Interior View Of Coaxial Reflectometer

The Reflectometer is a short section of transmission line containing two r-f voltmeters. Center conductor of line is a section of brass rod soldered to center pins of input and output receptacles. At either end of unit are the crystal diodes, bypass capacitors and terminals. Diode load resistors are at center of instrument, grounded to brass alignment rod.

reads the reflected component. The magnitude of standing wave ratio on the transmission line is the ratio of the incident component to the reflected component, as shown in figure 14. In actual use, calibration of the reflectometer is not required since the relative reading of reflected power indicates the degree of match or mis-match and all antenna and transmission line adjustments should be conducted so as to make this reading as low as possible, regardless of its absolute value.

The actual meter readings obtained from the device are a function of the operating frequency, the sensitivity of the instrument being a function of transmitter power, increasing rapidly as the frequency of operation is increased. However, the reflectometer is invaluable in that it may be left permanently in the transmission line, regardless of the power output level of the transmitter. It will indicate the degree of reflected power in the antenna system, and at the same time provide a visual indication of the power output of the transmitter.

Reflectometer The circuit and assembly in-Circuit formation for the reflectometer are given in figure 15. Two diode voltmeters are coupled back-to-back to a short length of transmission line. The combined inductive and capacative pickup between each voltmeter and the line is such that the incident component of the line voltage is balanced out in one case and the inductive component is balanced out in the other case. Each voltmeter, therefore, reads only one wave-com ponent. Careful attention to physical symmetry of the assembly insures accurate and complete separation of the voltage components by the two voltmeters. The outputs of the two voltmeters may be selected and read on an external meter connected to the terminal posts of the reflectometer.

Each r-f voltmeter is composed of a load resistor and a pickup loop. The pickup loop is positioned parallel to a section of transmission line permitting both inductive and capacative coupling to exist between the center conductor of the line and the loop. The dimensions of the center conductor and the outer shield of the reflectometer are chosen so that the instrument impedance closely matches that of the transmission line.

Reflectometer A view of the interior of the Construction reflectometer is shown in figure 16. The coaxial input and output connectors of the instrument are mounted on machined brass discs that are held in place by hrass alignment rods, tapped at each end. The center conductor is machined from a short section of brass rod, tapered and drilled at each end to fit over the center pin of each coaxial receptacle. The end discs, the rods, and the center conductor should be silver plated before assembly. When the center conductor is placed in position, it is soldered at each end to the center pin of the coaxial receptacles.

One of the alignment tods is drilled and tapped for a 6-32 bolt at the mid-point, and the end discs are drilled to hold 1/2-inch ceramic insulators and binding posts, as shown in the photograph. The load resistors, crystal diodes, and bypass capacitors are finally mounted in the assembly as the last step.

The two load resistors should be measured on an ohmmeter to ensure that the resistance values are equal. The exacit value of resistance is unimportant as long as the two resistors are equal. The diodes should also be checked on an ohmmeter to make sure that the front resistances and back resistances are balanced between the units. Care should be taken during soldering to ensure the diodes and resistors are not overheated. Observe that the resistor leads are of equal length and that each half of the assembly is a mirror-image of the other half. The body of the resistor is spaced about Vs-inch away from the center conductor.

Testing the The instrument can be ad-Reflectometer justed on the 28 Mc. band.

An r-f source of a few watts and nonreactive load are required. The construction of the reflectometer is such that it will work well with either 52- or 72-ohm coaxial transmission lines. A suitable dummy load for the 52-ohm line can be made of four 220 ohm, 2 watt composition resistors (Ohm-ite "Little Devil") connected in parallel. Clip the leads of the resistors short and mount them on a coaxial plug. This assembly provides an eight watt, 55 ohm load, suitable for use at 30 Mc. If an accurate ohmmeter is at hand, the resistors may be hand picked to obtain four 208 ohm units, thus making the dummy load resistors exactly 52 ohms. For all practical purposes, the 55 ohm load is satisfactory. A 75 ohm, eight watt load resistor may be made of four 300 ohm, 2 watt composition resistors connected in parallel.

