January 1938 Radio-Craft
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
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charging is the phenomenon whereby adhesion forces between two
surfaces causes the dislodging of electrons from nearby atoms, with
those electrons being attracted to the material with the highest
positive potential as the interface attempts to neutralize itself.
Relative contact motion (friction; e.g.,
walking across a carpet) is most often the cause of triboelectric
charge transfer, but simply pulling apart two dissimilar surfaces
can also be the mechanism (e.g., pulling
a wool sweater off or lifting a polymer type fabric blanket away
from a bed sheet) for charge transfer.
(ESD), a manifestation of triboelectric
charging, can damage or destroy electronic components. Another effect
caused by triboelectric charging is static in communications systems
that are contained within moving vehicles like cars, boats, airplanes,
rockets, etc. Much research has been performed to figure out how
to mitigate the problem. By the late 1930s, radio static was inhibiting
airborne communications to the point that serious action needed
to be taken ... and it was. This story, the last of a three-part
series, summarizes the findings and the remedies. One interesting
aspect is the table that tells how a radio operator perceives static
at various static voltage levels.
"Snow Static" Being Beaten by "Flying Laboratory"
This article has been presented here in order to show radio
men contemplating aircraft-radio work as a livelihood some of the
problems encountered in obtaining noise-free radio reception for
increased flying safety.
H. M. Hucke Part III
United Air Lines Communications Engineer H, M. Hucke
under the nose of the company's "Flying Laboratory" with
several of the experimental devices installed prior to flights
to determine their efficiency in reducing static.
Last Month the various effects noticed when different types of
electrodes were placed .in the static field, of the "flying laboratory"
during its many test flights, were discussed. Just how do these
points now shape up with respect to each other?
A grouping of these points suggested by Professor Starr gave
the most orderly results. This group consisted of a pointed 2-ft.
rod in the disturbed air at the tail; a pointed 2-ft. rod on the
nose projecting into the undisturbed air ahead of the plane; and,
a plate on the nose to record the impacting water particles.
A study of our data on all the points has resulted in the following
(1) That the plane may be either positive or negative with respect
to the surrounding cloud.
(2) That at any instant one wing may be in positive cloud particles
while other is in negative.
(3) That at any instant the nose of the plane may be in positive
particles while the tail is in negative or vice versa.
The maximum cross-flow measured from wing-to-wing was about .500
microamperes though undoubtedly larger flows are possible. The maximum
would constitute a stroke of lightning. There are many records of
lightning strikes on all-metal planes which indicate wing-to-wing
flows of several thousand amperes. During our flights we encountered
one condition in a thundercloud in which the plane's magnetic compass
moved 10 deg. with respect to the gyrocompass for a period of several
minutes. This may have been due to a strong magnetic field in the
cloud or to a cross-flow of current in the plane structure. Ground
tests indicated that a wing-to-wing flow of about 45 D.C. amperes
was required to produce the same compass deviation. A nose-to-tail
current of 125 amperes produced the same effect, This would vary
with the p6sition of the plane with respect to the earth's magnetic
field. Further tests with special wing constructions are needed
to establish the magnitude of current flow through the plane.
Electric Charge is Due to 6 Variables
It is known that a negatively-charged point will go into corona
about 50 per cent more readily than a positively-charged point.
It is also known that the action of the propeller in cutting up
water particles at a top speed of 800 ft. per second will produce
an electric charge. It is reasonable to believe that the wing of
a plane moving at 260 ft. per second will break up water particles
and produce a charge. The electric charge recordings are, therefore,
the summation of at least the following 6 variables:
(1) The plus or minus charges of the water particles in the cloud
which are collected by the wing foil.
(2) The generation of charge due to the wing sections splitting
(3) The generation of charge due to the propeller splitting water
(4) Foreign matter in the water particles (Portland, Oregon,
tap water split by the rotating propeller gives a positive charge
while Cheyenne, Wyoming, tap water gives a negative charge).
(5) The rectification action of the test points with different
polarity of the plane charge.
(6) Cross-current flows due to the plane short-circuiting sections
of cloud having different potentials.
From the above it is obvious that the mechanism by which the
plane gathers an electrostatic charge is quite complex. Rather than
spend valuable flight time trying to reach an orderly conclusion
from this group of variables, it was believed best to proceed on
to possible solution. In any case, it appeared probable that whether
the plane became plus or minus it eventually reached a sufficiently
high potential for corona discharges to appear on wing tips or any
sharp projecting points. As a check on this assumption a tracing
oscillograph connected to any of the test points gave typical corona
discharge tracings whenever the characteristic sounds were heard
in the plane's radio set.
