We have a Panasonic 1300W microwave oven. It causes a tremendous amount of
static in weak (5000W) AM radio stations. Even on a battery operated portable
playing in the other end of the house. Is this an indication of a problem with
the microwave, or it's normal, is there a trap, or something that can be done
to eliminate the interference?

TOM KAN PA ) writes:
We have a Panasonic 1300W microwave oven. It causes a tremendous amount of
static in weak (5000W) AM radio stations. Even on a battery operated portable
playing in the other end of the house. Is this an indication of a problem with
the microwave, or it's normal, is there a trap, or something that can be done
to eliminate the interference?


Sounds like leakage, usually around the door. Use the portable radio as a
detector, put the oven on low power, the radio tuned away from a station,
and move it all around the oven, esp. the edges of the door. If it's the door,
it's either not closing properly, or food particles are preventing a good
seal.

"TOM KAN PA" wrote in message
...
We have a Panasonic 1300W microwave oven. It causes a tremendous amount of
static in weak (5000W) AM radio stations. Even on a battery operated portable
playing in the other end of the house. Is this an indication of a problem with
the microwave, or it's normal, is there a trap, or something that can be done
to eliminate the interference?

My four year old Panasonic (full size) microwave oven causes no such
interference with AM signals. Put a cup of water in the oven and listen the
static when the oven is on full power and on low power. If you hear a
modulated sound on low (rapid pulses of static) then there is likely a poorly
bonded connection in the path between the magentron and the oven chamber. The
intense microwaves will cause arcing at any point where there is a sufficiently
poor bonding (electrical connection) between any of the metal parts involved in
the microwave path. This arcing will create a spark oscillator type source of
broad band radio noise which can get into the house mains or directly radiate
from the metal cabinet.

Fixing the problem by trying to filter out the noise created by a defective oven
would appear to be the wrong solution, better to eliminate the source. Since
microwave ovens are so cheap now I would just get another one and relegate this
one to the junk pile or someone you have a grudge against.

Regrads,

John

I have had two over the range Microwave ovens, a Quasar and recently a GE
Profile. When running either they create noise on the AM of my old 1972
Panasonic tabletop radio on the top of the fridge.

The current crop of dept store AM/FM radios are known to have crummy AM
reception. It is just built in so they can say it is an AM/FM model. The
designs are not very good at rejecting noise.

My easy solution is to turn off the radio when the microwave is on.

If you have to listen to it with the microwave on you can try the
following..

------------------

Low Noise Antenna Connection
From: (John Doty)
Newsgroups: rec.radio.shortwave
Subject: Low Noise Antenna Connection
Date: 26 Nov 1993 16:55:24 GMT



----------------------------------------------------------------------------
----


It doesn't take very much wire to pick up an adequate signal for anything
but the crudest shortwave receiver. The difference between a mediocre
antenna system and a great antenna system isn't the antenna itself: it's the
way you feed signals from the antenna to the receiver. The real trick with a
shortwave receiving antenna system is to keep your receiver from picking up
noise from all the electrical and electronic gadgets you and your neighbors
have.

The Problem:
Any unshielded conductor in your antenna/ground system is capable of picking
up noise: the antenna, the "lead-in" wire, the ground wire, etc. Even the
widely recommended cold water pipe ground can pick up noise if it runs a
significant distance before it goes underground

Symptoms of this problem include buzzing noises, especially at lower
frequencies, clicks as appliances are turned on or off, and whines from
motorized devices. Sometimes the problem can be reduced by running the radio
from batteries.

The Solution:
The solution is to keep the antenna as far as possible from houses, power
lines, and telephone lines, and to use a shielded (coaxial) transmission
line to connect it to the receiver. To get this to work well, two problems
must be avoided: noise currents on the shield must be kept away from the
antenna, and, if you want to listen to a wide range of frequencies, the
cable must be coupled to the antenna in a non-resonant way.

You can keep noise currents away from the antenna by giving them a path to
ground near the house, giving antenna currents a path to ground away from
the house, and burying the the coaxial cable from the house to the antenna.
Resonance can be avoided by coupling the antenna to the coaxial cable with a
transformer.

