For the variable capacitor there are a number of options: vacuum-variable, air-variable, or switched fixed-capacitor. for large values of inductance) I'll use coils wound on iron powder toroids.ĥ. To maximize Q, I'll like to have as many of these inductors as possible be air wound. So instead I'm going to follow the same route Elecraft uses with their KAT500: individual fixed inductors whose values increase by powers of 2 and are selected via relays. These can have low Q's (see: ), and I never know if I should short the unused portion of the coil or leave it open (leaving it open can result in very high induced voltages at the open end, and shorting it might reduce Q, although this seems to be open to debate). Because I want to minimize power loss, I don't want to use a roller inductor. What component values will I need? Here's a table I generated showing the max L and C values required to match an impedance anywhere on a specific "Smith Chart SWR Circle" to an SWR of 1:1.Ĥ. So, the L-Network is the way to go, in my opinion.ģ. One of my goals is to minimize power loss inside the tuner. The T-Network can have smaller-valued components, but there's a tradeoff associated with this - more power can be wasted within the T-Network tuner for a given load (assuming L and T networks have equivalent component Q's). If you've read my previous Antenna Tuner posts, you'll see that I much prefer the Lowpass L-Network topology to, say, T-Networks. This will save me from adding a large inductor that would be required for 160 meters.Ģ. I don't operate on 160 meters (nor on 6 meters), so I'll focus on 80 through 10 meters (and if 6 meters happens to become included, great). So let's get to started! First step - nail down the initial tuner specifications.ġ. After all, this design is a work in progress, and from my experience, rarely does the finished design match its initial specifications - there are tradeoffs that will come up during the design process, and the design will change to accommodate those tradeoffs. In this first post I will set out the initial Tuner specifications and why I chose them.īut you should expect the design to change from post to post. I've now started the design! This blog post is the first of a series of posts I have planned in which I will log the progress of the design as well as my design process. Specifically, I would like to build an Automatic Antenna Tuner. For the past months I've been looking at various tuner topologies and their tradeoffs, my ultimate goal being to actually build a tuner.
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