If you catch a grizzled old radio amateur propping up the bar in the little hours, you will most likely get the gravelly-voiced wisdom of the Ancients on impedance matching, antenna tuners, as well as LC networks. Impedance at RF, you will learn, is a Dark Art, for which you requirement a lifetime of experience to master. as well as presumably a taste for bourbon as well as branch water, to protect the noir aesthetic.
It’s not strictly true, of course, however it is the situation that impedance matching at RF with an LC network can be something of a pain. You will determine as well as simulate, however you will always discover a hold of other environmental elements getting in the method when it comes down to achieving a match. much tweaking of values ensues, as well as most likely a bit of estimating just exactly how poor a specific voltage standing wave ratio (VSWR) can be for your circuit.
If LC circuits aren’t for you as well as you have lots of RF power to play with, there is of program one more method to protect impedance matching, as well as it’s one in which you’ll never have to modify a recalcitrant inductor nog een keer. just utilize a resistive attenuator, as well as put in sufficient power to compensate for the truth that a few of it will be lost as heat. Your impedances are set by resistor values, which are reliably offered over a big range.
Pi network attenuator circuit. SpinningSpark (CC BY-SA 3.0).
A Pi network attenuator is a easy three-resistor circuit, as shown in the diagram to the right. Both input as well as output are terminated by resistors, in this situation R1 as well as R3, as well as the degree of coupling, or attenuation depending which method you want to look at it, is set by R2. From an impedance point of view each end sees an impedance equivalent to its termination resistor in parallel with a resistor made by the other two resistors in series. In method for high degrees of attenuation in which R2 is rather large, the overall impedance as seen from the outside has a tendency towards that of the terminating resistor, so for example if R1 as well as R3 were each 50 Ohm, as well as R2 was sufficiently large, the impedance seen at each end would still be quite close to 50 Ohm.
This residential or commercial property of Pi networks in which R2 is much larger than R1 or R3 likewise has a side impact which is the point of this article. If you were to eliminate R3, the impedance as seen across R1 would be equal to R1, while if you were to short R3 totally the impedance as seen across R1 would still be quite close to the value of R1. therefore not only does a resistive Pi network offer impedance matching at the cost of attenuation, it likewise offers a determine of isolation in the event of a considerable impedance mismatch. therefore you can utilize a Pi-network attenuator to isolate your RF generator from the adverse VSWR impacts of a serious mismatch, as well as produce a bench RF source that is efficiently bulletproof as well as can be linked to any type of impedance without damage. The injection clamp shown in our February function on EMC testing utilizes an attenuator for just this purpose.
So exactly how do you determine those resistor values? The formulae are easily available, as for example on the Wikipedia page, so there is bit point in regurgitating them right here as if we weren’t just pretending to be an authority while simply cut-and-pasting them. Of much more utilize though are a hold of on the internet calculators that are just a Google browse away. many of them will enable you to input favored resistor values, as well as to modify for the very best results to in shape your needs.
If you’ve never utilized an attenuator for this purpose, we hope you’ve had your eyes opened to the possibilities they offer, as well as we’ve liberated you from the tyranny of the LC circuit when it concerns quick matching of RF sources on your bench. perhaps in a far-off future where grizzled old subspace amateurs down synthehol in a holodeck bar, the wisdom Of The Ancients will include attenuators for a somewhat lazier version of impedance matching.
Attenuator image: Miikka Raninen [Public domain].