Charging the Plane to 100,000 Volts
The plane was charged up by a small Wimshurst machine while standing
on the ground and by bringing a pointed ground wire near its structure,
the characteristic snow-static sounds could be duplicated. Since
this experiment was limited by (1) the insulation of the rubber
tires, (2) the A.C. modulation of the Wimshurst disc, and (3) the
general variability of such a generator, a more substantial arrangement
was desirable. Through the courtesy of the Westinghouse Company
and Stanford university, we were able to borrow high-voltage insulators
and assemble a 100,000 volt D.C. ray power supply. The plane was
set up on these insulators in a large metal hangar and charged up
to either plus or minus 100,000 V.
With this arrangement engineers could remain inside the all-metal
plane with all radio equipment operating and use the test equipment
in the same manner as was possible in flight. The tests further
substantiated the corona discharge theory. The power was sufficient
to make the anti-static loops and regular antennas inoperative in
the same general ratio as had been observed on the test flights.
The characteristic snow-static sounds were present.
Source of "Crying" Snow-Static. Static noise in the receivers
with regular antennas began as low as 30,000 V., depending upon
local humidity and the proximity of the artificial ground plane
to the various points on the plane. The "crying" snow-static sounds
usually began at about 55,000 V. and occurred more readily when
the plane was positive with respect to ground. This crying phenomenon
was readily traced to a corona discharge from some point on the
plane. Artificial points were set up for its study and we concluded
that the space charge in the ionized air around the point breaks
down at an audio-frequency rate. This rate varies with the amount
of moisture in the air and the voltage gradient at the point. Under
controlled conditions it will produce any audio-frequency note.
For example, on one test the nose produced followed the order shown
in Table I.
55,000 noise like frying
60,000 frying noise begins
to pulse at about 10 cycles per second
62,000 frying noise pulses
at 100 cycles
64,000 frying noise pulses
at 500 cycles
66,000 frying noise pulses
at 2,000 cycles
68,000 frying noise pulses
at 8,000 cycles
70,000 frying noise pulses
at 15,000 cycles
72,000 frying noise pulses
at above audibility
At about this point some other point on the plane begins the
same sounds and goes on up through the musical scale.
At any one time it will be possible for a number of points to
produce this musical corona in any order. This, then, is the cause
of the characteristic snow-static sound.
A study of the plane structure indicates that antenna masts,
rivet heads, cotter keys, on aileron hinges and tail wheels, the
antennas themselves and any sharp points on the plane are the focal
points of the corona discharges and consequently the source of snow-static
radio interference while in flight.
Unless these discharge points are quieted snow-static cannot
be eliminated. Covering them with an insulator, reducing their sharpness,
or covering them with a well-rounded corona shield will only allow
the plane to build up to a still higher potential until some other
point starts corona.
There are two solutions open: (1) reduce the ability of the plane
to gather or generate charges; and (2), admit that the plane cannot
be prevented from gathering charges and work out a means for discharging
it which will not cause radio interference.
The second solution offered the best possibilities although several
plans for accomplishing the first will shortly be tested. It is
probable that a partial solution of both will eventually be used.
Suppressor Resistors Help Reduce Static Effects
A study of the noise indicates that it has a very short wavelength
and that its attenuation with distance is rapid. The field pattern
caused by a point in the corona at the rear of the airplane is shown
in Fig. 1. Note how the area of interference production is continuous
with the trailing edges of the airplane. When a resistor was added
in series with the point the interference was materially reduced
by a change in the noise field pattern to a location in the rear
of the airplane and comparatively isolated from it, as illustrated
in the lower portion of the diagram. Curves run on resistors indicate
that at least 100,000 ohms and in some cases up to 10 megohms are
necessary. Moving the point away from the plane takes advantage
of the rapid attenuation and gives a better pattern.
This indicated that a trailing discharging point as far as possible
behind the plane with suitable suppressor resistors had possibilities
for discharging the plane. Up to 1 milliampere discharge at 50 ft.
could be obtained with 100,000 V. without disturbance in the radio
set using the regular antenna. A 25 microampere discharge from a
point without suppressors 2 ft. from the plane prevented radio reception!
Since the mechanical troubles of a trailing wire are not desirable
a second version of this idea was tried. Here a series of 17 3-ft.,
3/1,000- in. dia. wires having a 5-megohm resistor in each was attached
to suitable points on the wing and tail surfaces. Test flights of
these dischargers are still in progress. Results in the air have
verified the test made on the ground. The single trailing wire appears
superior to the individual short wires though tests are not yet
conclusive. The dischargers are still considerably short of a commercial
cure and to date will only clear up radio range reception in about
15 per cent of the conditions encountered. Apparently the rate of
discharge is not yet fast enough when the plane enters areas where
the water particles have too high a potential. Although this system
is not yet commercially practical, we feel that it is the first
step on the road to a final solution.
