Preventing On-the-Air Tuneup QRM R, P, Haviland, W4MB Amateurs have recently increased the practice of tuning up on the air, which has reached such a level that it's often impossible to work a DX station or participate in a net. Many tune-up signals are weak, but others are far stronger than the desired signal. Such practices are not only annoying, they're also illegal. An excerpt from the International Telecommunications Union regulations (693) states in part "All stations are forbidden to carry out ... unnecessary transmissions... the transmission of superfluous signals and correspondence . . . (and) the transmission of signals without identification. " Observation and inquiry show that three factors are involved in the development of on-the-air tune up prac- tices: the characteristics of modern transmitting equip ment, lack of knowledge about corrective steps, and the "me first" approach too common today. This article is devoted to the second factor, corrective steps, with some discussion of the reasons why they are necessary. One of the characteristics of many modern commer- cially built exciters, transceivers. and linear amplifiers is that they lack dial scales. Some may have a mark at 9. 12. and 3 o'clock, or even at every hour position, but many are blank. As a result, it's almost impossible to preset operating controls with any degree of accuracy. The only way to obtain proper performance is to tune up with reference to meter readings. Since amateurs want full output, it's no wonder that on-the-air tuneup has become so common. The solution to this problem is simple -- install useful dials. Here, useful means a dial that can be read to about one degree of are or so. It certainly means one that's large enough to get your fingers on, is easy to read, and has no backlash. For many controls it also means a vernier knob. Most users of commercially built equipment have put up with the lack of useful dials from fear of spoiling equipment resale value. This shouldn't be a worry, be- cause it isn't necessary to change appearance, drill holes. or take other steps that will reduce resale value. Also no great amount of work is necessary. On most transmitters or amplifiers only three controls are involved: typically drive, tune and load. A couple of hours of work should take care of most cases. The simplest approach is to remove the old knobs and replace them with dials having calibration marks close enough to be useful, typically 0-100 over an arc of 180 degrees. Many styles are available in junk boxes, surplus outlets, and stores. However, it's difficult to find small dials with the necessary calibrations. If you're fortunate enough to obtain such dials, put the removed knobs in a cloth bag and tie it inside the equipment where it won't get lost. This saves no end of trouble when it's time to sell or trade. Another simple approach is to install a calibrated dial plate under the control mounting nut. These plates were common for many years but are scarce now. They can be made using dry-transfer kits similar to those for lettering, or the center portion of a piece of polar graph paper, suitably lettered, can be used to give calibrations each 2 degrees. Dial plates can also be hand drawn. Best appearance is obtained by making the drawing several times full scale then reducing it photographically. Both these methods have one disadvantage: they don't have vernier action. Small vernier dials are usually available from Radio Shack, Olson or Lafayette in 1'/2. 2, and 3 inch (3.8, 5.1, and 7.6cm) sizes. These dials can be mounted on a false panel section using countersunk screws. The assembly can be held in place by the control nut or by an adjacent nut as shown in Fig.1 by bending around the edge of the panel, or by a drilled hole hidden by the original knob. OST's "Hints and Kinks" contain other mounting ideas. Another method, which involves considerable work but which offers the opportunity to obtain a "custom station," is to make a complete dummy front panel. Custom features such as combinations of equipment, special lettering. engraved calls, and so on can be had by this method. It's a good method for organizing the clut- ter of auxiliary controls that most stations accumulate. Whatever the method used, an additional item needed is a setting log or card giving the proper setting of each control for each preferred frequency or for band seg- ments on the low frequencies and bands on the high frequencies. Plastic envelopes are convenient for these logs. Antenna Tuner The ability to return to a known dial setting is a big help but doesn't fully compensate for another character- istic of modern transmitters and amplifiers, the fact that they're designed to work into a 50-ohm load with an swr no greater than 2:1. An easy way to see how restrictive this can be is to consider the effect of antenna Q. The Q will be around 70 for a typical close-spaced Yagi on 7 MHz. The response will be nominally within 3 dB for a change of +/-50 kHz. However, the Q response curve must be multiplied by a factor, k, which takes into account the allowable swr. The way this factor varies with swr is shown in Fig. 2. The maximum range of transmitter adjustment is used up with a frequency change of only +/-37 kHz. Even with low-Q antennas such as a wide- spaced Yagi with a Q of around 7, where the transmitter adjustment range covers 350 kHz or so, there will be measurable difference in transmitter performance with a frequency shift of 30 kHz. No wonder on-the-air tuneup has become so common! If we look at the reason for this, we find that the culprit is the reactance caused by operating away from resonance. For example. the resistance and reactance values for a typical dipole are shown in table 1. For a one-per cent change in frequency, (70 kHz at 7 MHz). the radiation resistance changes by about 6 ohms, or less than ten per cent. The reactance change is over six times as large