Metal to metal contact generates RF?

[quietguy]

I fly (or crash) radio controlled model planes as a hobby.

Engine speed is controlled by a servo which operates a piano wire
control rod which attaches to the throttle lever on the engine.

I am advised by the cluey and experienced guys at my club that metal to
metal contact in glow planes is a no no - ie the metal throttle control
rod mustn't attach to the metal throttle lever on my engine, as it will
cause glitches etc - ie interference with the radio control operation

However, on thinking a bit about it I don;t see how this can happen as
there is no potential difference between the motor throttle lever and
the control rod - in my current case the other end of the rod as is
usual just attaches to a plastic/nylon/whatever lever on the throttle
servo.

I am not sure that RF 'noise' can be generated just by rubbing two bits
of metal together when there is no potential difference between them

I note that in other situations - eg mobile phones in cars, car radios
etc there is LOTS of metal to metal contact surounding these devices,
and is not a source of interference (ignoring the common problems of
earth loops etc which are a different matter)

If it is relevent the RC operates on 36.xxx megs

So, I am thinking that maybe the problem doesn't really exist - in the
situation as I have outlined above - but I have been wrong before,
so....

If you believe it could be a problem I would appreciate a short
explanation of how the interference is generated, and whether your view
is based on personal experience or information gleaned elsewhere

Cheers and thanks in advance

David

 

[Tom Biasi]

message

I fly (or crash) radio controlled model planes as a hobby.

Engine speed is controlled by a servo which operates a piano wire
control rod which attaches to the throttle lever on the engine.

I am advised by the cluey and experienced guys at my club that metal to
metal contact in glow planes is a no no - ie the metal throttle control
rod mustn't attach to the metal throttle lever on my engine, as it will
cause glitches etc - ie interference with the radio control operation

However, on thinking a bit about it I don;t see how this can happen as
there is no potential difference between the motor throttle lever and
the control rod - in my current case the other end of the rod as is
usual just attaches to a plastic/nylon/whatever lever on the throttle
servo.

I am not sure that RF 'noise' can be generated just by rubbing two bits
of metal together when there is no potential difference between them

I note that in other situations - eg mobile phones in cars, car radios
etc there is LOTS of metal to metal contact surounding these devices,
and is not a source of interference (ignoring the common problems of
earth loops etc which are a different matter)

If it is relevent the RC operates on 36.xxx megs

So, I am thinking that maybe the problem doesn't really exist - in the
situation as I have outlined above - but I have been wrong before,
so....

If you believe it could be a problem I would appreciate a short
explanation of how the interference is generated, and whether your view
is based on personal experience or information gleaned elsewhere

Cheers and thanks in advance

David

Hi David,
I have heard of pieces of metal, long metal cables, etc. when in the
presence of an RF field can arc at the junction in the case of a high field.
Even in a weak field the junction can act as a diode and may produce some
broadband noise but I never heard of the warning WRT RC planes.
Regards,
Tom

 

[Mark Fergerson]

message

I have heard of pieces of metal, long metal cables, etc. when in the
presence of an RF field can arc at the junction in the case of a high field.

I'm pretty sure the level of RF in a model plane is unlikely to
approach that needed to produce arcs as it's receiving, not
transmitting. OTOH long conductors can also look like
multiwavelength antenna elements, absorbing RF and storing it
temporarily assuming just the right lengths, and if their length
changes at just the wrong time that energy will dump out as sparks,
producing interference with desired operation. But this smells
improbable; one would have to work at it.

The situation with phones etc is the opposite; engineers stay up
late making sure this kind of thing can't happen.

Even in a weak field the junction can act as a diode and may produce some
broadband noise but I never heard of the warning WRT RC planes.

This sounds like a more reasonable concern, especially as the
metals' work functions may be different allowing rectification. This
applies even if the metals are the same as one bit may be more or
less oxidized than the other.

A somewhat less catastrophic version of the first idea seems more
reasonable; that the moving connected metal parts will look like a
variable antenna element parasitic to the actual antenna on the
receiver, reducing/reinforcing the field strength seen by the
receiving antenna making reception vary wildly as if the plane were
jumping large fractions of a mile from the transmitter as the parts
move.


