Thursday, June 25, 2015

Antenna Auto-tuner Design, Part 1: Preliminary Specification

For the past months I've been looking at various tuner topologies and their tradeoffs, my ultimate goal being to actually build a tuner.  Specifically, I would like to build an Automatic Antenna Tuner.

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.

In this first post I will set out the initial Tuner specifications and why I chose them.

But you should expect the design to change from post to post.  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.

So let's get to started!  First step -- nail down the initial tuner specifications...

1.  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).  This will save me from adding a large inductor that would be required for 160 meters.

2.  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.  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).

One of my goals is to minimize power loss inside the tuner.  So, the L-Network is the way to go, in my opinion.

3.  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. 

(click on image to enlarge)

4.  Because I want to minimize power loss, I don't want to use a roller inductor.  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).

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.  (Their power-loss numbers are very nice.  See this review:

To maximize Q, I'll like to have as many of these inductors as possible be air wound.  But where necessary (e.g. for large values of inductance) I'll use coils wound on iron powder toroids.

5.  For the variable capacitor there are a number of options:  vacuum-variable, air-variable, or switched fixed-capacitor.  Because of its inherent high Q, the vacuum-variable is tempting, but -- as you can see in the table above, a 10:1 SWR on 80 meters can require a maximum capacitance of about 2700 nF to match it (depending where on the Smith Chart's "Circle of Constant SWR" the load lies).  2700 nF is much larger than any of the vacuum variables I have in my junk box.  So I plan, again, to follow the Elecraft (and SGC, and LDG) route -- fixed capacitors selected via relays.  Just like the inductors.

6.  As for power ratings -- I don't operate really high power, so to keep inductor and capacitor specifications reasonable I'd like them to meet the max voltage and current requirements for an operating power of 200 watts continuous (e.g. AM carrier), 800 watts peak (e.g. SSB) at an SWR of 10:1, which should easily meet my operating requirements.

Here's a table showing those voltage and current requirements:

(click on image to enlarge)

For the inductors, I will assume that heating of the iron powder toroid cores is the main issue, so max Vrms is the important value (if using a program such as Mini Ring Core Calculator to determine power loss and flux density).

Capacitor Voltage Rating is usually in terms of Vpeak across the cap, so I need max Vpeak and also max RMS current (Irms) -- the latter affects capacitor heating (i.e. power loss).

7.  And finally, this tuner will be an Automatic Antenna Tuner (Auto-tuner).  I'll probably use a PIC processor to control tuning, which will give me a chance to learn to program one and play around with tuning algorithms!

So, to summarize:

K6JCA Auto-tuner, Preliminary Specifications:
  • Frequency Range:  3.5 - 30 MHz, continuous (and perhaps to 54 MHz, if it's free)
  • Matching Network Topology:  Lowpass L-Network
  • Load Matching Range:  Match to 1.2:1 (or better) for any load with a 10:1 SWR.
  • Input Power:  200 watts average, 800 watts peak.  ICAS operation.
  • Internal Tuner Power Loss:  (Unspec'd.  Design to minimize).

Now to select the tuning network inductors and capacitors.  I'll go through this process in the next post.  It can be found here:

Links to my blog posts in this Auto-tuner series:

Part 1:  Preliminary Specification

Part 2:  Network Capacitor Selection

Part 3:  Network Inductor Selection

Part 4:  Relays and L-Network Schematic (Preliminary)

Part 5:  Directional Coupler Design

Part 6:  Notes on Match Detection

Part 7:  The Build, Phase 1

Part 8:  The Build, Phase 2 (Integration of Match Detection)

Part 9:  The Build, Phase 3 (Incorporating a Microcontroller)

Part 10:  The Final Schematics


Inductors, W8JI discussion:
Inductors, Roller, W8JI (part way down this post):

Antenna Tuners, W8JI:
Antenna Tuners, G3YNH:

Review, Elecraft KAT500:

Software, Toroid Inductor Calculator (including loss), Mini Ring Core Calculator:

My Other Tuner posts:

A quick tutorial on Smith Chart basics:

The L-network:

A correction to the usual L-network design constraints:

A look at highpass T-Networks:

More on the W8ZR EZ-Tuner:  (Note that this tuner is also discussed in the highpass T-Network post).

The Elecraft KAT-500:
 The Nye-Viking MB-V-A and the Rohde Coupler  :

Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And so I should add -- this design and any associated information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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