Laboratorio de microondas_Medicion en lineas de tx usando lineas ranuradas

8/10/2019 Laboratorio de microondas_Medicion en lineas de tx usando lineas ranuradas

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ECE 451

Automated Microwave Measurements Laboratory

Experiment No. 2

Coaxial Transmission Line Measurements Using a Slotted Line

Introduction

In this experiment we will check the response of a crystal detector by using it to detect

energy picked up by the probe in a coaxial slotted line, and in the process explore some of

the problems with slotted line measurements.

We will check the crystal detector by using some basic transmission line theory. Recall that

the voltage at a point on an uniform lossless line at a distance d from a termination ZL, can be

expressed as:

( ) j d j dL LV d V e V e +

= +

where +LV and

LV are the voltage waves, incident upon and reflected from the termination,

evaluated at the termination.

For ZL

= 0(a short),+

= LL VV

( )( ) 2 sinj d j dL LV d V e e jV d + += = and

( ) in dB 20 log(2 ) 20log sinLV d V d +

= +

20log sinK d= +

If the slotted line is terminated in a short circuit and the amplitude of the field recorded over

at least one quarterwavelength from a minima, a plot of V(d)in dB vs 20log|sind| should be

a straight line, and for a square-law detector the slope of this line should be one. In order to

make the measurements necessary to check our detector, assemble equipment as shown

pictorially in Figure 1 and schematically in Figure 2.

The Slotted Line

The Type 874-LBA Slotted Line is designed to measure the voltage standing wave pattern

produced by any load connected to it. Its characteristic impedance is 50 ohms. The outer

conductor is slotted for a length of approximately 50 centimeters, and a small shielded probe

extends into the region between the two conductors. The probe is mounted on a carriage,

which slides along the outside of the outer conductor. The penetration of the probe into the

line and, hence, the capacitive coupling between the probe and the line, can be adjusted over

a wide range by means of a screw adjustment. Cross-sectional views of the probe

arrangment are shown in Figure 3. Since the probe is capacitively coupled to the line, the

voltage induced in the probe circuit is proportional to the voltage existing between the inner

and outer conductors of the line at the probe position. The RF voltage induced in the probe

circuit is detected by a crystal in the probe carriage.


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Figure 1

Tuning the Crystal Detector

The crystal rectifier built into the carriage is tuned by means of the adjustable stub, which is

effectively connected in parallel with it in order to increase the sensitivity and to provide

selectivity. The stub is adjusted until maximum output is indicated by the detector.

Figure 2

The General Radio Type 874-LBA slotted line and cross-section views of the carriage

showing the crystal mount and the probe are shown in Figure 3.


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Figure 3

Be sure the stub is not tuned to a harmonic of the desired signal rather than to the

fundamental. Confusion may result in some cases if the tuning is done with a high VSWR on

the line, as the minima of the harmonics may not be coincident with the minima of the

fundamental and, consequently, the harmonic content of the signal picked up by the probe

may be greater than that present in the local oscillator output. To minimize the possibility of

mistuning, the probe should be tuned with a low VSWR on the line, for instance, with the

line terminated in a Type 874-WM 50 Ohm Termination. As a check, the distance between

two adjacent voltage mimina when the line is terminated in a short circuit can be measured.

If the stub is tuned correctly, the spacing should be a half wavelength.

Probe Penetration Adjustment(Please do not adjust)

The probe penetration should be adjusted for adequate senstivity as well as insignificant

effect on the measured VSWR. The presence of the probe affects the VSWR because it is a

small admittance in shunt with the line. It has the greatest effect at a voltage maximum,

where the line impedance is high.

To adjust the probe penetration, remove the tuning stub connected to the left-hand connector

and turn the small screw found inside the inner connector (See Figure 3). Clockwise rotation

of the screw increases the coupling. In most cases in which moderate VSWR's are measured,

a penetration of about 30% of the distance between the two conductors gives satisfactory

results. For this experiment, your probe penetration has been initially adjusted to give

approximately 30% coupling as indicated below.