Construction:
My antenna and feed system are built with television antenna system
components and other common hardware. These parts are inexpensive and easily
obtainable in most places.

The transformer is built around a toroid extracted from a TV "matching
transformer". If you're a pack rat like me, you have a few in your basement:
you typically get one with every TV or VCR (or you can buy one). Pop the
plastic case off and snip the wires from the toroid (it looks either like a
tiny donut, or a pair of tiny donuts stuck together). The transformer
windings should be made with thin wi I use #32 magnet wire. The primary
is 30 turns while the secondary is 10 turns. For a one-hole toroid, count
each passage of the wire down through the hole as one turn. For a two-holer,
each turn is a passage of the wire down through the right hole and up
through the left.

Mount the transformer in an aluminum "minibox" with a "chassis mount" F
connector for the coax cable and a "binding post" or other insulated
terminal for the antenna. Ground one end of each winding to the aluminum
box. Solder the ungrounded end of the primary to the antenna terminal, and
solder the ungrounded end of the secondary to the center conductor of the
coax connector.

Drive a ground stake into the earth where you want the base of your antenna
to be (well away from the house). Mount the transformer box on the ground
stake: its case should make good contact with the metal stake. Drive another
ground stake into the earth near the place where you intend for the cable to
enter the house. Mount a TV antenna "grounding block" (just a piece of metal
with two F connectors on it) to the stake by the house. One easy way to
attach hardware to the ground stakes is to use hose clamps.

Take a piece of 75 ohm coaxial cable with two F connectors on it (I use
pre-made cable assemblies), connect one end to the transformer box, the
other end to the grounding block. Bury the rest of the cable. Finally,
attach a second piece of 75 ohm coax to the other connector on the grounding
block and run it into the house. Use waterproof tape to seal the outdoor
connector junctions.

Attach one end of your antenna to the antenna terminal on the transformer
box and hoist the other end up a tree or other support(s) (don't use the
house as a support: you want to keep the antenna away from the house). My
antenna is 16 meters of #18 insulated wire in an "inverted L" configuration
supported by two trees.

If your receiver has a coaxial input connector, you may need an adapter to
make the connection; in any case, the center wire of the coaxial cable
should attach to the "antenna" connection, and the outer shield should
attach to the "ground" connection.

Multiple grounds and transformer coupling of the antenna should reduce the
danger posed by lightning or other electrostatic discharge, but don't press
your luck: disconnect the coax from the receiver when you're not using it.

How it works, in more detail:
Coaxial cable carries waves in two modes: an "outer" or "common" mode, in
which the current flows on the shield and the return current flows through
the ground or other nearby conductors, and an "inner" or "differential" mode
in which the current flows on the inner conductor and the return current
flows on the shield. Theoretically, outside electromagnetic fields excite
only the common mode. A properly designed receiver is sensitive only to the
differential mode, so if household noise pickup is confined to the common
mode, the receiver won't respond to it.

The "characteristic impedance" of the differential mode is the number you'll
see in the catalog or on the cable: 75 ohms for TV antenna coax. The
characteristic impedance of the common mode depends on the distance of the
line from the conductor or conductors carrying the return current: it varies
from tens of ohms for a cable on or under the ground to hundreds of ohms for
a cable separated from other conductors.

A wire antenna can be approximately characterized as a single wire
transmission line. A single wire line has only a common mode: for #18 wire
30 feet above ground, the characteristic impedance is about 620 ohms. For
heights above a few feet the characteristic impedance depends very little on
the height.

If the impedances of two directly coupled lines match, waves can move from
one line to the other without reflection. In case of a mismatch, reflections
will occur: the magnitude of the reflected wave increases as the ratio of
the impedances moves away from 1. A large reflection, of course, implies a
small transmission. Reflections can be avoided by coupling through a
transformer whose turns ratio is the square root of the impedance ratio.

The basic difficulty with coupling a wire antenna to a coaxial line is that
the antenna's characteristic impedance is a poor match to the differential
mode of the line. Furthermore, unless the line is very close to the ground,
the common mode of the line is a good match to the antenna. There is thus a
tendency for the line to pick up common mode noise and deliver it
efficiently to the antenna. The antenna can then deliver the noise back to
the line's differential mode.

