Tweaking the antenna system

July 1, 2008

The last link in the broadcast chain under the broadcast engineer's control is the transmitter and antenna system. From then on quality of reception is left in the hands of the listeners. Best quality includes not only audio, but also signal strength to overcome noise and interference. It's not very satisfactory to transmit the highest quality audio if the signal is weak and below the required field strength level.

Engineers in non-directional, one-transmitter stations should not have great difficulty ensuring the signal is up to par. In connection with audio modulation it is advisable to use a ‘scope on the transmitter output to judge modulating conditions visually. Flat topping on peaks will show clearly, as will excessive negative modulation. Sometimes unanticipated modulation artifacts are brought to light on a ‘scope.

It's always a good idea to remeasure the base operating impedance of the single vertical antenna when checking overall station performance. Although the original license may have shown a base operating resistance of 55 ohms, it may no longer be correct today. Sometimes guy insulators develop cracks, or may even break sufficiently to add a few feet to the antenna's electrical length. Base insulators have also been known to crack and affect antenna impedance.

Figure 1. A typical diplex installation. The F<sub>1</sub> and F<sub>2</sub> filters are set for pass (P) or reject (R).

Figure 1. A typical diplex installation. The F1 and F2 filters are set for pass (P) or reject (R). Click image to enlarge.

Sometimes a cracked insulator, or in a worst-case condition, a length of guy wire from a broken insulator added to the antenna, will cause intermittent noise in the signal and generally change the base operating impedance match. Such an occurance can change the operating condition of the final stage and may affect the RF output.

When satisfied with the operating conditions of the antenna system, it is time to check the efficiency of the transmitting system. I like to feel transmission lines for hotspots, which are good indicators of impedance mismatches in the RF system. Their significance varies with the power of the system. This is a situation where an inline operating bridge is an essential piece of equipment, as well as close visual inspection. Depending on what is discovered, it may be necessary to readjust the ATU.

The ATU should be examined carefully looking for overheated inductors, which can usually be spotted by their discoloration. Capacitors should be felt for over heating (be sure power is off first) and suspicious components should be replaced. Sometimes shorted turns in a matching network may get hot and waste power. In networks calling for negative reactance it is better to use a single capacitor of the desired value, if possible. The usual practice of using extra negative X and obtaining the desired value by inserting inductive X (i.e. an adjustable coil) to cancel the unwanted negative X can sometimes be less satisfactory. Unfortunately variable gas or vacuum capacitors are expensive.

Many engineers run quick mental checks by multiplying final plate voltage by plate current to find dc plate power. Then they check this against the antenna RF power using I2ant × Rant to determine final stage efficiency. If this is in agreement with previous figures and/or the manufacturer's, and the other checks are satisfactory, one can be reasonably sure the system is performing properly. It is important, when performing this check on a directional station, to be sure to use the correct FCC power percentage allowance for the transmitter power involved.

Directional antenna systems

Generally, fine tuning a directional system involves much the same work as in a non-directional except that there is more of it. Some of the critical measurement values involved are common point impedance and current. Their significance is similar to antenna-based operating impedance and current for a non-directional system. In each case the combination of current and impedance determines the RF operating output power. In the case of the non-DA system, adjustment of the ATU affects only the antenna match. However, in the case of a directional antenna, adjustment of a single ATU can impact the whole system, and affect the common point characteristics. It is very important to remember that almost any change in an ATU or the phasor settings will probably change the common point current and or impedance. It's also important to apply the correct FCC allowed operating power tolerance when you try to calculate RF power in a directional antenna system.

Non-directional multiple transmitter installations require adjustment of considerably more equipment. In addition to the basic transmission line system there will be an ATU for each frequency, together with additional pass and reject filters to be checked and retuned as necessary. This is an area where impedance mismatches can be catastrophic.

Additional filtering in the form of pass and reject filters will also be required. Figure 1 shows a typical diplex installation. Each ATU should have a separate equipment cabinet to prevent against stray RF pickup. A separate antenna lead will go to the output of each ATU.

At the input side of the ATU for transmitter 1 (F1) a pass filter tuned to transmitter 2 (F2) shunts any undesired F2 signal to ground. Between the ATU output and the antenna is a reject network tuned to F2. This prevents F2 signals from entering the F1 ATU. If the pass and reject filters are not precisely tuned to the required frequency, internal cross modulation will occur in the final stage of the transmitter.

The operation of the ATU and the filtering system for transmitter F2 is similar with the exception that the reject filter between the ATU output and the antenna and the pass filter at the ATU input are both tuned to F1 to prevent any F1 transmitter signals entering F2 transmitter. Occasionally additional pass or reject filtering is required depending on transmitter frequency difference and power.

Because two or more different frequencies are feeding the same antenna, it is particularly important to ensure there are no corroded antenna connections, or tower joints, that can operate as rectifiers and generate interference to both stations and others. It is very important to check for spurious signals and cross modulation products in the desired signals.

Multiplexed directional

Fine tuning multiplexed directional antenna systems requires the same degree of attention to filter network adjustment as for non-directional installations, but there is a lot more work. Each individual antenna will have its own ATU to be adjusted to meet the DA specifications plus pass and reject networks and its own phasor. When there are so many networks involved tuning interaction sometimes occurs and adds to the time and work involved.

It goes without saying that ATUs should be separated in view of the many possible undesired sum and difference signals that can be unintentionally generated.

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