January 1953 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
My introduction to passive intermodulation
(PIM) issues was while working on a BTS switching system back around the turn of
the century (yes, this century), where because of high power levels, the matrix
had to be guaranteed to not generate PIM products greater than a certain value.
About 1/3 of the units were not passing test. The cause turned out to be a high
power attenuator in one of the paths. The manufacturer of the attenuator was a very
well established and reputable firm. The PIM levels were intermittent and would
come and go after removing and then replacing the interconnecting cables. I fortunately
noticed that the N-type connector on one end of the attenuator was rotating slightly
during the process. According to our specification to the manufacturer (it was a
custom design), the connector bodies were to be pinned to prevent movement during
torqueing. They appeared to be built properly because the small hole where the steel
pin was driven had been backfilled with conductive epoxy. When the company was told
that the connectors were rotating, they immediately blamed us (me, actually) for
abusing the connectors, since there was no way the connectors would move with the
pins in place. I shipped some of the failed units back to them, received replacements,
and darned if some of the new ones didn't do the same thing. Again I was blamed,
since the QA inspection paperwork "proved" the connectors had been pinned by virtue
of the stamp at that process step. I was sweating big time by then, but knew I had
not abused the attenuators. Fortunately, the company I worked for had an x-ray inspection
machine, so I took a good and a bad attenuator to the shop and had the technician
have a look inside. Guess what? That's right, no steel pin was visible in the bad
attenuator. All of the failed units were x-rayed and shown to be missing the pins.
The carefully documented QA process of the supplier had been fudged. I'm not sure
what happened to the people responsible there, but I imagine it was not a happy
time since they had been so arrogant and adamant toward me. Bullet dodged.
Harmonic Radiation from External Nonlinear Systems
Fig. 1 - Double-conversion arrangement for identifying and
measuring harmonic signals.
Sources, Characteristics, and Methods of Detection
By Mack Seybold,* W2RYI
This article is the first comprehensive treatment of a type of harmonic generation
that represents the last barrier - and often a nasty one - to completely TVI-proof
operation in fringe areas. When shielding and filtering on the transmitter, and
high-pass filters on the TV receiver don't do a complete job, external rectification
is the probable reason. The author, an outstanding worker in the anti-TVI field
from the beginning of post-war TV broadcasting, presents here the results of a thorough
study of the problem and describes new methods for its detection and elimination.
The old filter-builder, Ralph, W2CVF, had been passing comments during many sessions
of the Gloater's Club. Most of the boys on the 29-Mc. net were right in there pitching
with him: "How can a guy spend two or three years out there in West Orange chasing
microvolts with a butterfly net!" Or, "All you have to do down here in Englewood
is put the rig and filter in a couple of dishpans back-to-back, and you're on the
air! Why fiddle with the plumbing in the kitchen sink?"
Ralph would agree that there was a remote possibility that sinks might be involved
in TVI. We all would agree that some sinks might be above suspicion, but, of course,
there are different kinds of sinks - big ones and little ones, clean ones and rusty
ones, city sinks and country sinks.
It finally took Santa Claus to get Ralph to believe in kitchen sinks. On Christmas
Day, in 1951, W2CVF went on the air to pass along his season's greetings and to
find out who had been able to cash in on the softened-up holiday atmosphere for
double-conversion superhets. The atmosphere didn't stay soft for long in West Englewood
that day, however. Channel 2 was taking a beating from the most filtered transmitter
east of the Hackensack.
How could a horrible thing like this happen to an ardent disciple of Phil Rand,
Russ Valentine, and George Grammer? How could it happen to the chief baiter of K2CR
and W2RYI for bigger and better filter designs?
Ralph admits that he remembered the kitchen sink as soon as the unusual occurred.
Actually, the plumbing wasn't involved, but it was the same proposition in a more
refined category. Santa Claus had seen to that! The most recent change in the household
was a Christmas present to the up-and-coming junior operator, a crystal set for
the pleasure of a Christmas toy, and possibly the beginning; of a devious road to
scientific achievement or fun and fortune on the amateur bands.
