To: file

From: Jon Hagen

Date: Feb. 1, 1999

Subject: 430 waveguide tests - reflections and loss

TDR measurements

Figure 1 shows how the HP network analyzer was used in time domain node, looking into the waveguide through a coax-to-waveguide transition on the mezzanine in the U-turn position before the harmonic filter. The analyzer was set to sweep from 420 to 450 MHz with 801 analysis points.

Figure 2 shows the TDR result when the Y switch is set to connect the line feed to the radio astronomy receiver, leaving the waveguide shorted. The large reflection at 3.852 microseconds is clearly due to this short.

In Figure 3 the Y switch is set to connect the line feed to the waveguide. The reflection is reduced considerably. It does not disappear altogether, however, so we see that the line feed is not perfectly matched to the waveguide. The distance corresponding to a round-trip propagation time of 3.852 microseconds is 1434 ft. This follows from the fact that the guide wavelength for WR2100 at 430 MHz is 0.922 and the fact that the product of the phase velocity and the group velocity is c2. The reflection near zero delay is probably due to the EH tuner but may also be partly due to the coax-to-waveguide transition used to connect the network analyzer.

Figure 4 was made with the waveguide again shorted by the Y switch. In addition, the EH tuner was adjusted to minimize the close-in reflection. The close-in reflection is still considerable. The analyzer was calibrated by disconnecting it from the waveguide so find the reference line for mag S11 = 0dB. The reflection from the Y switch has a relative amplitude of -7.1 dB. We expect the waveguide to have a round-trip loss of about 1 dB, not 7.1 dB. The measured value, 7.1dB is almost certainly influenced by the remaining uncorrected close-in reflection.

Figure 5, however shows log mag S11 vs frequency. Again the Y switch presents a short to the waveguide. The tuner is still in the position that minimized the close-in reflection. The close spacing of the ripples is due to the very long line.

Figure 6 shows log mag S11 over a smaller frequency range, 429-431 MHz. The ripples crest at - 1dB which is what we would expect if the round trip loss is 1 dB. (See the attached MathCad analysis where S11 is calculated for the shorted waveguide with a shunt impedance at the input. Lowering the value of the shunt impedance deepens the ripples; the crests remain at -1 dB.

This plot suggests that the line is in good condition. A direct power transmission test, made to verify this, is described below.

Direct transmission test shows 1.25 dB loss in waveguide system.

A Hewlett Packard HP8648 generator was set to produce a 430Mhz sine wave with an output level of +10dBm. A 10dB attenuator was added to ensure that the output impedance was 50 ohms. The generator was check at this level with a spectrum analyzer; 2nd and 3rd harmonics were down more than 30dB. A Hewlett Packard HP8481A power meter was used to read the power. Plugging the meter into the generator/attenuator produced a reading with an error of only .03 dBm. Reducing the generator power by 5 dB reduced the power meter reading by 4.97 dB. We could therefore trust that the power read at the platform would be accurate to better than .05 dB. The power meter was taken to the carriage house and connected to a waveguide-to-coax transition that temperorily replaced a directional coupler next to the line feed. With the meter head in place, the EH tuner was varied to minimize the relection seen at the input transition in the transmitter room. The EH tuner brought the reflection down to better than -15 dB. A simple half-inch stub at the generator reduced the reflection to -26dB. The system was thus tuned so that it would accept all of the available power i.e. 1 mW. Plugging in the generator, the reading in the carriage house was observed to be -1.66 dBm. From this we subtract 0.41 dB, the loss of the piece of RG214 connecting the generator to the waveguide transition to calculate a net waveguide loss of 1.25 dB.

Of this, we expect 0.48 dB from 1400 feet of WR2100. This leaves 0.77 dB to be accounted for. Presumable this 0.77dB loss is distributed among the remaining components: the harmonic filter, the corrugated waveguide sections, the rotary joint, the five-probe tuner, the slotted line, and the travelling collector. We should continue these measurements to identify these various losses..