Some antenna systems exploit the mismatch between the antenna's
characteristic impedance and the line's characteristic impedance to resonate
the antenna. If the reflection at the antenna/line junction is in the
correct phase, the reflection will add to the signal current in the antenna,
boosting its efficiency. While this is desirable in many cases, it is
undesirable for a shortwave listening antenna. Most shortwave receivers will
overload on the signals presented by a resonant antenna, and resonance
enhances the signal over a narrow range of frequencies at the expense of
other frequencies. It's generally better to listen with an antenna system
that is moderately efficient over a wide frequency range.

In my antenna system, grounding the shield of the line at the ground stakes
short circuits the common mode. The stake at the base of the antenna gives
the antenna current a path to ground (while the transformer directs the
energy behind that current into the coax). Burying the cable prevents any
common mode pickup outside the house, and also attenuates any common mode
currents that escape the short circuits (soil is a very effective absorber
of RF energy at close range). Common mode waves excited on the antenna by
incoming signals pass, with little reflection, through the transformer into
differential mode waves in the coax.

A major source of "power line buzz" is common mode RF currents from the AC
line passed to the receiver through its AC power cord. These currents are
normally bypassed to chassis ground inside the receiver. They thus flow out
of the receiver via the ground terminal. With an unshielded antenna feedline
and a wire ground, the ground wire is a part of the antenna system: these
noise currents are thus picked up by the receiver. On the other hand, with a
well grounded coaxial feed these currents make common mode waves on the coax
that flow to ground without exciting the receiver.

Performance:
A few years ago, I put up a conventional random wire antenna without a
coaxial feed. I was disappointed that, while it increased signal levels over
the whip antenna of my Sony ICF-2001, it increased the noise level almost as
much. I then set up the antenna system described above; in my small yard,
the base of the antenna was only 12 meters from the house. Nevertheless, the
improvement was substantial: the noise level was greatly reduced. This past
year I moved to a place with a roomier yard; with the base of the antenna
now 28 meters away I can no longer identify any noise from the house.

The total improvement over the whip is dramatic. A few nights ago, as a
test, I did a quick scan of the 60 meter band with the whip and with the
external antenna system: with the whip I could only hear one broadcaster,
unintelligibly faintly, plus a couple of utes and a noisy WWV signal. With
the external antenna system I could hear about ten Central/South American
domestic broadcast stations at listenable levels. WWV sounded like it was
next door.

I have also tried the antenna system with other receivers ranging from
1930's consoles to a Sony ICF-SW55. I've seen basically similar results with
all.



Remove "zz" from e-mail address to direct reply.



"JTM" wrote in message
...

"TOM KAN PA" wrote in message
...
We have a Panasonic 1300W microwave oven. It causes a tremendous amount
of
static in weak (5000W) AM radio stations. Even on a battery operated
portable
playing in the other end of the house. Is this an indication of a
problem with
the microwave, or it's normal, is there a trap, or something that can be
done
to eliminate the interference?

My four year old Panasonic (full size) microwave oven causes no such
interference with AM signals. Put a cup of water in the oven and listen
the
static when the oven is on full power and on low power. If you hear a
modulated sound on low (rapid pulses of static) then there is likely a
poorly
bonded connection in the path between the magentron and the oven chamber.
The
intense microwaves will cause arcing at any point where there is a
sufficiently
poor bonding (electrical connection) between any of the metal parts
involved in
the microwave path. This arcing will create a spark oscillator type
source of
broad band radio noise which can get into the house mains or directly
radiate
from the metal cabinet.

Fixing the problem by trying to filter out the noise created by a
defective oven
would appear to be the wrong solution, better to eliminate the source.
Since
microwave ovens are so cheap now I would just get another one and relegate
this
one to the junk pile or someone you have a grudge against.

Regrads,

John

wrote in
:

On Fri, 01 Aug 2003 22:36:41 GMT, "George"
wrote:


wrote in message
. ..
On 1 Aug 2003 18:37:44 GMT, (Tom Bach)
wrote:

TOM KAN PA ) writes:
We have a Panasonic 1300W microwave oven. It causes a tremendous
amount
of
static in weak (5000W) AM radio stations. Even on a battery
operated
portable
playing in the other end of the house. Is this an indication of a
problem with
the microwave, or it's normal, is there a trap, or something that
can
be done
to eliminate the interference?


