,
Dave W8NF
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