No sweeter demonstration of nonlinear systems could have been prepared on the
laboratory work bench: a 29-Mc. harmonic-free transmitter, an efficient crystal
rectifier on an isolated receiving antenna, and a television set tuned to a holiday
show. Ralph suspected the crystal and, sure enough, when the cat's-whisker was lifted
from the galena, TVI ceased entirely.
Not all amateurs have had the dubious pleasure of encountering harmonics radiated
from external nonlinear systems. Many in the TV areas have found the phenomenon
at work in the first r.f. stage of TV receivers, where a strong amateur signal can
swing the grid beyond linearity and produce multiple harmonics of an original pure
signal. A few amateurs have isolated other devices that proved to be the cause of
harmonics seemingly radiated from carefully shielded transmitters, but the complexity
of the problem has baffled careful workers, and many sources of spurious radiation
are yet to be discovered.
Table I - TVI from Nonlinear Systems
Table I lists some of the many sources from which harmonics produced in external
nonlinear devices were strong enough to interfere with television reception. There
are probably hundreds of locations where harmonics are produced at a level too low
to develop visible cross-hatching, but are strong enough to measure with simple
equipment. From the evidence at hand, it is somewhat perplexing that more of the
interfering variety have not been reported. Perhaps, in time, some of the low-level
producers will develop into efficient rectifiers, and stronger harmonics will be
radiated. This development may be particularly noticeable in cases similar to the
one listed in the table wherein corroded TV antenna fittings were found to be at
fault. Corrosion conditions are accelerated in humid regions, especially where salt
pray is in the atmosphere. Some industrial centers have a slightly acidic atmosphere,
which also enhances corrosion.
Signal Strength for Interference
As usual with TVI problems, a harmonic entering a receiver at a frequency near
that of the picture carrier will not produce cross-hatching unless it is at least
1/100 as strong as the TV carrier. A harmonic measuring 100 microvolts at TV-antenna
terminals might never be seen on a kinescope located within walking distance of
a TV transmitter. The same level of harmonic measured at a fringe-area receiver,
however, could very well be equal in strength to the picture carrier, and could
produce cross-hatching as black and white as the picture elements. If all amateur
transmitters were completely single-signal radiators, the fringe-area boys would
still have more TVI problems than the amateurs in the primary coverage areas, because
harmonics from external rectifiers are produced with equal facility in both areas.
Basically, any rectifier can produce harmonics if a signal is fed to the cathode-anode
circuit. The low-order harmonics are the strongest, and the high-order harmonics
become progressively weaker unless some common resonant circuit accentuates a specific
multiple of the fundamental frequency. A good rectifier is one which has the largest
ratio of forward-to-reverse-direction current flow. A device having a low rectification
ratio will be less efficient as a harmonic producer, the ultimate being a pure conductor
which passes current equally well in both directions, producing no harmonics, or
a nonconductor which will not pass current in either direction.
TVI from Nonlinear Systems
Table II - Nonlinear Systems
Rectifiers are called nonlinear devices because of the marked discontinuity in
the graphic presentation of their conduction characteristics. Triodes and pentodes
are also nonlinear devices because the output is not exactly proportional to the
input, and a graph illustrating the operating characteristics discloses a curved
line. The range in which an amplifier is normally used is represented by such a
limited portion of the curve that it is almost a straight line, and the harmonic
output is extremely low. If the range is increased by applying a large input signal,
the portion of the curve used no longer approximates a straight line, and the output
signal, therefore, is not an exact reproduction of the input signal. The resultant
wave-shape is a composite of strong harmonics.
Nature is a prolific producer of rectifiers. Oxides and other corrosion products
of metals will frequently pass current better in one direction than in the reverse.
The efficiency of most of these compounds is low, but some of nature's products
are excellent rectifiers. Lead sulphide is the galena crystal of early radio fame.
Silicon and germanium diodes are employed in many modern applications. Copper oxide
and selenium, produced under controlled conditions, are utilized extensively for
Because corrosion of such metals as iron, copper, and aluminum proceeds uncontrolled
in nature, the rectification ability of the corrosion by-products from these metals
varies immensely. If the material can pass current slightly better in one direction,
however, and if it is part of a circuit arrangement in which r.f. signals can be
admitted and released, harmonics will be produced. The strength of the harmonic
is a function of the r.f. voltage applied, the efficiency of the rectifier, and
the resonant frequency of the system.