Sounds like leakage, usually around the door. Use the portable
radio as a detector, put the oven on low power, the radio tuned
away from a station, and move it all around the oven, esp. the
edges of the door. If it's the
door,
it's either not closing properly, or food particles are preventing
a good seal.


Why low power? Microwave Ovens actually only have one "power" - the
lower settings merely cycle the Magnatron on and off. You should
never run a microwave while empty, in any case. Put a bowl of water
in it.

BB

Some newer designs don't use PWM and are actually variable power.


Really? Please tell me where I can find out more about this.

Thanks,
BB


Panasonic Inverter technology.They use a switching power supply to generate
the voltage for the Magnetron,and can vary the output DC voltage,thus
changing the power the tube generates.This also eliminates an expensive
iron core transformer,and reduces the weight of the oven.

Try the Panasonic website for info on this.

Only drawback I can think of is that the switching PS is more failure-
prone;more parts,more complexity= less reliability,and more vulnerability
to surges on the mains.

--
Jim Yanik,NRA member
remove null to contact me

Jim Yanik wrote:
Only drawback I can think of is that the switching PS is more failure-
prone;more parts,more complexity= less reliability,and more vulnerability
to surges on the mains.

Switching power supplies are notorious sources of birdies at every harmonic
of their (not very high) operating frequency, even when there's been some
effort to suppress them in design. I doubt a microwave oven has much suppression.
--
Ron Hardin


On the internet, nobody knows you're a jerk.

In article ,
Ron Hardin wrote:

Switching power supplies are notorious sources of birdies at every harmonic
of their (not very high) operating frequency, even when there's been some
effort to suppress them in design. I doubt a microwave oven has much
suppression.

Try plugging the radio into an outlet that's on a different circuit, you
may find one that's more isolated from the microwave. Or if possible,
run the radio from batteries, and see if that makes a difference.

--
Larry Weil
Lake Wobegone, NH

"Ron Hardin" wrote in message
...
Jim Yanik wrote:
Only drawback I can think of is that the switching PS is more failure-
prone;more parts,more complexity= less reliability,and more
vulnerability
to surges on the mains.

Switching power supplies are notorious sources of birdies at every
harmonic
of their (not very high) operating frequency, even when there's been some
effort to suppress them in design. I doubt a microwave oven has much
suppression.

My old microwave oven had a strong arc and died. I replaced it with a
Panasonic which uses the "inverter" technology. They must have done
something clever to reduce noise from the switching power supply because
there is no noise on any of the nearby radios or cordless phones etc. The
oven also performs as advertised regarding thawing and cooking. Their claim
is that thawing is uniform and that the edges of food are not overcooked as
compared to an oven which uses PWM.


--
Ron Hardin


On the internet, nobody knows you're a jerk.

"George" wrote in
:


"Ron Hardin" wrote in message
...
Jim Yanik wrote:
Only drawback I can think of is that the switching PS is more
failure- prone;more parts,more complexity= less reliability,and
more
vulnerability
to surges on the mains.

Switching power supplies are notorious sources of birdies at every
harmonic
of their (not very high) operating frequency, even when there's been
some effort to suppress them in design. I doubt a microwave oven has
much
suppression.

My old microwave oven had a strong arc and died. I replaced it with a
Panasonic which uses the "inverter" technology. They must have done
something clever to reduce noise from the switching power supply
because there is no noise on any of the nearby radios or cordless
phones etc. The oven also performs as advertised regarding thawing and
cooking. Their claim is that thawing is uniform and that the edges of
food are not overcooked as compared to an oven which uses PWM.


--
Ron Hardin


On the internet, nobody knows you're a jerk.




Regular MW ovens don't use PWM control,they vary the duty cycle of the
magnetron,on for 50%,off for 50% for half-power,for example,or 25/75% for
1/4 power.The magnetron runs at full power for the time it's on.Those ovens
use an iron-core transformer and rectifier to generate the HV for the tube.

--
Jim Yanik,NRA member
remove null to contact me