To: file

From: Jon Hagen

Date: 8-23-99

Subject: Waveguide tuning with HFSS

A half-height WR2100 waveguide was partially obstructed by a 4" diameter x 2.5" high metal pill, placed at the center of the broad wall. The center of the pill was 5" from one of of the waveguide (Port 2) and 55" from the other end of the waveguide (Port 1). For this two-port, HFSS found S22 = .4772 @ 150.494 degrees and S11 = .4772 @ 123.518 degrees. Note that if we use S22 to predict S11 we get 150.581 - 2(55-5)/36.311 + 720 = -120.854 degrees, where 36.311" is the guide wavelength at 430 MHz, the operating frequency. The HFSS results are consistent to about 3 degrees. (With the pill removed, HFSS finds S12 to be 1.000 @ 124.152 which compares with -(60/36.311)360 +720 = 125.13 to a difference of about one degree).

I. Capacitive Tuning

The Smith Chart at the right shows that if we

move out 32.012/2 degrees from Port 2, we will

arrive at the G=1 circle in the top (inductive)

portion of the chart. It should be possible to

install a shunt capacitance - another pill - at this

point that will tune out the original obstruction.

A 2" diameter pill was put at this point, on the center

line of the waveguide. Varying the height of the tuning

pill produced the following HFSS results:

Ht. of tuning pill	S22

2.5"	-11.98 dB

3.0"	-19.612 dB

3.5"	-33.5 dB

Note: the distance from the center of the cylindrical obstructing post to the center of the tuning pill is 6.614". This is 6.614/36.311 = 0.18 , showing that this obstruction is not equivalent to a shunt inductance at one point on the guide.

II. Inductive Tuning.

If we move a distance of 268.963/2 degrees from Port 2, the reflection coefficient hits the

G=1 circle in the bottom (capacitive) half of the

Smith Chart. At this point we should be able to

install a shunt inductance (a post spanning the

waveguide) that will tune out the reflection of the original obstruction. The physical distance

from the port should be

([268.963/2]/360)(36.311") = 13.57".

Putting a 1" diameter post at this distance and varying its distance from the side wall produced the following results:

Post axis-to-sidewall distance S11

2"	-7.14 dB

3"	-8.66 dB

4"	-10.25 dB

5"	-12.6 dB

6"	-17/7 dB

6.5"	-31.1 dB

7"	-24.8 dB

8"	-11.39 dB

III. Close-in Inductive tuning

The inductive tuning post was moved in by half a guide wavelength. This puts it so close to

the original obstruction that we expect the tuning to suffer. HFSS came up with a still respectable -20 dB. Moving the post another half inch inward (putting it almost exactly alongside the obstruction improved the return loss to -30 dB. The conclusion seems to be that the tuning device can be in the near field of the obstruction and still work.