All of the elements required to produce a harmonic-generating system are present
in the houses and structures in the vicinity of amateur stations. If a house could
be observed with Superman's X-ray eyes, the metallic structure standing there would
be identical to the conductors that a radio wave would recognize as it passes through
the maze of pipes, wires, ducts, and fixtures. An r.f. signal produces standing
waves on the various elements of this complex receiving antenna, and wherever a
rusty or corroded joint has formed between pipes or lines crossing each other, rectification
takes place. The harmonics produced are reradiated by the same metallic maze.
If a joint is an efficient rectifier, if the r.f. voltage applied is high enough,
and if the resonant frequency of the metallic structure is appropriate, a nearby
TV antenna may receive a harmonic signal sufficiently strong to interfere with the
picture. The poker joint listed in Table I produced a 1000-microvolt 57-Mc. signal
on the feeder of a TV antenna located 30 feet from the joint and 80 feet from the
28.5-Mc. transmitting antenna.
Because manufactured rectifiers are usually the most efficient, any equipment
containing them may be suspected of producing harmonics. The access of r.f. signal
to equipment rectifiers is the limiting factor in TVI production. Other tubes, vacuum
and gas, triodes and pentodes, can also produce harmonics if sufficient r.f. reaches
them in the equipment suspected.
Locating Nonlinear Devices
Fig. 2 - Bypass filters for exposed tube circuits where
rectification can occur. L = 30 μh.; C = 1000 μμfd.
Table II lists some of the places where trouble may be encountered. Locating
natural nonlinear joints is sometimes difficult. Watching the interference pattern
on a TV receiver while pushing exposed pipes, air ducts, BX cables, etc., may disclose
the location of a joint. The intensity of the interference changes rapidly as the
joint is jarred by pushing the pipe or pounding the floor or "all behind it.
When brute-force methods fail, more exacting techniques are required, especially
when the system is located between floor and ceiling, or sealed behind plaster walls.
Fig. 1 shows a 6-inch pick-up loop fed into a TV tuner. Readings are taken
on the S-meter of the communications receiver tuned to the output of the tuner.
As a joint is approached with the loop, the S-meter on this double-conversion equipment
will indicate an increase in signal strength. The loop will not always pin-point
the source, however, because the harmonic signal may be radiating from a considerable
length of metal, such as pipe or BX. comprising the joint circuit.
As a matter of fact, the complexity of the circuits involved led to the experiments
at W2RYI to learn more about the behavior of harmonic-producing joints. If a nonlinear
system cannot be located by direct methods, knowledge of its characteristics is
helpful in the detective work necessary to isolate and eliminate the source of the
The groundwork for these experiments had been laid in 1947, '48, and '49; completely
shielded transmitters and conservatively designed low-pass filters were used, and
the worst of the natural nonlinear systems were eliminated. All vacuum and gas tubes
in exposed power supplies and control circuits had been by-passed, as shown in Fig. 2
and Table III. Some of the preliminary search for nonlinear joints had been conducted
with a portable broadcast receiver. Inasmuch as these external rectifiers will mix
signals as well as produce harmonics, the sum and difference frequencies of two
signals transmitted from antennas in the back yard are produced in the nonlinear
house circuits. Simultaneous transmissions on 27.0 and 28.4 Mc. produced a 1400-kc.
signal which could be picked up on the battery portable. In this instance, too,
the general location of the rectifying joint could be determined.
Table III - Procedure for Analysis of Harmonic Radiation
Another phenomenon proved helpful early in the proceedings. A small 60-cycle
voltage normally present on the BX sheath modulated the harmonics produced in joints
common to the BX cable, and the hum could be heard in the double-conversion set-up
or could be seen as horizontal bands having variable intensity in the interference
pattern. Joints common to water and gas pipes did not exhibit the modulation, so
one more bit of information was available to aid in isolating rectifiers.
