Suppression of EMI from Spark Gap

[Vince]

My heating system uses a constant ignition wiring configuration at the
oil burner. This is causing an annoying high level noise in a HF
receiver.

I would like to kill the source by improving shielding if that is
required. The electrodes are within a metallic housing (e.i., within
the burner assembly).


How can the spark gap EMI be suppressed? Would a connection from the
burner housing to earth ground be helpful?

Thanks.

 

[default]

My heating system uses a constant ignition wiring configuration at the
oil burner. This is causing an annoying high level noise in a HF
receiver.

I would like to kill the source by improving shielding if that is
required. The electrodes are within a metallic housing (e.i., within
the burner assembly).


How can the spark gap EMI be suppressed? Would a connection from the
burner housing to earth ground be helpful?

Thanks.

The oil burner should already be grounded via the mains connection to
it. But a ground if there isn't one should help.

A differential and/or common mode filter will do wonders for noise.
My Electric stove was triggering the "lightening detector" and causing
my modem to drop out. I got a cheesy filter from some surplus company
and ditched the toroid they had and rewound a pair of large ferrite
cores with heavy wire to carry the 30 amps the stove can suck. No
problems.

I found my communications receiver worked much better with a dedicated
ground (10' of 1/2" copper pipe) and not the building ground.. I was
getting all kinds of switching transients in the building ground.
Receiver ground is at the antenna and less than 5 feet from the
receiver with nothing else on it. Building ground was a serpentine
path through the attic to the outside service and over 60' of wire.

Direct RFI radiation is seldom the problem - the oil burner ignition
is not designed to be an efficient radiator - most of the problem is
likely to be via the power lines. For that the source needs a filter
and that filter has to be grounded.

 

[John Popelish]

My heating system uses a constant ignition wiring configuration at the
oil burner. This is causing an annoying high level noise in a HF
receiver.

I would like to kill the source by improving shielding if that is
required. The electrodes are within a metallic housing (e.i., within
the burner assembly).


How can the spark gap EMI be suppressed? Would a connection from the
burner housing to earth ground be helpful?

Make sure the spark transformer case is bonded to the
firebox to provide an HF path for capacitive spark current
back to the grounded center tap of the HV winding.

Add an X rated (across the line AC rated) capacitor across
the transformer primary and a ferrite bead over both primary
leads, or add a line filter package to the transformer
primary, if space permits.

 

[Vince]

When I disconnect the antenna lead from my transceiver, the
interference is gone. Thus, I believe that the problem involves
"radiation" across the RF spectrum and does not involve the AC mains.

There is no center tap at this transformer that I know of. The
secondary winding terminals in this application are actually spring
terminals, that touch the electrodes used for the spark gap only when
the (hinged) transformer housing is closed and resting upon the burner
chassis (held down by two pressure screws).

I wonder if the underside of the transformer housing flange needs to
be scraped to remove paint... to make a better ground connection. I
will check that out soon.

 

[[email protected]]

When I disconnect the antenna lead from my transceiver, the
interference is gone. Thus, I believe that the problem involves
"radiation" across the RF spectrum and does not involve the AC mains.

What if the mains are not conducting the noise to the transciever, but
radiating it to the antenna?

There is no center tap at this transformer that I know of. The
secondary winding terminals in this application are actually spring
terminals, that touch the electrodes used for the spark gap only when
the (hinged) transformer housing is closed and resting upon the burner
chassis (held down by two pressure screws).

I have the same thing. You can check the center tap grounding with an
ohm meter from each spring to case. Make sure you unplug the burner
power before risking this. Mine is center grounded, to balance the
high voltage between the two electrodes, to discourage an arc from
either electrode to the surrounding burner housing.

I wonder if the underside of the transformer housing flange needs to
be scraped to remove paint... to make a better ground connection. I
will check that out soon.

Mine has some screws on the sides. For this type, I would adsd a
jumper wire from one of these to some bolt on the burner box, to
guarantee a high frequency path around the painted hinges. There is
also a slotted tab that gets clamped down, and a star washer might cut
the paint and do the same job.

 

[JoeBloe]

Make sure the spark transformer case is bonded to the
firebox to provide an HF path for capacitive spark current
back to the grounded center tap of the HV winding.

Add an X rated (across the line AC rated) capacitor across
the transformer primary and a ferrite bead over both primary
leads, or add a line filter package to the transformer
primary, if space permits.

The spark itself emits. Radiates like a little antenna.

 

[JoeBloe]

What if the mains are not conducting the noise to the transciever, but
radiating it to the antenna?

More likely that the spark itself is doing that. The radiation it
produces is far more broad banded. For the mains to do that, high
currents at those frequencies would need to be on them.