Mark L. Fergerson

 

[quietguy]

Thanks for the replies - much appreciated

David

 

[kell]

I fly (or crash) radio controlled model planes as a hobby.

Engine speed is controlled by a servo which operates a piano wire
control rod which attaches to the throttle lever on the engine.

I am advised by the cluey and experienced guys at my club that metal to
metal contact in glow planes is a no no - ie the metal throttle control
rod mustn't attach to the metal throttle lever on my engine, as it will
cause glitches etc - ie interference with the radio control operation

However, on thinking a bit about it I don;t see how this can happen as
there is no potential difference between the motor throttle lever and
the control rod - in my current case the other end of the rod as is
usual just attaches to a plastic/nylon/whatever lever on the throttle
servo.

I am not sure that RF 'noise' can be generated just by rubbing two bits
of metal together when there is no potential difference between them

I note that in other situations - eg mobile phones in cars, car radios
etc there is LOTS of metal to metal contact surounding these devices,
and is not a source of interference (ignoring the common problems of
earth loops etc which are a different matter)

If it is relevent the RC operates on 36.xxx megs

So, I am thinking that maybe the problem doesn't really exist - in the
situation as I have outlined above - but I have been wrong before,
so....

If you believe it could be a problem I would appreciate a short
explanation of how the interference is generated, and whether your view
is based on personal experience or information gleaned elsewhere

Cheers and thanks in advance

David

Those guys could be wrong about the WHY, but right about the
interference. You could get interference coming from the ignition. Do
you have a high voltage coil in the plane to give you spark? If there
is a conductive path from the engine to the radio-controlled
servomotor, you might get interference from the high voltage spikes
generated by the ignition coil.

 

[quietguy]

Good thought Kell, but most RC planes - all of mine - use a glow plug so
there is no coil or spark etc\\\

David

 

[Chris]

I fly (or crash) radio controlled model planes as a hobby.

Engine speed is controlled by a servo which operates a piano wire
control rod which attaches to the throttle lever on the engine.

I am advised by the cluey and experienced guys at my club that metal to
metal contact in glow planes is a no no - ie the metal throttle control
rod mustn't attach to the metal throttle lever on my engine, as it will
cause glitches etc - ie interference with the radio control operation

However, on thinking a bit about it I don;t see how this can happen as
there is no potential difference between the motor throttle lever and
the control rod - in my current case the other end of the rod as is
usual just attaches to a plastic/nylon/whatever lever on the throttle
servo.

I am not sure that RF 'noise' can be generated just by rubbing two bits
of metal together when there is no potential difference between them

I note that in other situations - eg mobile phones in cars, car radios
etc there is LOTS of metal to metal contact surounding these devices,
and is not a source of interference (ignoring the common problems of
earth loops etc which are a different matter)

If it is relevent the RC operates on 36.xxx megs

So, I am thinking that maybe the problem doesn't really exist - in the
situation as I have outlined above - but I have been wrong before,
so....

If you believe it could be a problem I would appreciate a short
explanation of how the interference is generated, and whether your view
is based on personal experience or information gleaned elsewhere

Cheers and thanks in advance

David

Hi, David. Unless you have a brushless DC servomotor controlling the
throttle, you might want to replace your brushes (and make sure you
have the right ones -- the wrong brushes, even when new, may not make
good contact). If they're worn, that can generate plenty of RF noise.
I believe that's the most common cause of interference with the RC
control.

If you want to follow up on the question of static generation, look at
any insulators rubbing against each other. That's a much more likely
cause of charge generation (like shoes on a dry carpet, or a comb on
wool). Sometimes, putting a tiny amount of dry graphite lubricant
between the rubbing insulators can help.

Good luck
Chris

 

[quietguy]

Thanks Chris - actually I am not having any problem with interference - the
plane in question works fine and no glitches noticed. However, experienced
club members (and lots of other guys on a RC plane newsgroup) suggested I
not contine to use metal to metal contact in a new plane I built - so I was
wondering about the how and what and such of this potential problem

But I appreciate your reply

David