Several techniques were used at W2RYI to eliminate nonlinear joints after they
had been located. When possible, the junction was insulated or separated mechanically.
Several BX-box connectors had to be scraped and soldered. The water connections
to the kitchen sink were inaccessible, and neither insulating nor soldering could
be accomplished without ripping up the wall, so a third method was required. Where
the pipes emerged under the sink, short copper straps were connected to by-pass
the iron-enamel-rust pipe junction.
How long can an effective shorting strap be? How far away from the transmitter
can difficulties be expected from harmonics produced by nonlinear systems? What
effect does pipe length have on production and radiation of harmonics? Does a joint
produce harmonics in direct proportion to transmitter-power input? These were some
of the questions that needed answers in the form of specific data, and the experiments
were performed to obtain those data.
Tests with Artificial Joints
A 1N34 crystal diode was utilized to simulate a relatively efficient rectifying
joint. The measurements were taken on the double-conversion set-up after it was
calibrated with a borrowed signal generator; a well-filtered TV receiver was used
to reveal the interference effects. The receiving antenna used for the measurements
was of the high-band, low-band type, with folded dipoles and reflectors. This antenna
was located 15 feet off the ground and 50 feet from the discone transmitting antenna.
The artificial nonlinear systems were also placed 50 feet from the discone, and
were located 20 feet behind the TV antenna. The picture carriers of Channels 2,
4, and 5 from New York averaged 1000 microvolts at the feeder terminals while measurements
were being taken, which makes the threshold of interference with this particular
set-up approximately 10 microvolts.
Fig. 3 - Effect of power input on harmonics produced by
Fig. 3 shows the effect of transmitter power input upon harmonic output
of the 1N34 system. The dotted lines represent exact proportionality between transmitter
power input and harmonic signal output. The deviation exhibited is small enough
to indicate that the measurements were reasonably accurate and that the voltages
applied to the crystal were not causing breakdown or saturation. Extrapolation of
the data for these curves shows that the power input to the final amplifier would
have to be reduced to 0.003 watt to stay below the threshold of interference with
an isolated nonlinear system of this efficiency.
Fig. 4 shows the effect of various lengths of wire connected to the crystal
diode. Here, as in Fig. 3, the rectifier was placed at the end of a half-wave
conductor, but the wire appendage connected to the other diode terminal was shortened
progressively as the readings were taken. It is possible that there were changing
lobes in the radiation pattern, but the major effect of appendage length is clearly
shown. That second harmonic comes whompin' out at a wide variety of appendage lengths,
and the third harmonic is not far behind!
The optimum conditions for harmonic production occur when the rectifier and appendage
are connected at a point of maximum voltage at the fundamental frequency and the
appendage is one-half wavelength at the harmonic frequency. It is reasonable to
expect the optimum level of the second harmonic to be 6 db. above the optimum 3rd
harmonic. It is interesting, however, to note that a strong 2nd harmonic is radiated
at a wide range of appendage lengths, whereas a strong 3rd is produced in a restricted
A set of conditions in which a strong 3rd harmonic exceeded the 2nd is shown
in Fig. 5. A 3rd-harmonic appendage was moved along a fundamental 1/2-wavelength
conductor. A reasonably strong 2nd harmonic was available at a number of positions,
but in a region between two and three feet from each end of the conductor the 3rd
harmonic radiation exceeded the 2nd by a voltage ratio of 4 to 1. Knowledge of the
conditions under which higher-order harmonics can exceed the strength of lower-order
radiation is valuable in sifting evidence of nonlinear systems, especially when
the systems concerned are concealed in walls or ceilings.
Fig. 4 - Harmonic radiation as a function of appendage length.
Fig. 6 is similar to Fig. 5, except that a 2nd-harmonic appendage was
employed in taking the radiation data. Naturally, the 2nd harmonic was easily produced,
but it was somewhat surprising to find that the relatively low-level end-point radiation
of the 3rd harmonic was not exceeded at other positions along the conductor.
Other combinations of conductor and appendage were tested, but the three described
above give the basic characteristics of the systems. Various-sized loops with a
crystal inserted were tried. Reasonably strong signals were produced, but loop circumferences
varying from 16 feet down to a few inches produced no extreme resonances. The largest
of the loops produced harmonic output in the 1000-microvolt range.