 

[JoeBloe]

But that spark is inside a metal fire box.

With gaps big enough to let out a LOT of HF noise. Certainly not a
faraday cage. These sparks are pretty big spikes amplitude wise.

 

[John Popelish]

The spark itself emits. Radiates like a little antenna.

But that spark is inside a metal fire box.

 

[notmyreal.email.net]

With gaps big enough to let out a LOT of HF noise. Certainly not a
faraday cage. These sparks are pretty big spikes amplitude wise.

I can really be sure because I can't see what he up against.

Can you get a coffle can to go around it, to shield it for test?

You can also use copper tape or any source of tin.

 

[[email protected]]

When I disconnect the antenna lead from my transceiver, the
interference is gone. Thus, I believe that the problem involves
"radiation" across the RF spectrum and does not involve the AC mains.

.. . .

If it's radiating through space, then I would think you need a "Faraday
cage". This doesn't necessarily have to be made out of solid metal. It
can be made out of some sort of conductive mesh screening.

A rule of thumb that I used to use is that the holes in the Faraday
cage can be no larger than 1/8 wavelength. So, assuming a maximum
frequency of 300 Mhz, for example, you can have holes as large as 4.68".

 

[JoeBloe]

. . .

If it's radiating through space, then I would think you need a "Faraday
cage". This doesn't necessarily have to be made out of solid metal. It
can be made out of some sort of conductive mesh screening.

A rule of thumb that I used to use is that the holes in the Faraday
cage can be no larger than 1/8 wavelength. So, assuming a maximum
frequency of 300 Mhz, for example, you can have holes as large as 4.68".

Ok fine. Now think about how utterly stupid it would look to walk up
to someone with a mesh box made from nearly 5 inch mesh, and say, look
at my RF shielded cage!

In fact, I cannot think of a single time I have ever seen such big
gaps in a device, and have it still be called or considered to be
"shielded".

 

[[email protected]]

Ok fine. Now think about how utterly stupid it would look to walk up
to someone with a mesh box made from nearly 5 inch mesh, and say, look
at my RF shielded cage!

In fact, I cannot think of a single time I have ever seen such big
gaps in a device, and have it still be called or considered to be
"shielded".

Science and engineering are not always intuitive. Some people use 1/8
wavelength and some use 1/10 wavelength as a rule of thumb. The theory
is that it requires 1/4 wavelength to make an antenna and 1/8 and 1/10
are significantly below that. Do you work as a compliance engineer?
What sort of rule do you use?

What aperture size would you allow for a microwave oven at 2450 Mhz?
What aperture size would you allow for submarine ELV signals at 76 Hz?

One reason electronic devices have smaller holes is that they are
concerned with much higher frequencies.

 

[Michael A. Terrell]

Science and engineering are not always intuitive. Some people use 1/8
wavelength and some use 1/10 wavelength as a rule of thumb. The theory
is that it requires 1/4 wavelength to make an antenna and 1/8 and 1/10
are significantly below that. Do you work as a compliance engineer?
What sort of rule do you use?

What aperture size would you allow for a microwave oven at 2450 Mhz?
What aperture size would you allow for submarine ELV signals at 76 Hz?

One reason electronic devices have smaller holes is that they are
concerned with much higher frequencies.

At 2.4 GHz I would prefer solid aluminum walls with the seams bonded,
like an old portable shielded computer room. At the least, I would want
a double shielded room made of fine woven bronze screening, and a double
layer copper clad door with finger stock all around the seams and the
two shields bonded only where the power comes in. I've worked in both
types of shielded rooms. I designed a foil shielded room built on 2"
foam panels and covered with wood paneling, but I haven't built it yet.
I have to put a new roof on my shop, first.


Do you know the wavelength of 76 Hz?


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[[email protected]]

At 2.4 GHz I would prefer solid aluminum walls with the seams bonded,
like an old portable shielded computer room. At the least, I would want
a double shielded room made of fine woven bronze screening, and a double
layer copper clad door with finger stock all around the seams and the
two shields bonded only where the power comes in. I've worked in both
types of shielded rooms. I designed a foil shielded room built on 2"
foam panels and covered with wood paneling, but I haven't built it yet.
I have to put a new roof on my shop, first.

If you were building a microwave oven, though, what is the largest
aperture you would allow to seal the door or the window, for instance?

If you think you might ever run a generator in your shop, by the way, I
would put sheet rock on the ceiling (walls also) and fiberglass
insulation.

Do you know the wavelength of 76 Hz?

