Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - What about the low-frequency harmonics?
A serious problem for the amateur newcomer.
Although most hams these days worry about TVI from their v.h.f. harmonics, W2VMX points out that even more serious trouble can be caused by the low-order (second, third, etc.) harmonics that may interfere in channels used by important communications services. This should be of special significance to the Novice working on the 80- and 40-meter bands, since the second harmonics fall outside the bands assigned to amateurs.
The problem of harmonics in amateur radio is by no means limited to the matter of TVI. The v.h.f. harmonics which so irk the XYL and the neighbors have simply received more attention in recent years. Yet there are many hams on the air today radiating strong signals, outside amateur bands, that do not bother TV sets. Too often, the first inkling of trouble in these cases is a letter from the FCC. ARRL Official Observers are sending many post cards to amateurs in this category. It is sincerely hoped that this service will save many hams from FCC notices. Our station alone has sent cards to several hundred different amateurs heard outside amateur bands in the last few years.
If you would have first-hand knowledge of the situation, sweep the spectrum from 7350 to 7500 kc. on any Sunday afternoon. We have heard harmonics so thick in this region that some stations couldn't be copied because of harmonic QRM from others.
Granted that a problem exists, what are we going to do about it? Let's begin by asking where this unwanted energy comes from. A harmonic frequency is simply an integral multiple of some given frequency called the fundamental. For instance, if your transmitter operates on 3705 kc., the second harmonic is at 7410 kc., and the third is at 3 × 3705 or 11,115 kc. Regardless of the kind of transmitter you use, whether homemade or store-bought, small amounts of energy at these harmonic frequencies are present in your final tank along with the useful energy at the fundamental frequency. Such energy can create problems whenever (1) there is a large amount of harmonic energy generated or (2) when conditions exist under which the harmonic emergy may be easily radiated. Either condition alone may be bad; the combination is an invitation for FCC action.
The files of our station contain quite a number of letters and comments from the stations to whom we have sent ARRL Official Observer cards. These letters show that the chief reasons for the harmonics we have heard are, in order, carelessness, ignorance of the situation, and the inability to correct the problem. Let's go through the most common mistakes and see whether we can avoid these pitfalls.
The first question in chasing low-frequency harmonics should be this: To what band is the final tank circuit tuned? Many Novices are building bandswitching rigs. Many others have built rigs in which the final tank capacitor, without any change in the tank inductor, will tune, for instance, to both 3.7 and 7.4 Mc. In the case of the bandswitching transmitter, the danger always exists that the operator will, without thinking, leave a 3.7 Mc. crystal in place when the bandswitch is turned to the 7 Mc. band. A frequency-doubling action takes place and the full output of the transmitter is then radiated on a frequency outside the amateur bands. In the case of the transmitter which will tune both bands without replacing the inductor, the operator must at all times beware of the resonance point which uses the lesser capacitance of the tuning capacitor. A special case of this same trouble may exist where the transmitter is operated from a crystal or VFO in the 160-meter region. In such a transmitter it is sometimes possible to tune a multiplier stage to the third harmonic around 5.4 Mc., instead of the intended second harmonic near 3.7 Mc. Again, it sometimes happens that the output of a doubler stage tuned to 3.7 Mc. may contain sufficient energy at 5.4 Mc. to drive a final so that it will show a pronounced dip at resonance at 5.4 Mc.
What can be done to make sure the final is tuned to the right band? The FCC regulations provide that the frequency of the transmitter must be checked from time to time. Unfortunately, we can't rely on the receiver to tell us about harmonics. If we have a 3.7 Mc. transmitter right next to the receiver, we will probably hear a signal every 3.7 Mc. right up the dial. The instrument we need to be sure of the right band is called a wavemeter. An excellent unit that can be built for just a few cents was described in QST recently, complete with a cut-out dial.(1) A more elaborate and more sensitive instrument was described by another author in the February issue of this year.(2) The latter can be used to indicate the presence of very small amounts of harmonic energy at 5.4 and 11 Mc.
If we find that the final tank is not tuned to the right band, the necessary correction is easy enough. If the tank is tuned to the right place but there is enough unwanted energy to give an indication on the wavemeter, this is another problem. Then we must find out whether the unwanted signal is coming from the final or from some previous stage. Always correct the trouble on the lowest power level.
High-Q tank circuits will minimize harmonics in lower-level stages. Harmonic generation in amplifier stages can be minimized by reducing drive to the lowest practical level. Consult the Handbook for proper grid drive, and do not exceed this figure. These are just two of the ways to help cut harmonic generation in the transmitter. After we have worked on this angle, we will probably want to ask if we can cut down the radiation of unwanted frequencies. The answer in most cases is yes.
Let's make up our mind that some sort of antenna-tuning device is always in order, not only to get the greatest efficiency from the radiator, but also to cut down spurious signals. Good designs are given in the Handbook,(3) and many excellent commercially-built units and kits are on the market. Where tuners provide for a grounding arrangement, this should always be the very best and most direct ground. One special precaution is in order, based on reports we have received. Don't overcouple the transmitter to the antenna or tuner! Beginners sometimes get the idea that the more turns there are in a link, the more r.f. will be coupled to the antenna. As a result, we have heard of links of ten and twenty and even thirty turns. Remember that such an arrangement is not only a link but a very effective capacitive-coupling device - something harmonics love like mice love cheese. Any two pieces of metal separated by an insulator form a capacitor of sorts, whether these be flat plates (as in an air variable) or rolled foil (as in a paper capacitor) or in coil form. Notice commercially-made coils for the 40- and 80-meter amateur bands. Usually these have only two turns, or three at the most.(4)
Measures which you have probably already taken in connection with TVI will also help with your low-frequency harmonics. For example: A Faraday shield in the final tank link, coax from this link to the shielded antenna tuner, and a shielded link in the tuner unit itself, grounding of center taps of coils in balanced systems, and the like, are all beneficial. A low-pass filter will not have any effect whatever on low-frequency harmonics, since it is usually designed to pass everything below 30 Mc.
There are always a few special cases, and it may be that your rig seems to radiate unwanted signals despite everything that you have done to it. This is a good place for a huddle at the local club, as two heads are usually better than one at this point. If it is a manufactured item, write telling the maker exactly what happens, and follow his suggestions carefully. If your rig is built from QST or Handbook plans, you might write to ARRL. However, since ARRL has not tried and tested the rigs described in contributed articles, you will probably save time by writing directly to the authors of such articles, whose addresses appear in the author's footnotes.
Good hunting, OM, and be careful to stay in the bands!
Charles L. Wood, W2VMX.