To: File

From: Jon Hagen

Date: 5-25-00 expanded 10-20-00

Subject: Fine tuning the waveguide collector

Preliminary measurements of the new collector indicate that its return loss is better than 15dB.

We want to fine-tune the device to make it reflectionless at 430 MHz. The tuning device will be an inductive post, installed in the output stack. The location for this post is determined as follows: The collector is placed so that it is free to move in a spare section of slotted waveguide. The input end of the slotted guide is terminated with an ordinary WR2100-to-coax transition and a coaxial load. We can safely assume that the return loss of this termination is better than 15 dB , i.e. | |< .18. We calibrate the network analyzer with our half-height WR2100 cal kit (three waveguide shorts: 0", 6", and 12") and then measure the reflection coefficient seen at the plane of the stack's flange.

Because the termination of the slotted line is not perfect, the observed reflection coefficient will vary as we move the collector along the slotted guide - it will trace out a circle whose center is effectively at S11 of the collector, the parameter we need to measure in order to determine the depth of the tuning post behind the flange. This "sliding load" technique is analyzed in Appendix 1.

Once we have located S11 on the reflection plane, we rotate counterclockwise around the origin, until we intersect the g=1 circle r = - cos ) in the lower (capacitive) half plane. If this rotation is, for example, 200 degrees, we would insert the post 100 electrical degrees behind the flange or 36.3" x 100/360 = 10.1". (The guide wavelength at 430 MHz is 36.3"). This procedure accurately determines the longitudinal position of the post. The lateral position must be determined experimentally, moving the post laterally and holding it in place with a large C-clamp outside the waveguide while the measurement is being made. When the post is in the correct position, the reflection coefficient will orbit the origin of the reflection plane as the sliding load is moved. The tuning can be confirmed by moving the network analyzer to the transition on the slotted line and installing a perfect load on the collector. (The cal kit can be used to prepare a load that is perfect at 430 MHz.) As the collector is moved along the slotted guide, the observed reflection should remain stationary in the reflection plane.

Appendix 1: Sliding load technique

In the figure below, the 2-port device with labeled S-parameters represents the collector. The sliding load has a reflection coefficient , which orbits the center of the reflection plane as the load is extended, i.e. as the collector is pushed along the slotted waveguide..

Let us find as a function of : By inspection we see that the forward wave into the DUT, V1 , can be expressed as

It follows that

If we let represent the position of the collector (extension in electrical length - radians) we can write

as | |ej2 . The observed reflection coefficient will therefore be

This expression forms a circle in the complex reflection plane. One can confirm, using Mathcad for example, that the circle is very well centered at S11, as long as | | and |S22| are both less than .2. i.e. less than about -14 dB.

Note added 10-20-00

The above procedure allows us to locate the proper position for a tuning bar in the collector. Once the collector has been tuned, its reflection can be measured in a more direct fashion as follows:

Use the network analyzer to prepare a good half-height waveguide load for the collector. This has usually been one of our half-height trasistions (the green one) with a 50 ohm coaxial load and a single type N

right-angle bend. Putting the bend between the transition and the load lowers the reflection loss measured at the waveguide flange from 22dB to 26 dB. Install the load on the output stack of the collector.

Connect the network anlyzer to the full height transition at the end of the piece of slotted line. Calibrate the analyzer as follows:

Select the the WR2100 user cal kit. This kit asks for three shorts, "S1" is a flat plate "S2" is a 6-inch bucket, and "S3" is 12-inch bucket. While we don't have such shorts for slotted guide, we can

fool the system. Remove the coaxial load from the transistion on the collector stack. The resulting open circuit will provide a 100% reflection. As one moves from the collector back into the slotted guide,

there will be planes that appear as perfect shorts. That is, the open collector can be considered a sliding short, even if we don't know the position of the short. Pick any position and call it S1. Move the collector 6" farther from the analyzer and call that position S2. Move another 6 inches and call that position S3. Calibrate the analyzer using these three "shorts". By not knowing the real positions of these shorts, we have effectively inserted an unknown length of line between the analyzer and the device under test, the collector. The trace on the polar chart will simply be rotated about the center. Radial distances all remain the same, however, so the "log mag" display will be correct.