R-f power is coupled to the reflectometer and the dummy load is placed in the "output" receptacle. The indicator meter is switched to the "reflected power" position. The meter reading should be almost zero. It may be brought to zero by removing the case of the instrument and adjusting the position of the load resistor. The actual length of wire in the resistor lead and its positioning determine the meter null. Replace the case before power is applied to the reflectometer. The reflectometer is now reversed and power is applied to the "output" receptacle, with a dummy load attached to the "input" receptacle. The second voltmeter (forward power) is adjusted for a null reading of the meter in the same manner.

If a reflected reading of zero is not obtainable, the harmonic content of the r-f source might be causing a slight residual meter reading. Coupling the reflectometer to the r-f source through a tuned circuit ("antenna tuner") will remove the offending harmonic and permit an accurate null indication. Be sure to hold the r-f input power to a low value to prevent overheating the dummy load resistors.

Using the The bridge may be used up to Reflectometer 150 Mc. It is placed in the transmission line at a convenient point, preferably before any tuner, balun, or TVI filter. The indicator should be set to read forward power, with a maximum of resistance in the circuit. Power is applied and the indicator resistor is adjusted for a full scale reading. The switch is then thrown to read reflected power (indicated as A, figure 14). Assume that the forward power meter reading is 1.0 and the reflected power reading is 0.5. Substituting these values in the SWR formula of figure 14 shows the SWR to be 3. If forward power is always set to 1.0 on the meter, the reflected power (A) can be read directly from the curve of figure 14 with little error.

If the meter is adjusted so as to provide a half-scale reading of the forward power, the reflectometer may be used as a transmitter power output meter. Tuning adjustments may then be undertaken to provide greatest meter reading.

34-7 Measurements on Balanced Transmission Lines

Measurements made on balanced transmission lines may be conducted in the same manner as those made on coaxial lines. In the case of the coaxial lines, care must be taken to prevent flow of r-f current on the outer surface of the line as this unwanted component will introduce errors in measurements made on the line. In like fashion, the currents in a balanced transmission line must be 180 degrees out of phase and balanced with respect to ground in order to obtain a realistic relationship between incident and reflected power. This situation is not always easy to obtain in practice because of the proximity effects of metallic objects or the earth to the transmission line. All transmission line measurements, therefore, should be conducted with the realization of the physical limitations

Figure 17

SKETCH OF THE "TWIN-LAMP" TYPE OF S-W-R INDICATOR

The short section of line with lamps at each end usually is taped to the main transmission line with plastic electrical tape.

of the equipment and the measuring technique that is being used.

Measurements on One of the most satisfactory Molded Parallel- and least expensive devices Wire Lines for obtaining a rough idea of the standing-wave ratio on a transmission line of the molded parallel-wire type is the twin-lamp. This ingenious instrument may be constructed of new components for a total cost of about 25 cents; this fact alone places the twin-lamp in a class by itself as far as test instruments are concerned.

Figure 17 shows a sketch of a twin-lamp indicator. The indicating portion of the system consists merely of a length of 300-ohm Twin-Lead about 10 inches long with a dial lamp at each end. In the unit illustrated the dial lamps are standard 6.3-volt 150-ma. bayonet-base lamps. The lamps are soldered to the two leads at each end of the short section of Twin-Lead.

To make a measurement the short section of line with the lamps at each end is merely taped to the section of Twin-Lead (or other similar transmission line) running from the transmitter or from the antenna changeover relay to the antenna system. When there are no standing waves on the antenna transmission line the lamp toward the transmitter will light while the one toward the antenna will not light. With 300-ohm Twin-Lead running from the antenna changeover relay to the antenna, and with about 200 watts input on the 28-Mc. band, the dial lamp toward the transmitter will light nearly to full brilliancy. With a standing-wave ratio of about 1.5 to 1 on the transmission line to the antenna the lamp toward the antenna will just begin to light. With a high standing-wave ratio on the antenna feed line both lamps will light nearly to full brilliancy. Hence the instrument gives an indication of relatively low standing waves, but when the tic (|) |lL IL I t 1 =

Figure 18

OPERATION OF THE "TWIN LAMP" INDICATOR

Showing current flow resulting from inductive and capacitive fields in a "twin lamp" attached to a line with a low standing-wave ratio.

standing-wave ratio is high the twin-lamp merely indicates that they are high without giving any idea of the actual magnitude.

Operation of The twin lamp operates by the Twin-Lamp virtue of the fact that the capacitive and inductive coupling of the wire making up one side of the twin-lamp is much greater to the transmission-line lead immediately adjacent to it than to the transmission-line lead on the other side. The same is of course true of the wire on the other side of the twin-lamp and the transmission-line lead adjacent to it. A further condition which must be met for the twin-lamp to operate is that the section of line making up the twin-lamp must be short with respect to a quarter wavelength. Then the current due to capacitive coupling passes through both lamps in the same direction, while the current due to inductive coupling between the leads of the twin-lamp and the leads of the antenna transmission line passes through the two lamps in opposite directions. Hence, in a line without reflections, the two currents will cancel in one lamp while the other lamp is lighted due to the sum of the currents (figure 18).

The basic fact which makes the twin-lamp a directional coupler is a result of the condition whereby the capacitive coupling is a scalar action not dependent upon the direction of the waves passing down the line, yet the inductive coupling is a vector action which is dependent upon the direction of wave propagation down the line. Thus the capacitive current is the same and is in the same phase for energy travelling in either direction down the line. But the inductive current travels in one direction for energy travelling in one direction and in the other direction for energy going the other direction. Hence the two currents add at one end of the line for a wave passing toward the antenna, while the currents add at the other end of the twin-lamp for the waves reflected from the antenna. When the waves are

Figure 19 SWR BRIDGE FOR BALANCED TRANSMISSION LINE

A double bridge can be used for two wire transmission lines. Bridge Is inserted in line and may be driven with grid-dip oscillator or other low power r-f source.

strongly reflected upon reaching the antenna, the reflected wave is nearly the same as the direct wave, and both lamps will light. This condition of strong reflection from the antenna system is that which results in a high standing-wave ratio on the antenna feed line.

Use of the The twin-lamp is best

Twin Lamp with suited for use with an-

Various Feed Lines tenna transmission lines of the flat ribbon type. Lines with a high power rating are available in impedances of 75 ohms, 205 ohms, and 300 ohms in the flat ribbon type. In addition, one manufacturer makes a 300-ohm line in two power-level ratings with a tubular cross section. A twin-lamp made from flat 300-ohm line may be used with this tubular line by taping the twin-lamp tightly to the tubular line so that the conductors of the twin-lamp are as close as possible on each side of the conductors of the antenna line.

34-8 A "Balanced" SWR Bridge

Two resistor-type standing wave indicators may be placed "back-to-back" to form a SWR bridge capable of being used on two wire balanced transmission lines. Such a bridge is shown in figures 19 and 21. The schematic of such an instrument (figure 20) may be compared to two of the simple bridges shown in figure 13. When the dual bridge circuit is balanced the meter reading is zero. This state is reached when the line currents are equal and exactly 180 degrees out of phase and the SWR is unity.

As the condition of the line departs from the optimum, the meter of the bridge will

Figure 19 SWR BRIDGE FOR BALANCED TRANSMISSION LINE

A double bridge can be used for two wire transmission lines. Bridge Is inserted in line and may be driven with grid-dip oscillator or other low power r-f source.

show the degree of departure. When the line currents are balanced and 180 degrees out of phase, the meter will read the true value of standing wave ratio on the line. If these conditions are not met, the reading is not absolute, merely giving an indication of the degree of mis-match in the line. This handicap is not important, since the relative, not the absolute, degree of mis-match is sufficient for transmission line adjustments to be made.

Bridge A suggested method of con-

Construction struction of the balanced bridge is illustrated in figures 19 and 21. The unit is constructed within a box measuring 4" x 6" x 2" in size. The 0 - 200 d.c. microammeter is placed in the center of the 4" x 6" side of the case. The input and output connectors of the instrument are placed on each end of the box and the internal wiring is arranged so that the transmission line, in effect, passes in one side of the box and out the other with as little discontinuity as possible. The input and output terminals are mounted on phenolic plates placed over large cutouts in the ends of the box, thus reducing circuit capacity to ground to a minimum value. The "SWR-CAL" switch Si is located on one side of the meter and the "Calibrate" potentiometer Ri is placed on the opposite side.

The transmission line within the unit is

Was this article helpful?

0 0

Post a comment