Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/articles/tuneup/qrm.txt

Text File | 1996-06-30 | 12KB | 256 lines

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.