Matchable Area

A tuner's primary characteristic of interest is its matchable area, i.e., the range of reflection coefficients G_L that can be perfectly matched by the tuner. The typical matchable area of a three-stub tuner is composed of two subareas (Fig. 31), one matchable by the combination of stubs 1 and 2, the other by the combination of stubs 2 and 3, stub 1 being the one closest to the generator.

Fig. 31.  Three-stub tuner matchable area.

When increasing the frequency, the two subareas can overlap (as shown in the figure), touch, or there may be a gap between them.

The radial extension of the matchable area grows with increasing the maximum stub insertion depth h_max. This defines the maximum modulus |G_L|_max of reflection coefficient that can be perfectly matched irrespective of its phase.

If h_max is high, there may be portions of the matchable area with very high reflection coefficients that can still be perfectly matched – by extremely inserted tuning stubs (areas A and B in Fig. 31). This means that the full generator power is being pumped into a space with nearly no losses, which results in an extreme buildup of the internal field strengths and surface currents. Such a situation thus represents an acute danger of electric breakdown, arcing and tuning stub structure overheating, and thus severely limits the maximum safe working power. Fore more information about these problems, please refer to the following publication:

V. Bilik and J. Bezek, “High Power Limits of Waveguide Stub Tuners,” Journal of Microwave Power and Electromagnetic Energy, vol. 44 (2010), No. 4, pp. 178-186.

The best way to avoid the described risk is to construct your applicator such that its reflection coefficient never exceeds a certain magnitude, e.g., |G_L| < 0.9. This is sometimes challenging. Next, we will discuss other options.

Limiting Stub Insertion

One method of reducing the risk of arcing is limiting the maximum possible stub insertion h_max. This can be done in two ways: as a factory setting, or as a user-definable setting.

Factory Setting

The absolute maximum h_abs of stub insertions is defined in Homer's Tun.mem file. The value can only be changed by the Homer manufacturer. The user, after receiving a modified Tun.mem file, has to run the Reinstall procedure to apply the new settings.

User-Defined Setting

For a given h_abs, the user can further reduce h_max by editing the line

MaxInsert_mm=22.7

in the Tun.cfg configuration file (for safety, this is the only available method in the live operational HomSoft version). For more details on how this can be done, see topic Homer Internal Files.

For studying the effect of varying h_max on your individual Autotuner, see topic Studying Matchable Area.

Unfortunately, as you might have observed, limiting the maximum stub insertion shrinks the whole matchable area, thus also reducing the useful and safe matchable range. Fortunately, the Homer firmware provides a way of removing only the risky portions of the matchable area by its truncating, as explained next.

Truncating Matchable Area

A full matchable area can be software-trimmed to a certain radius R_max by the following line in the Tun.cfg configuration file (for safety, this is the only available method in the live operational HomSoft version):

MaxMag_mU=850  

This defines the maximum magnitude in milliunits (mU) to be used for autotuning. In the above example line, R_max = 850 mU = 0.85. If the magnitude |G_L| of the actual load reflection coefficient

G_L = |G_L| exp(jj_L)

exceeds R_max, the tuning algorithm uses

G_M = R_max exp(jj_L)

for its input instead of G_L, as shown in Fig. 32. Thus, the critical regions of the matchable area are avoided while maintaining the full tuning performance elsewhere.

Fig. 32.  Truncating the matchable area to R_max.

For studying effects of truncating the matchable area of your particular Autotuner, see the topic Studying Matchable Area.

Studying Matchable Area

The matchable area of your particular Autotuner can be varied and studied using the HoSim simulation program.

• For customizing HoSim to behave as your tuner, see the topic Real Tuner Performance.

• To display and observe the matchable area, activate Plot in the DUT page of the Homer Model Window.

• To change the frequency for which the plots are valid, use the page More of the Homer Model Window.

Effect of Limiting Stub Insertion

The value of h_max can be changed on the Motors page of the System Info window by clicking the Max Steps/Travel row (this is only available in the HoSim simulation). The effect is immediately seen if the matchable area is displayed. You can even set the insertion deeper than is physically possible. To return to default, set zero as the insertion.

Note that this function is disabled in the live operational HomSoft version and is available only in the HoSim simulation.

Effect of Truncating Matchable Area

The value of R_max can be changed in the Autotune page of the System Info window by clicking the row Max tune Mag [milliunits]. This is only available in the HoSim simulation. The effect is immediately seen if the matchable area is displayed.

Examples

Below, Fig. 33 shows examples of matchable areas of the 2450-MHz WR340 Homer Autotuners for the following cases:

• Default stub insertion (h_max = 22.7 mm), without truncation (R_max = 1000 mU). For this case |G_L|_max = 0.77 (VSWR = 7.7).

• Stub insertions limited to 20 mm, without truncation (R_max = 1000 mU). For this case |G_L|_max is only 0.65 (VSWR = 4.7).

• Default stub insertion (h_max = 22.7 mm), the matchable area truncated to R_max = 850 mU. For this case, |G_L|_max has remained 0.77 as in the default case but the critical regions have been removed.

h_max = 22.7 mm, R_max = 1000 mU

h_max = 20 mm, R_max = 1000 mU

h_max = 22.7 mm, R_max = 850 mU

Fig. 33.  Effects of limiting the maximum stub insertions (middle) and of truncating the matchable area (bottom).