According to an online calculator the wavelength for 76 Hz is
12,947,368.421 feet (2452 miles). The wavelength for 2450 Mhz,
incidentally, is 4.82 inches. I've always wondered, incidentally, if my
subwoofer would sound better if I built a room 1/4 wavelength long at
it's lowest frequency. I haven't done it yet, though :>

http://www.onlineconversion.com/frequency_wavelength.htm

Me too--3 years--U.S. Army. I've lost everything else over the years,
including all my year books, but I still have my DD214. I'm not sure
why. Maybe subconciously I wanted to make sure they didn't call me up
accidently :>

 

[Michael A. Terrell]

If you were building a microwave oven, though, what is the largest
aperture you would allow to seal the door or the window, for instance?

If you think you might ever run a generator in your shop, by the way, I
would put sheet rock on the ceiling (walls also) and fiberglass
insulation.

No, the foam panels are quite rigid, and they carry the weight of
each panel. Heavy aluminum foil is glued on each side, and over the edge
where the panels join. You leave at least a half inch gap all the way
around the edge to be sure they don't touch. Then you glue thin
paneling or hardboard to each side to protect the foam and foil. The
corners and edge joints are the secret, they hold everything together
and connect the inside and outside foils into a pair of separate but
continuous shields. Finally, you bring power in somewhere and bond the
two shields. You can make a pair of holes for air conditioning, as long
as you are careful to maintain the shielding with bronze screening. I'll
put plenty of pictures on my website as I build it. You can build a 12'
* 8' for well under a grand, as opposed to over 20 grand for a solid
aluminum booth.

Sheet rock if fairly heavy, so you paint the ceiling panels white, or
another light color.



A generator, as in Gasoline powered? It would be in a separate
building, by the automatic transfer switch.

According to an online calculator the wavelength for 76 Hz is
12,947,368.421 feet (2452 miles). The wavelength for 2450 Mhz,
incidentally, is 4.82 inches.

A simple steel box will stop 76 Hz.

I've always wondered, incidentally, if my
subwoofer would sound better if I built a room 1/4 wavelength long at
it's lowest frequency. I haven't done it yet, though :>

It would cause a resonance over a narrow range of frequencies and
sound like crap, unless all you want is a vibration that will shake the
walls apart.

Me too--3 years--U.S. Army. I've lost everything else over the years,
including all my year books, but I still have my DD214. I'm not sure
why. Maybe subconciously I wanted to make sure they didn't call me up
accidently :>

You can get full copies of your military records by applying to the
VA directly, or through your local Veterans Service Officer. A local
DAV post should have at least one, and sometimes the county government
has one, too. I have both, locally, and have had help from both at
different times.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Fred Abse]

At 2.4 GHz I would prefer solid aluminum walls with the seams bonded,
like an old portable shielded computer room. At the least, I would want
a double shielded room made of fine woven bronze screening, and a double
layer copper clad door with finger stock all around the seams and the
two shields bonded only where the power comes in. I've worked in both
types of shielded rooms. I designed a foil shielded room built on 2"
foam panels and covered with wood paneling, but I haven't built it yet.
I have to put a new roof on my shop, first.

A scrap shipping container (welded steel) makes a good start for a
shielded room, if you've got the real estate.

Do you know the wavelength of 76 Hz?

My rule of thumb is: if the wavelength is farther than you'd want to walk,
it's VLF

 

[Michael A. Terrell]

A scrap shipping container (welded steel) makes a good start for a
shielded room, if you've got the real estate.

I'm not zoned for it, or I would already have a couple of them. I would
have to put them in my driveway and my neighbors would complain, loudly.

My rule of thumb is: if the wavelength is farther than you'd want to walk,
it's VLF

Some days that would be up around 100 MHz.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[jasen]

No, the foam panels are quite rigid, and they carry the weight of
each panel. Heavy aluminum foil is glued on each side, and over the edge
where the panels join. You leave at least a half inch gap all the way
around the edge to be sure they don't touch. Then you glue thin
paneling or hardboard to each side to protect the foam and foil. The
corners and edge joints are the secret, they hold everything together
and connect the inside and outside foils into a pair of separate but
continuous shields. Finally, you bring power in somewhere and bond the
two shields. You can make a pair of holes for air conditioning, as long
as you are careful to maintain the shielding with bronze screening. I'll
put plenty of pictures on my website as I build it. You can build a 12'
* 8' for well under a grand, as opposed to over 20 grand for a solid
aluminum booth.

how do you intend to bond the edges of the aluminium sheets ?

do you have some sort of spot-welder?

Bye.
Jasen

 

[Michael A. Terrell]

how do you intend to bond the edges of the aluminium sheets ?

do you have some sort of spot-welder?

Bye.
Jasen

Aluminum tape with conductive glue.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida