High-power RF Tuning

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Dec 3, 2010 3:44 PM, By Russell Brown

Even a newly commissioned delivery line is not perfect. You shaft together the new sections properly, setup all ammo and tie all the bolts, but still impedance imperfections result in neglected VSWR or reflected power. Imperfections exist in every elbow, prong connection and transition, and these imperfections are voiced in impedance discontinuities. The best delivery line would present an expect impedance tie in to the RF outlay (transmitter) and to the bucket (antenna). In that situation, all power from the outlay would be eliminated to the bucket with usually the loss from attenuation next to to the length of the delivery line run. With a best impedance match, no power would be mislaid as it was reflected back to the source and no high voltage spots would be generated by a high voltage station call proportion (VSWR), that can result in bend inside of the delivery line.

Unfortunately, best impedance matches do not exist; the many that may be completed is a shut tie in where many of the appetite is eliminated to the bucket and the VSWR is kept to a minimum.

Tuning a line

A delivery line's impedance is commanded by its earthy parameters, i.e., the size and spacing of its components. Because of this, to melody or arrange the impedance of a indicate inside of a delivery line, its earthy make up must be altered.

At low frequencies, such as those used for AM radio, the wavelengths are really long, that requires considerable gadgets to start the RF signal. To announce the AM signal, the whole tower(s) is used to illuminate the RF signal, and infrequently just a siren acts as the delivery line to the tower. As you pierce up in frequency, the wavelengths obtain shorter and simpler to correlate with or alter. Rigid persuade and waveguide may be tuned by putting a hole in them. Striking a delivery line with a produce will unquestionably start its impedance since it changes the earthy measure of the line. This is a of the reasons why waveguide may be tough to work with: Its considerable size creates it simpler to damage whilst installing it, mainly the thin-wall variety. Rigid persuade is harder to hole due to its thick walls and turn shape, but if it is dented, its impedance is altered at that spot.

In essence, all you have to do to change the impedance of a delivery line is to hole it. That hole changes the spacing of the externa; to middle transmitter in persuade or between the tip and bottom bowls of the waveguide. Interacting with the rest of the components of the delivery line, station waves may be created, consequent in aloft internal voltages and loss of power eliminated to the load.

To manage the impedance to emanate a improved tie in to the subsequent to components, you must be able to manage the change and the frequencies affected. This is where delivery line tuners advance in.

Measurements

To know where to place the tuners, measurements must be done to establish where the impedance mismatch is occurring. A network analyzer is used for this purpose; it is specifically written to measure the impedance and, thus, the reflected power inside of an RF device such as a delivery line.

In general, an RF network analyzer uses two various types of displays when creation measurements. The initial is formed on the Smith map out display impedance, and the second uses an X-Y display to uncover SWR (reflected power). These tests are performed using a assessment vigilance that covers the operation of frequencies routinely carried in the delivery line, such as your TV channel.

The Smith map out display shows a array of circles that indicate by size and chain the impedance of the part being tested. The tighter and more centered the circles, the closer the impedance tie in is.

The X-Y display shows the width of the SWR on the Y scale and time or stretch on the X scale. In this mode, you would routinely see all the prong connectors as tiny peaks at the bottom of the display, spaced out at periodic intervals. Any considerable VSWR errors would uncover up as incomparable straight peaks. By knowing the magnitude of the assessment signal, the stretch may be distributed to any disparity from the monitoring point, thus pinpointing the complaint area. (See Figure 1.)

The X-Y display display SWR is used to examine delivery lines to the receiver when seeking for bad ammo in stiff persuade or unsuited prong connections. This sort of assessment is performed when a new delivery line network is commissioned to authorize its performance. You would moreover wish to perform this assessment if you saw the reflected power levels stepping up on your transmitter for no strong reason.

The Smith map out display is used for adjusting the tuning or relating of the delivery line. The many familiar places tuning is compulsory is where a array of elbows are installed, at transitions from waveguide to persuade or vise versa, at the finish of a long run of delivery line and at the submit to an antenna. All of these areas usually require impedance relating to upgrade their ability to give as ample RF power as probable to the subsequent to device by the use of RF tuners.

Tuners

Once the mark that requires relating is located, you must be able to arrange the impedance of the delivery line inside of the bandwidth of fascination in a tranquil way (i.e., not denting it). This is where tractable tuners are used. Tuners are nothing more than steel probes that are extrinsic in to the delivery line with tractable height and spaced is to magnitude being examined.

The tuners are usually placed 4 in a quarrel spaced one-fourth wavelength apart. For example, on Channel 33, whose magnitude rope is 584MHz-590MHz, the core magnitude is 597MHz with a one-fourth wavelength of 4.94in. (See Figure 2.)

Once the division of delivery line just before where the mismatch occurs is removed, a set of 4 tuners would be commissioned in to the delivery line spaced 4.94in apart. Not all the tuning holes will be used, but it's ample simpler to setup all four, casing an whole wavelength, and then examination to see that ones will start the relating the most. One way to assessment this is to use a screwdriver. The screwdriver is extrinsic in to the holes whilst the analyzer's display is monitored, and as long as the screwdriver is heartwarming the externa; steel of the delivery line, it will start the relating as would the real tuner. Once the scold location of the tuners is determined, the real tuners are commissioned and practiced for depth. Many times, however, the real tuners are commissioned and practiced to find the scold location without initial contrast with a screwdriver. (See Figure 3.)

On stiff coax, the tuners are commissioned with the use of 3/8in nuts soldered over the holes. Sometimes just a shaft is screwed in to deed as the tune, but other times a special paddle tuner is used. A second bulb is used to lock the shaft in place. If the line is pressurized, you have to be certain the nuts are soldered all the way around and rubber washers are used to make the shaft wise airtight. The new holes are hermetically sealed with partial bolts to keep mud out and nitrogen in. (See Figure 4.)

For waveguide, special tuners are used that are ample bigger, but the way of designation and composition is the same.

Conclusion

The expertise compulsory to investigate and arrange high-powered RF systems takes years of practice and experience, but do not let that stop you from learning. If you have access to a network analyzer, pick up how to use it and it will help you to improved comprehend the world of RF.