numerically. The solution to the problem introduced by this reac- tance change is simple - use an antenna tuner. Further- more, since antenna reactance causes most of the swr change. we can use a single variable element. The other antenna tuner elements can be fixed. The improvement possible by this reactance control method is evident from Fig. 3. The upper curve is the swr expected with a typical wire dipole fed with a 50-ohm line. The swr reaches 2:1 for a one-per cent change in frequency. By simply cancelling the reactance, the swr seen by the transmitter changes to the lower curve. Its minimum swr is now lower, reaching 1.0; and 2:1 is reached only after a 5 1/2 per cent frequency change. Note that the minimum swr occurs at a "different frequency as far as the trans- mitter is concerned. Note also that the swr on the line does not change: it reaches about 7:1, causing increased voltage across the line, which may cause breakdown with small line and high power. The high swr also causes a small increase in line loss, usually entirely negligible. The gain in frequency flexibility is far more important than these small problems. The easiest way I've found to attain this single con- trol action is shown in Fig. 4. The pi network is conven- tional, with the arms continuously adjustable. The dif- ference is in its use. L, C1 and C2 are preset to the band used and only C1 varied as frequency is shifted within the band. The indication for proper adjustment is a zero reading on the reflected power meter, which is an ARRL Handbook meter with only the reflected power ele- ments connected. C1 is first preset to the table value, then adjusted as required when transmission begins. For a matched load, proper adjustment occurs with the reac- tance of each arm equal to the line impedance. This also gives the element values for a switched arm network. Dummy Loads Good dials and a single-control matching unit are a big help in solving the tune-up problem, but it's still comforting to know that everything is tuned up on the nose. We can have this capability without radiating by adding another element - a dummy antenna automat- ically inserted into the transmitter output when the transmitter tune control is activated. A circuit for this is shown in Fig. 5. The diodes prevent interaction between the external relay and the internal circuits. The switch allows the test signal to be radiated, primarily to deter- mine initial matching network settings. The dummy antenna can be a commercial or kit unit. but the large units rated at 1 kW are not needed in this service provided that tuneup is held to the few seconds needed to touch up preset controls. A dummy antenna isn't hard to build; the basic components are a handful of resistors. The following discussion is based on use of Allen-Bradley resistors. Units of other manufacture may be satisfactory, but the rating values should be secured from the manufacturer. A 2-watt A-B resistor of the twenty-per cent series is rated for a continuous load of 2 watts for 100,000 hours when the resistor is mounted with 1 inch (2.5cm) leads and has a body temperature of 212 F (100 C): this occurs when the ambient is 122 F (50 C). The life rat- ing increases by a factor of ten for a 122 F (50 C) reduction in body temperature. The allowable load in- creases by 40 per cent for a 10:1 reduction in life. For short-term loads the resistors, for the same mounting, ambient and life, are rated at 44 watt-seconds; i.e., 44 watts for one second. Suppose one of these resistors is used as a dummy antenna for a five-second tune-up checks. Over a ten- year period, if used ten times a day, the load would be applied for about 50 hours total. Since the required life under load is so short, the load can be increased accord- ingly. Also, the resistor body temperature can be decreased by mounting the resistor on heavy fins that touch the ends of the body and by immersing the fins and resistors in cooling oil. The body temperature will then be essentially ambient, certainly no greater than 122 F (50 C). As a result of these steps, the power input can be increased by 1.4 times for the reduction in temperature. and 1.44^4 times for the acceptable reduction in life, or by 5.35 times. The rating is now 10.6 watts continuous, or 47 watts for five seconds. Ten 510-ohm resistors in parallel will dissipate 470 watts for five seconds; easily full output for a 600-watt-input transmitter of the type that runs at full output on tune, and ample for a 1 kW unit that tunes at 50 per cent of full output. Forty resis- tors will handle a maximum legal input transmitter. A dummy load designed in this fashion is shown in the photo, Fig A. The container is a one-quart paint can. Fins are copper. The 2-watt resistors are separated by three body diameters in each direction, and are staggered for good oil circulation. The cooling oil is light mineral oil, but transformer oil or hydraulic fluid are usable; mineral oil is odor and stain free. The unit shown is eight years old and has been used for tests up to 30-seconds dura- tion with a 500-watt-input transmitter, repeated until the can became quite warm. Actually the rating method above seems to be quite conservative, since the change in resistance in this period has been much less than the 10-20 per cent expected. Now it is up to the operator With these three aids in place it's now possible to get on the air, correctly tuned up, without causing the slightest interference. The steps are: 1. Set the exciter, final amplifier and tuning dials to the logged settings for the band. 2. Activate the tune control, placing the transmitter on the dummy antenna, and touch up the tuning if readings aren't normal. 3. Start transmitting, observing the antenna tuner re- flected-power meter; adjust the antenna tuner if needed. These steps will result in the maximum possible signal and will also make life more pleasant for others on the channel.