Anti-Static Aircraft Antennas
Fig. 1. Resistors remove noise-field from plane.
Our antenna tests indicate that snow-static interference is considerably
worse at the rear than at the front of a plane. When the snow-static
noise was of average strength, the loop located in the tear drop
housing and the loop on the belly were both rotated and indicated
that the source of maximum disturbance was toward the rear of the
plane. When the static became extreme, rotating the loops indicated
static in all directions. Probably corona had started on the wing
tips and propellers in conditions of severe static.
In mild snow-static when beacon reception on the "V" antenna
was normal, the 2 rear beacon antennas were so noisy that no beacon
reception was possible. The vertical rear antenna had a 25 to 1
better signal pick-up due to polarization of the range signals,
but the snow-static pick-up was about the same on either. Both rear
beacon antennas were about the same length and spacing from the
fuselage. We concluded that the snow-static interference radiation
was not normally polarized.
Although the 40 ft. top antenna was far superior to the lower
"V" antenna, from a signal pick-up standpoint, in snow-static the
"V" antenna would pick up 5,000 kc. short-wave stations 1,000 miles
away when they were unreadable on the top antenna.
Although we did not test a trailing wire as an antenna, we did
conclude from our study that it should be about the worst form of
antenna for reception in snow-static. It would carry as high as
2 milliamperes of discharge current in vigorous "warm front" conditions.
The static leak connected across the input of the average receiver
is about 1/2-megohm; with a 2 ma. peak current the voltage drop
across the antenna input circuit of the receiver could be 1,000
V. The noise modulation on this D.C. voltage would be less than
1%, or only a few volts of random A.C.
During the tests we reeled out 150 ft. of steel No. 14 B. &
S. stranded aircraft cable. It had no resistance suppressors in
it and did not increase or decrease snow-static on the beacon frequencies.
The short-wave receiver, however, was tuned to a 60 meter wavelength,
hence, the 150 ft. cable plus the 65 ft. plane length was more than
one wavelength long. Reeling the cable in and out gave 2 nodes of
maximum snow-static and 2 nodes of minimum snow-static. The minimum,
however, was not sufficiently low to materially aid reception.
At the time we began our tests there were some snow-static theories
which presumed that the noise was due to charged particles striking
the antenna. To check this, a special pair of rod antennas were
constructed. These were hollow, 1 in. dia. tubes of bakelite and
the other of aluminum. The single wire antenna was held in the center
of these tubes by insulating discs and the lead-in completely enclosed
in metal tubing. With this arrangement no particle of any kind could
strike the antenna itself. The aluminum tube was grounded to the
plane at 3 points with 1/10-meg. resistors. The bakelite tube was
painted with a solution of airplane dope and graphite so that its
entire surface was a 10,000-ohm resistance leak to the plane. Good
beacon reception was obtained on either antenna, but they gave no
advantage over the regular No. 14 bare copper wire antenna exposed
to the snow and rain particles. We concluded that the impinging
particles were not sources of noise, or that the corona noise was
so great that the impinging particle noise was obscured.
In conditions when the plane is highly charged and corona appears
as St. Elmos' Fire at the propeller tips, the regular bare No. 14
antenna wires must also go into corona. Since the wires have a small
diameter they might discharge to the atmosphere sooner than other
points on the plane. To test this, the "V" beacon antenna was replaced
with wires having a diameter from 3/1,000-in. up to 1 in. dia. tubing.
As the diameter increased, the reception improved and the outgoing
corona current decreased. The curves indicate, however, that only
a very small advantage would be gained by increasing the present
wire diameter from No. 14 to No. 10.
It is of material importance to reduce all sharp points such
as cotter keys on antenna fittings, and to generally round off all
rough edges on antenna structures.
Horizontal and Vertical Dipoles
A pair of horizontal and vertical dipole antennas was tried on
the ground with the 100,000 V charging equipment. They were tuned
and coupled to the receiver by means of an electrostatically shielded
antenna transformer. Although "they gave a definite improvement
over corona static as compared to a single bare wire, their signal
pick-up was too poor for practical aircraft use. Resonating the
aluminum tube antenna previously discussed, gave some gain against
corona static, but not enough to warrant its use. Under the same
conditions, a receiver having a high-impedance antenna coupling
system was compared with another receiver having a low-impedance
antenna coupling system. The corona static to signal ratio was practically
identical on both receivers.
The metallically covered loop antennas gave the following advantages
over the regular bare wire beacon antennas:
(1) The advantage varies with the intensity of the corona discharge.