A feeder signal of 4000 microvolts collected on an antenna 20 feet from the radiating
system can be expected to produce about 2000 microvolts at 40 feet, and 1000 microvolts
at 80 feet. Interference to the TV receiver at W2RYI was, of course, appreciable
during the tests of artificial nonlinear systems, and occasional complaints came
in from around the neighborhood when the signals were kept on for more than a few
seconds during the major-program hours. No attempt was made to determine the exact
sphere of influence, but some interesting tests were run with Paul Schneider, W2CYZ,
to estimate the range of interference.
Paul's house is located 900 feet from W2RYI. At that distance, the signal from
the artificial nonlinear system should have been able to produce about 100 microvolts
on his TV feeder. We had planned to take the equipment over to check the level,
but after looking at the interference pattern on his receiver, we decided the measurements
were unnecessary. When W2RYI was operating on 28 megacycles, with the artificial
nonlinear system 50 feet from the discone, and Paul's TV receiver 900 feet away
tuned to Channel 2, the cross-hatching in the picture was objectionable.
Table IV - Some of the harmonics from the" artificially-corroded"
TV antenna located 50 feet from the transmitting antenna. TV feeder is 35 feet long.
Input to transmitter = 50 watts.
In the reverse direction, when W2CYZ was transmitting and the nonlinear system
was 20 feet from the TV antenna at W2RYI, no visible interference was produced -
but that was easy to fix! A 1N34 crystal clipped in series with an 8-foot wire loop
across the TV feeder at W2RYI was all that was needed to generate enough harmonic
output from Paul's harmonic-free signal to massacre Channel 2. Numerous other combinations
of crystal and TV feeder were tried using the signal from 900 feet, and most of
them produced objectionable interference. Two combinations that were particularly
bad were set up to simulate conditions that could actually occur in standard TV
installations. One represented a rusty lightning arrestor; the other, a corroded
The "lightning-arrestor" joint consisted of a 1N34 diode clipped to one side
of the 300-ohm line about 10 feet from the receiver. An 8-foot appendage from the
diode was strapped to the water main where it emerged from the ground.
The "corroded antenna" had an artificial joint that represented one of the many
that can develop from weathering. The joint selected was located between the feeder
terminal on the low-band folded dipole and the adjacent bolt from the steel support
arm. A 1N34 diode clipped between these two points produced heavy Channel 2 interference
in the tests with the signal from W2CYZ, and demonstrated the effectiveness of nonlinear
joints that can develop on TV antennas.
Fig. 5 - Harmonic intensity as a function of the position
of the third-harmonic appendage along a half-wave conductor. Frequency, 27 Mc.;
power input, 320 watts; transmitting antenna 50 feet from harmonic-generating system;
receiving antenna 20 feet from system.
Fig. 6 - Harmonic intensity as a function of the position
of the second-harmonic appendage along a half-wave conductor. Same conditions as
Fig. 5, except that appendage length is 102 inches.
The artificially corroded TV antenna was also utilized to make measurements of
the intensity of high-order harmonics produced by an efficient rectifier These measurements,
shown in Table IV, indicate that the high-order harmonics are comparatively weak.
Analysis of the data also indicated that the 35-foot TV feeder was not sufficiently
long at 3.5 and 7 Mc. to develop optimum fundamental voltage on the rectifier. The
optimum transfer of amateur fundamental signal to harmonics available at the receiver
terminals occurs with a length of TV feeder that is resonant at the amateur fundamental,
placing a voltage maximum at a rectification point common to both the feeder and
a harmonic-resonant element in the TV antenna.
Another optimum condition for nonlinear production of harmonics occurs when an
amateur transmitting antenna has a rectifying joint. An "ideal" system would be
available if a rectifier were substituted for the insulator at the end of a half-wave
doublet, with a wire lanyard 1/2 wavelength for the second harmonic. Some day, someone
is going to experience an exact duplicate of this condition - an old insulator with
soot and rust and corroded copper wire - but almost any antenna or supporting structure
is capable of developing a corroded joint at a point close enough to the optimum
to produce potent harmonic signals.