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To adjust the coupling to 30%, increase the coupling until the probe strikes the center

conductor of the slotted line; then back it off six full turns of the screw. The point of contact

between the probe and the center conductor is most easily measured by connecting an

ohmmeter between the inner and outer conductors of the line with the standing wave

indicator connected as shown in Figure 1. Using the 2000 ohm range on the ohmmeter, notethe point at which the resistance suddenly drops from a very high value to a reasonably low

value. The crystal is in series with this circuit, so the resistance will not drop to zero. No

indication will be obtained if the crystal has been removed. Do not screw the probe down

tight against the center conductor, as it will damage the probe.

The amount of the probe penetration can be visually checked by looking at the probe though

the slot from one end of the line.

The effect of the probe coupling on the VSWR can be determined by measurement of the

VSWR at two different degrees of coupling. If the measured VSWR is the same in both

cases, the probe coupling used has no significant effect on the measurement. If the measuredVSWR's are different, additional measurements should be made with decreasing amounts of

probe penetration until no difference occurs. However, as pointed out in the previous

paragraph, a 30% coupling usually gives satisfactory results except when the VSWR is

high, which usually requires a large coupling.

The SWR Meter

The standing wave indicator, or SWR meter, is a low-noise tuned amplifier-voltmeter

calibrated in dB and SWR. The amplifier has a 3 dB bandwidth of approximately 25 Hz

centered at approximately 1000 Hz. The RF source must be modulated with a square wave

having a frequency which corresponds to the center of the bandpass of the SWR meter. TheSWR meter is a linear amplifier and linear voltmeter with the meter scales calibrated for use

with a square-law detector. Hence a two to one change in voltage to the SWR meter is

shown as a 3 dB change; this would be a 6dB change on a conventional voltmeter.

Procedure

A) Perform the following seven measurements at each frequency setting. ULUT=Unknown

Load Under Test.

1) Location of nulls with slotted line (SL) terminated by a short

2) Location of nulls with SL terminated by unknown load under test (ULUT)

3) VSWR with SL terminated by unknown load under test (ULUT)

4) Location of nulls with SL terminated by 15 ft cable + short

5) VSWR with SL terminated by 15 ft cable + short

6) Location of nulls with SL terminated by 15 ft cable + (ULUT)

7) VSWR with SL terminated by 15 ft cable + ULUT


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B) You now have all the data you need for the following calculations:

Calc#1: Impedance of ULUT measured at the SL

Calc#2: Impedance of ULUT + cable combined measured at SL

Calc#3: Impedance of load with the effect of the 15 ft cable removed

Calc#4: Return loss and cable loss

Note: Think carefully about the reference plane for each calculation

C) Compare the results of Calculations #1 and #3 by plotting all the points on the same Smith

Chart

D) The velocity of propagation in RG 8/U is less than the velocity of propagation in free

space. One way of finding the relative velocity of propagation of a given coaxial cable is

to compare the number of half wavelengths at a given frequency in the cable to the

number of half wavelengths in the same cable when the velocity of free space (0.3m/nsec)

is assumed. Attach the male end of the 15 foot coaxial cable to the slotted line andterminate the female end of the cable with a male (Type N) short. At a frequency of 600

MHz, find and record the location of the first null from the 15 foot cable end of the slotted

line. Then increase the frequency slowly until again a null appears at the same point on

the slotted line (remember to retune the stub). From knowing that at 600 MHz (f1), you

have (N) half wavelengths from the short at the end of the cable to the first null on the

slotted line; and at a second frequency (f2), you have (N+1) half wavelengths over the

same distance, you can calculate the value of N (an integer).

)( 12

1

ff

fN

=

This is the total number of half wavelengths in the slotted line and coaxial cable at 600MHz. You now have to determine the number of half wavelengths in the cable. To do

that, terminate the slotted line with the same short (Type N) that was used on the end of

the cable. Again at 600 MHz, find and record the location of the first null on the slotted

line (don't forget to retune the stub). Calculate the distance between the two nulls (L) in

centimeters. The relative velocity of propagation in the coaxial cable is given by the

following formula:

]

]600][2[

sec/103)1[(

]54.2][12][15[10

L

MHz

cmxN

cminftVr

=

If the null with the slotted line shorted is nearer to the end of the slotted line than the null

with the cable shorted, use the negative sign. If the opposite is true, use the positive sign.

Laboratorio de microondas_Medicion en lineas de tx usando lineas ranuradas

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