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I designed a Colpitts oscillator with a single transistor amplifier stage. I now have to connect the antenna to the circuit through a coaxial cable. Do I have to redesign the output impedance of the circuit to match the impedance of the coaxial cable? Or is it better to just add a buffer stage and design that to match the coaxial?

By match, I mean Input Impedance = Output Impedance.

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For maximum power transfer to the antenna you should match your output circuit to the impedance that your circuit "sees" at the output: cable + antenna.

Coaxial cables have an impedance of 50/75/100 Ohm no matter how long the cable it is.

Antennas have an impedance of 50/75 Ohm only if you install the antenna according to the manufacturer's specifications. For example you may be asked to put a ground plane.


Example:

if your "cable + antenna" system has, at the frequency of your oscillator, an impedance Z:

Z = 44 + 12j

Than you have to add an impedance matching circuit that has an impedance Z*:

Z* = 44 - 12j

Use a coaxial connector to bring the signal from the circuit to the coax cable.

The matching circuit is usually made of L and C components.


There are many programs (some are free, some are open source, some are on-line, some need a commercial license) that can build your matching network.

Optenni Lab

MatchCalc™ RF Design Calculator (QORVO)

RF Impedance Matching Calculator (Analog Devices)

You may also Google this: "RF matching circuit synthesis program" or "Synthesis of passive LC networks to match complex loads to sources"

Optenni is considered the king of all.

Radio Amateurs have their program but I forgot the name.

I got no affiliation to any of the companies I mentioned.


To measure the impedance of your "cable + antenna" system you need an antenna tester or a Vector Network Analyzer.

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