If an artificial joint 50 feet from a transmitting antenna can produce TVI at
900 feet, less efficient joints common to the antenna should certainly be suspected
when TVI at close range is developed.
Experiments with Joints as Mixers
Getting back to the TV antenna, the diode connected to the support arm was utilized
in examining the behavior of joints as mixers. Natural joints had been investigated
with two signals transmitted simultaneously at W2RYI on several occasions, but data
from controlled tests were needed for a better understanding of the results.
At times, two separate transmitting antennas were used, but results were identical
when the two signals were fed through a coax T-fitting to the discone transmission
line, so the measurements made to produce the data for Figs. 7 and 8 were taken
with a single source of radiation. One transmitter and a grid-dip oscillator had
been used while tracking down the high-level sink joint. The signal from the grid-dip
meter was fed into the BX, water, sewer, gas, and steam lines at various points
along the lines while the regular transmitter was on 27 Mc. The water and sewer
lines in the vicinity of the kitchen developed the strongest sum and difference
signals, the grid-dip oscillator being run at 28.4 and 3.0 Mc. to develop signals
at 1400 kc. for detection with the battery portable, and at 30 Mc. for checks in
the communication receiver.
Stronger and steadier mixed signals, of course, are developed when a second transmitter
is run instead of the dipper, and, if work such as searching the neighborhood areas
for joints is found necessary, a husky signal is advantageous. In any detecting
operation, whether at broadcast frequencies or in the amateur or television bands,
the receiver must reject the amateur fundamental. Traps or high-pass or low-pass
filters may be required to prevent the production of harmonics or mixed signals
in the receiving device.
Fig. 7 - Harmonic and heterodyne signals from artificial
joint on TV antenna. Two transmitters, each 50 watts input; transmitting antenna
50 feet from TV antenna.
Two-signal transmissions produced some interesting data. Fig. 7 shows the
major harmonic and sum-frequency signals between 50 and 100 Mc. that were developed
in the TV antenna with the artificial joint. The power input to each transmitter
was 50 watts; the transmitters were run individually at 27 and 29 Mc. and then simultaneously
at the two frequencies. Readings taken for the individual transmitters showed that
mixed signals developed between two and three times as much voltage as harmonics
in the same frequency ranges. The harmonics had less amplitude when both transmitters
were operating than when each transmitter was on by itself; the ratio of mixed-to-harmonic
signal when both transmitters are operating runs between 5 and 30 to 1. No set pattern
for the occurrence of various ratios is evident in the limited data represented
by this graph, but the general observation holds that the mixed signals are stronger
than the harmonics.
The two-signal method can be utilized to advantage if the existence of detrimental
nonlinear systems is questioned in a given location. If mixed signals are produced,
the evidence helps confirm the presence of rectifying joints, and the general ratio
of harmonic-to-mixed signal can be employed to approximate how much of the harmonic
radiation is due to the transmitting equipment and how much is caused by the external
Another clue to the presence of rectifiers is available in locations where there
is a strong local broadcast station. Mixed signals from the amateur and broadcast
transmitters are sometimes available. With the artificially corroded antenna, modulated
signals in the 10-to 20- microvolt range can be found 930 and 770 kilocycles from
the 2nd and 3rd harmonics of 27 Mc. These signals are W2RYI mixture products of
WPAT and WJZ, which are located, respectively, 5 and 10 miles away.
On rare occasions, an audible arc may form at a corroded joint. Some of the "old
timers," especially 'phone men, have had experience with talking bathtubs and hot-air
ducts that called CQ. This type of modulated arc can produce r.f. harmonics as well
as sound waves, but the sound may be easier to follow than the r.f. path when tracking
down the source of the disturbance. There is one more example of sound indicating
r.f. trouble: If the telephone picks up modulation from a transmitter, rectification
is taking place, and r.f. harmonics are probably being produced.
A New Phenomenon
Fig. 8 - Harmonic and heterodyne signals from natural joints
at W2RYI. Two transmitters, each 300 watts input; signals picked up by TV antenna.
