In both VHF and UHF single-station and multistation antennas, the use of two transmission lines for feeding the upper and lower halves of the antenna is common. While most of these systems work well, some experience coverage issues caused, in most cases, by an incorrect differential phase.
A mistake that broadcasters often make is to assume that if the two transmission lines are installed with the same overall lengths, then they must be in phase. Physical measurements will not necessarily provide confirmation of the relative phase between the lines. An electrical measurement is the only accurate method for confirming the relative phase.
In some cases, electrical measurements are made by using a network analyzer and installing a short at the top of the transmission lines. This is a reflected system, in which the phase of one line is subtracted from the other and divided by two to determine the differential phase.
There are two main concerns with this type of system. First, the type of short used is typically a Type N coax screwed onto a reducer-mounted N coax connector. Ideally, the short should instead be bolted to the transmission line and consist of a flat slab with an inner connector bolted to it. Second, the reflected signal may see a different phase going down than it does going up to the short, particularly if the measurement includes several elbows.
A more accurate procedure, a “direct” or “through” measurement system, involves feeding a signal from a signal generator to both transmission lines, in phase, at the antenna end. A network analyzer is connected to the two transmission lines at the transmitter end. The analyzer is operated in the external source auto mode. In this mode, the analyzer locks to the received signal from the signal generator. The analyzer is in phase format, and the zero degree reference line is the middle of the display.
The frequency of the signal is displayed in megahertz on the analyzer display, allowing the user at the transmitter end to verify that the correct frequency is being used at the antenna end. This is important in multistation systems where several frequencies are used.
Regardless of the process used, it is necessary to know if there is one or more multiples of a wavelength in either transmission line referenced to the other. Time domain measurements of each transmission line can be used to initially determine the approximate physical lengths of the lines.
The measuring system
The basics of a through measuring system include a signal generator,power divider, transitions from N female connectors to the correct transmission line size and impedance, and a network analyzer. A UPS power source is used to power the generator if 110V is unavailable. A battery-powered signal generator is also an option. A block diagram of the basic system is shown in Figure 1.
Having the smaller, lightweight signal generator at the antenna end of the line and the network analyzer at the transmitter end makes it easier to work with. The UPS supply at the antenna end can be smaller due to lower power requirements. Because a signal generator takes less expertise to operate than a network analyzer, it's more suited for the antenna end, where a nontechnical person is likely to be doing the work.
Any signal generator capable of generating a CW signal and output of 3dBm is all that is required. When measuring a multistation antenna, however, it is nice to be able to set up and store the required frequencies when calibrating the system on the ground. This allows the tower worker to choose preset buttons to change frequency — a much easier process than setting each frequency via a knob on the tower's signal generator.
The following description walks you through an actual measurement on a three-station DTV antenna system.
The two transmission lines are measured using time domain measurements to determine the length in feet of each line. Figures 2 and 3 are expanded plots of these measurements. Note that the lower transmission line is 24in longer than the upper transmission line. This data will determine how many degrees longer, at each of the three frequencies, the lower transmission line is compared with the upper transmission line.
The measuring system was set up on the ground and calibrated at the three station center frequencies. The measuring system consists of an Agilent N9310A signal generator with a Type N power divider connected to the signal generator output, four transitions from the transmission lines to N connectors, and an Agilent 8753ES Option 10 and 11 network analyzer. A notebook computer, connected to the analyzer, stores the measurement data.
The transitions, cables and instruments have been match-marked in such a manner that the system could be connected on the tower and at the transmitter end in the same manner as when calibrated on the ground. The transitions are labeled upper transmission line and lower transmission. Figure 4 displays the signal generator with the power divider and two cables installed. Also note the display showing the three stored station frequencies.
The upper and lower transitions were connected together for calibration. The signal generator had the three frequencies stored, and the network analyzer was set up in phase format and external generator auto. The system was calibrated at all three frequencies, and this information was stored in the analyzer.
At this time, the individual who was going to be on the tower went through the selection of the frequencies as he would when on the tower. Once everyone understood the process, the system was disassembled and taken to the tower. In this sample case, 110V was available on the tower, so the UPS supply was not required.
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The two transitions at the input of the two transmission lines were installed, and the network analyzer was connected to them. With the signal generator powered up on the tower, the engineer on the ground directed the tower worker to select a frequency. This frequency was also selected from the calibration file in the network analyzer. The differential phase then appeared on the network analyzer display. This procedure was repeated for the other two channels. The measurement data was stored in the computer and labeled differential phase as found. (See Figures 5, 6 and 7.)
The time domain measurements indicated that there was an extra wavelength in the lower transmission line at all channels. Knowing this and the differential phase, we calculated the need for a 22.6in section of transmission line in the upper line. Once this was installed, we repeated the differential phase measurements on each channel.
Ideally, the differential phase should be zero degrees at each channel. However, due to the number of frequency-sensitive components (such as tuned elbows), the variation was not unexpected and, when compared with calculated plots of the antenna patterns using these phase differences, the variation was negligible and acceptable. (See Figures 8, 9 and 10.)
When to make measurements
Since the through measurement system displays the differential phase in real time, any change is immediately reflected on the network analyzer display. When making the measurements in the example measurement above, a drift in the differential phase over time was noted. Investigation revealed that the sun exposure was on the transmission line feeding the lower antenna half, while the upper half of the transmission line was shielded. (See Figure 11.) As the sun moved across the sky, the phase increased 20 degrees and then gradually returned to normal.
Because the sun cannot be turned off like a light bulb, the only other option was to make the measurements on a cloudy day or at night. The effect of this condition on signal strength can be calculated. This was done in our case, and it was determined that it was not a problem, as shown in Figure 12.
Each situation can be different, so it is recommended that the measurements be made at night or on a cloudy day so that the “as found” measurements and “final” measurements are done at the same temperature.
In the example measurement above, the sun affected the lower transmission line due to the shielding on the tower legs. This resulted in downward beam tilt. In other systems, the same scenario could have caused upper line expansion and upward beam tilt. In other systems it could affect both transmission lines, causing the beam tilt to swing up and down.
Regardless of the environmental circumstances of an antenna installation, the through measurement system provides more accurate measurements than other measuring systems that use shorts, line stretchers and physical measurements.
Dean W. Sargent is president of D W Sargent Broadcast Service.
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