One of the major lamentations of the small- to mid-size market engineer is the dearth of test equipment available for their use. While this problem may not always affect a contract engineer with a robust portfolio, or a group chief, it tends to be especially acute for our brothers and sisters functioning as employees. This month we look at some of the test gear used on the RF side of AM antenna systems.
I have always thought of monitoring a directional antenna system as something akin to a three-legged stool: When all three legs are there, and in good condition, you can plant your keister with confidence.
The three simplest pieces of test gear for a directional AM antenna system are the current meter, the field strength meter and the phase monitor; and although no longer required under commission rules, well-engineered stations will maintain and rely on their base current meters. Regular measurements with these items will ensure the health of the array and help you to spot problems.
|The AIM 120 is a fits-in-your-briefcase VNA that works in conjunction with a laptop or PC.
Let’s take a look at each of these items.
Changes in current meter values, with no corresponding power change, are indicative of an impedance change. If noted at the common point, the impedance change could be something simple in the trim circuit, which matches the actual common point to the transmitter impedance; or it could be a more complex issue elsewhere in the array.
From the base currents, inferences to the ground system and seasonal shifts can sometimes be made from the absolute values, while the current ratios are indications of pattern shape.
Baseline impedance values measured at the phasor output and ATU inputs can also serve to aid in quickly identifying the location of a problem should it develop. In other words, when the array is all tuned up and working nicely, make reference impedance measurements at those locations, and note them in the log. They could be very useful down the road.
Phase monitor inputs can occasionally be affected by lightning. An easy check on phase monitor health is to take the reference input and parallel it to the other inputs with tee adapters and short lengths of coax. A healthy monitor will indicate very similar magnitude readings across all inputs, with some minor variations in phase. Several degrees of change in phase are to be expected with multiple adapters and short cable lengths.
Monitor point values at variance from typical norms, or in excess of licensed values, should be investigated. In some cases, the array may be experiencing seasonal variations. In others, environmental factors may have changed in the vicinity rendering the current limit invalid.
|Jim Hatfield Sr., taking field measurements for KIRO, about 1941.
The use of the impedance bridge further quantifies the condition of the system. Bridges come in two different flavors, which can be referred to as “cold” and “hot” or “operating” versions. The two most well known cold bridges are the General Radio models 1606-A and 916-AL. Both of these bridges require the use of an external driver and detector, and are to be used only by the very low power levels provided by generator/detector combos. The two most common models of that are the Delta Electronics RG-4B and Potomac Instruments SD-31, which provide both in one box. Alternately, an RF signal generator may be utilized as a driver, with a field strength meter being used as a detector.
In order to measure impedance values under operating conditions, a “hot,” or operating impedance bridge is utilized. Several varieties of these are manufactured by Delta Electronics including the Common Point Bridge and OIB families. The CPB family has three different power levels (5, 50 and 100 kW) available, intended for permanent installation at the common point of the phasor. These models have a measured resistance range of 30 to 100 ohms and a reactance range of plus/minus 50 ohms at 1000 kHz.
On the portable side is the OIB family. The initial model is the OIB-1, which covers resistance values of up to 400 ohms, and reactance values of up to 300 ohms at 1000 kHz. The newer version is the OIB-3, which expands the resistance range to 1000 ohms either side of a short, and the reactance range to +/- 900 ohms at 1000 kHz. Both of these models will handle up to 5 kW of modulated power, or up to 10 kW dead carrier, and also function as a cold bridge with a RF driver/detector combo.
Since the GR and OIB bridges follow slightly different designs, they present reading values differently. The OIB requires no user balancing, and provides direct readouts based on the dials and a combination of extender switches. The indicated dial value for reactance must be multiplied by the frequency in MHz, while no conversion is necessary for resistance. Similarly, the GR bridges require no conversion for resistance, but the reactance conversion is one where the dial reading is divided by the frequency in MHz.
|The AIM 120 not only collects the data but also facilitates the generation of
graphs — helping the user through its analysis.
Additionally, the GR bridges require balancing at the frequencies of operation, and the reactance reading is relative to the balance point. A typical balance of the bridge occurs by setting the resistance to zero ohms, and the reactance dial to some value that will cover the expected measurement range. For example, assuming a station at 1000 kHz and a measured reactance range of +/- 300 ohms, we would set the dial for 300 ohms, and then with the bridge grounded, adjust the initial balance knobs for a null. This means that an impedance of 0+j0 ohms is indicated by the bridge as 0+j300 ohms. Put another way, 300 must be subtracted from the measured reactance value to obtain the actual reactance. Since we are testing at 1000 kHz, no additional conversion of the reactance is required.
As they become more cost effective, the vector network analyzer is finding more and more use in the AM station of today. Perhaps the most cost effective VNA for AM use is the Array Solutions family. In particular, the Power AIM-120 is specifically designed for use for broadcast measurements. The analyzer itself, which runs almost forever on a gel cell battery, is slightly larger than two packs of smokes laid side-by-side, and is controlled by a PC platform computer such as a laptop or tablet. The unit can also function as an impedance bridge using tones, has distance to fault, and generates Smith Charts. Due to the single port configuration and software design, this unit is generally limited to a cold bridge type of measurement.
The balancing of the GR bridges discussed functions like calibration. While the OIB bridges do not require user calibration techniques, the network analyzers do. Calibration of the analyzer establishes the reference plane, and eliminates effects of connectors and cabling up to that point. Typically, calibration standards come with a fixed connector, such as the “N” type. Thus, when calibrating for AM use, which more often than not utilizes alligator type clips, attaching the clips to the standard directly with care is care is a good practice. Although some stray inductive and capacitive effects remain due to clip placement, their impact will tend to be much smaller than connecting the standards directly to the analyzer port, and ignoring the cable entirely. This can become very critical in situations where high reactance values are present, such as on skirted towers, or those with multiple isocouplers.
From the simplest current meter all the way to the most complicated network analyzer setup, a story is told.
Network analyzers are verbose and provide much detail, but like a complexly woven tale, details can be overlooked or misinterpreted. A current meter, like a haiku, lacks details, but conveys a powerful message. Even if the library is incomplete, much can still be learned from what is available.
Ruck is the principal engineer of Jeremy Ruck and Associates, Canton, Ill.