An unusual phenomenon was immediately apparent when the readings were being taken
on the mixed signals. Fig. 7 shows the effect as a differential between the
radiation levels of a given harmonic when first one and then two fundamental signals
are introduced to the diode. The 3rd harmonic of 27 Mc. was recorded as 1500 microvolts
when the 27-Mc. transmitter was on the air alone. As soon as a 29-Mc. signal was
added, however, the 81-Mc. signal dropped from 1500 microvolts down to 50. This
was not a case of poor regulation! The power output of the 27-Mc. transmitter remained
constant. It was a weird experience to raise the 29-Mc. power slowly from zero to
maximum and watch the 81-Mc. signal drop inversely from maximum to minimum. Other
harmonics shown on the graph behaved similarly, but the 3rd of 27 happened to be
the most striking.
The mechanism of this phenomenon may be cancellation by phase displacement or
shift in the operating range of the crystal, or some other derangement that obeys
the law of energy conservation. Whatever the cause, some day some application will
come along that can utilize this screw-ball effect. In the meantime, it can be used
as part of the evidence to convict kitchen sinks.
Two Signals on Natural Joints
Fig. 8 shows the normal two-signal radiation field at W2RYI. When this graph
was made, the artificial joints had been tucked away in boxes on the shelf, and
specific "bad" natural joints had been eliminated. The general trends in the graph
pattern are similar to those in the artificial joint pattern of Fig. 7, adding
to the evidence that the residual harmonic level is predominantly from external
Occasionally, the natural joints at W2RYI produce signals at a lower level than
that shown in Fig. 8, and at other times the intensity increases. Joints that
produce heavy interference appear on an average of about once a year. As mentioned
previously, some of the bad joints are difficult to eliminate, so shunting them
with a conductor is an alternative. The accessibility of the junction determines
the length of the shunt that can be employed, but shunts that are long have too
Fig. 9 - Effect of length of shunt across optimum second-harmonic
Shunt lengths are evaluated in Fig. 9. The data plotted represent the radiation
characteristics of the optimum 2nd-harmonic system with various lengths of No. 14
copper wire connected across the diode. The actual shunt path includes the distance
along the conductors on both sides of the crystal. If attenuation of more than 10
to 1 is desired, it is apparent that a total shorting path of less than 15 inches
is necessary. Readings were also taken with an optimum 3rd-harmonic system. The
curves for the 2nd and 3rd harmonics were identical in shape, but were 6 db. apart.
No obvious resonances were found during the shorting tests. The shunt evidently
does a straight-forward job of reducing the fundamental r.f. voltage across the
crystal. There may be some resonance tricks that can be performed on the lines common
to an inaccessible joint, but the. possibilities seem remote. There are, however,
other tricks to try that are based on known procedures. For instance, a beam antenna
could be so situated that the heading toward the house would be the one most seldom
used. Improvement in the order of 10 to 20 db. could be derived. If the shack is
on a farm, putting the antenna far away - and high - will reduce the fundamental
field. Sometimes changing the polarization from horizontal to vertical, or vice
versa, will reduce the strength of the signal reaching a specific, troublesome conductor.
Perhaps a ground-plane antenna placed on top of the house would reduce the r.f.
field below sufficiently to eliminate TVI from inaccessible joints; this is an opportunity
for the antenna boys to come up with data on a super signal squirter with a high
There are probably other tricks that expediency will bring forth. Building a
new house with all components designed to obviate metallic contacts might be the
ultimate, but that would be the last resort! The number of amateurs that would go
to such an extreme is limited. As a matter of fact, the number that will experience
nonlinear system difficulties may be very small.
It is difficult to predict, from the meager statistics available, what the trend
will be. A few stations have had one or more proven cases. A favorable number have
apparently had no difficulties. A group of unknown size having partially TVI-proofed
transmitters mayor may not know that they are having nonlinear system difficulties.
And a happy-go-lucky gang, the boys who start activities after the last local television
station lowers its flag for the night and plays the "Star-Spangled Banner," may
* c/o Tube Dept., RCA Victor Div., RCA, Harrison, N.J.
Posted November 24, 2022
(updated from original
post on 7/15/2016)