I am using a twin lead folded dipole antenna for an FM receiver. I have a few questions regarding impedance matching. I know impedance matching is needed to have maximum transfer of power between stages, such as from the antenna to a band pass filter stage before the signal is feed to a RF stage. I know that the antenna has an impedance of 300 Ohms, so if I connect it a coaxial cable of 50 Ohms via n:1 balun transformer which will be connected to my pcb by a connector, do I still need to worry about impedance matching from the output of the coaxial cable connector to the input of my band pass filter? If so, do I place a 50 ohm resistor(50ohm from inductor or capacitor) before the input of the next stage to match, Or can I just connect the signal to the input of the next stage an no additional impedance network ? Also is it possible to just take the feed line of the antenna (300 Ohm) and match it with an impedance network to connect to the input of the next stage on a pbc, or would this be unwise and ineffective?
Electronic – Impedance Matching Antenna
antennaimpedance-matchingpcb-antennaRF
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Best Answer
For matching RF connections, receive or transmit, don't, whatever you do, start sprinkling resistors around thinking you will do good, because they will absorb and lose you more power than you would ever lose through mismatch reflection.
Treat each end of the transmission line independently. Match the antenna to the downlead. Match the downlead to the receiver. Obviously this will be easier if the downlead impedance is equal to at least one of those, and easiest if it equals both.
Given a choice between 50 and 75 cables for a radio downlead, 75ohms is invariably chosen. This is because it's lower loss than 50 ohm cable. It also happens to be a 4:1 impedance ratio with the 300 ohms of a folded dipole, which means an easy-to-build 2:1 balun will do the job. 300 ohms twin wire is a good choice too, if the antenna and receiver are 300 ohms.
What's the input impedance of your PCB filter? If it's intended for FM reception and it's single ended, it's almost certainly 75 ohms, if balanced it's 300. Many FM receivers have a built-in balun, so you can connect 75 coax or 300 ohm twin wire downleads.
Could you skip the filter and connect it to the next stage? What's the impedance of the next stage? Most filters are designed between equal impedances, but some are designed to match impedances from input to output over a small range of frequencies. What's yours doing?
If you don't have an FM receiver, but instead some general purpose test gear, it's more likely to have a 50 ohm input impedance. In this case, 50 and 75 are so close that you will lose little signal by connecting them directly, probably less than if you used a 50:75 ohm transformer, which is only 1.22:1 in voltage terms.