(2) In mild snow-static the advantage as measured by R.M.S. static
output of the receiver may be 20 or 30 to 1.
(3) In heavy snow-static the advantage drops to 5 or 10 to 1.
(4) In very heavy snow-static no range reception can be heard
on any loop antenna even when flying within 2 or 3 miles of the
On one test trip in a Pacific tropical-marine, warm air mass
front no range reception was possible when any of the anti-static
loops were used for a period of 25 minutes. Had we remained in this
air mass layer we could have been without range reception for several
hours, since the front was parallel to the airway. With the assistance
of our meteorologists such conditions can normally be avoided, and
this particular flight represents an extreme case. It does appear,
however, that the anti-static loops must be coordinated with discharging
systems and meteorological guidance if a complete solution is to
It seems that the advantage of the metallically shielded loops
lies in their metal covering. An experimental, wooden nose was installed
on the plane, and covered with copper foil. The foil was cut at
suitable points to make a Faraday shield. An unshielded loop in
this nose gave practically the same results as the loops with the
metal immediately surrounding the wire. The loop in the tear drop
housing gave practically the same results as the nose ring or metallically
covered loop on the plane belly. The nose ring loop was usually
about 5 per cent better than the loop on the plane belly, probably
because it was farther forward. A low-impedance metallic loop with
an impedance-matching network gave the same results as a high impedance
of the same metallically covered construction. The wooden nose without
cooper foil was painted with a mixture of dope and graphite so that
it had an average resistance of 20,000 ohms to the plane structure.
Signal pick-up dropped about 15 per cent for loops inside this nose.
No change in snow-static advantage occurred. An unshielded loop
in this nose suffered from snow-static, while a metallically covered
loop in the same place gave the usual advantage. Position about
the nose of the plane seemed to have very little bearing on the
Miscellaneous Sources of Static
Any insulated surfaces such as windshield, de-icers and non-metallic
loop housings can charge up with respect to the plane. When the
charge on them becomes high and the plane suddenly flies into a
higher or lower charged cloud area, these insulated surfaces will
spark to the plane structure. Painting the loop housing with dope
and graphite stops this source of noise. If, however, the plane
flies through an icing area, an ice cap will form on top of the
graphite paint. This ice cap is an insulator which charges up and
sparks over in the same manner as the insulated surface. Thus in
ice, the special paint does not accomplish its purpose. The answer
is to streamline loop housings so that ice does not form.
For some time we have been using bakelite stubs instead of the
egg-type insulators on transmitting antennas to avoid ice troubles.
These stubs have always followed the usual streamline form with
the blunt forward edge and tapering rear edge. They also gather
a thick layer of ice on the blunt forward edge. As a result of our
loop housing work, we are now constructing stubs with a sharp front
edge, which should completely and finally solve the antenna icing
During the course of our flights we found that the bonding on
one of the ring cowls had broken. This ring cowl, resting on leather
pads, charged up in snow-static and sparked over at regular intervals.
In average charged clouds, this sparking caused a headphone noise
sounding like pebbles falling in a metal pail. Any other exposed
metal parts on a plane which are not bonded would cause a similar
noise. The first steps toward improving plane reception in snow-static
should include a thorough inspection of all bonding.
Our work with the loops gave rise to considerable speculation
as to why a shielded loop attenuated the corona radiation and an
unshielded loop did not. Four theories have been advanced, but none
have been carried far enough to date to warrant discussion here.
All, however, must consider that the wave front of the interference
is exceedingly steep as compared to that of the beacon signal. Dr.
O'Day is working on a mathematical approach to the problem, which
we hope will clear up this peculiarity. Once it is understood, the
way may be open to a new type of antenna which does not have the
disadvantages of the loop.
The loop type of beacon antenna has no cone of silence, and is
practically "unflyable" when within 5 miles of a loop-type DOC radio
range. To overcome this, we might assume that we can always change
over to the regular antenna when close to the station so that a
normal cone of silence can be obtained. It is assumed that the range
signal will override the snow-static when close in. Actually, however,
we have records of a number of cases where the static directly over
the range station was strong enough to make changing over impossible
since even the loop unable to receive through the static.
In closing this paper, I wish to express our appreciation of
the assistance, advice and loan of equipment which made this work
possible. A number of men gave of their time without compensation,
and the manufacturers their personnel and equipment without hope
The author of this article on the problem of snow static as it
affects aircraft-radio reception, and discussion of methods being
developed to counteract snow static, is Supt. of Communications
Laboratory, United Air Lines. The subject matter of this series
of articles was recently presented at Denver, Colo., before the
Inst. of Aeronautical Science, and the American Assoc. for the Advancement
Posted October 9, 2014