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Old Feb 01, 2011, 06:56 PM
k2k
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United States, CA, Oceanside
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First hand experience - I had a 3.6 meter Ventus with flat wire to the elevator servo in the fin. 72 MHz radio. The elevator servo would oscillate once or twice around neutral after giving an input. Everyone said bad pot. New servo, same thing. Changed the extension to twisted wire and the problem disappeared.
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Old Feb 01, 2011, 07:21 PM
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Originally Posted by target View Post
Here's a question, and I'm looking for facts, not opinion!

Does it really matter if the servo wire is twisted or flat for the wings in our F3X planes, and the fuselages leading to the receiver?
Target, you've heard some facts, some well-informed opinions, and some nonsense in this thread. You're probably sick of it by now.

But I'll give you a fact you can easily check out. You know that network cabling that is used to connect computers? Ethernet cable, also known as Cat 5, Cat 6, and so on? Grab a spare scrap of it, and cut off the insulation with a sharp hobby knife or other suitable tool. Inside you'll find eight wires, grouped into four pairs - and every pair is twisted together.

There are tens of thousands of miles of this stuff in use all over the world, and you can bet your boots nobody would bother to twist it if it were not necessary. In fact, the twisting is MANDATORY to get enough immunity from RF noise for the network to actually work.

This is so important that the wires are colour-coded, and this is used to ensure that whatever current goes out one wire always comes back through its mate, the one twisted around it. This twisting largely cancels out the magnetic fields generated by the current in the wire, which is what reduces its tendency both to emit stray RF noise, and to pick it up.

If you mis-match the wires, so the signal goes out through one wire and comes back through a different one that is *not* wrapped around it, network performance drops drastically, sometimes to the point of not being able to send any data at all. This particular wiring error is serious enough to have a technical name all to itself - it's called a "split pair".

So here's the fact: twisting together the wires that carry a signal out and back WILL reduce RF emission, and RF pick up, substantially.

As to whether that matters for your application or not, you be the judge. Personally, I wouldn't bother re-wiring a servo (or an ARF) with twisted wire if it performed satisfactorily and range-tested okay with untwisted wire. But, as some of the posts in this thread attest, there are locations and environments where using twisted wire is the difference between being able to fly, and only being able to crash.

-Flieslikeabeagle
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Old Feb 01, 2011, 07:53 PM
Detail Freak
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Harbor City, CA
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K2K and F.L.A.B.-

Both great posts, thank you.

While the last one doesn't deal with RC planes specifically, It does help clarify and justify the twisting of wires.
And the previous post is indeed, as close to proof positive as you can get, that twisting the wires does work in some applications, which is what I was after.

Do I NEED to use twisted wire in an F3B plane for the aileron install? Probably not. But, knowing that it actually DOES SOMETHING is also really helpful.

I hate to waste my time building a plane, especially for someone else, by not doing the right thing. That being said, saving time is also important to me as well. And saving weight in sailplane wings, especially at the tips, is always a good idea.

I'm really interested in what Roman posted about the wire weigh differences between twisted and flat servo wires (factory supplied, not custome made of different gauges to save weight).
Obviously, if I take flat wire, and split it into 3 strands, and twist it, it HAS TO BE heavier per length than when it is flat.

But, I have never thought to weigh factory twisted wire vs same gauge factory flat wire; I just errantly assumed that the twisted wire has to be heavier, since more copper is involved. Interesting!

R,
Target
PS. I hope that you all realize that all the contributors to this thread are very intelligent, and although some of you may disagree, we are all looking for the same solution to the question, so please play nice.
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Old Feb 01, 2011, 10:46 PM
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Sorry... the analogy for network structured cabling and our servo wiring doesn't work. Cat 5 - 5E - 6 cable pairs are twisted so their signals don't interfere with the other pairs in the same sheath... and it has not much to do with RFI. In fact, if you look real close, you'll see that each of the pairs is twisted differently... i.e. some are twisted with more turns per inch than others. The data rates are much greater than we're dealing with while sending signals from the decoder section of the Rx to the servo amps.

Jack Womack - RCDD - I do networking on the physical level as part of my living... Oh... and lotsa RF, too...

Not trying to diss anyone... but that's the real poop on that one.
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Old Feb 02, 2011, 05:16 AM
Roman Vojtech
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Quote:
Originally Posted by target View Post

This is very interesting, I had not thought of this at all. Is that because the wires joined flat uses more material in the jacket to join the wires together??

Or, is the outer material itself lighter somehow??

Now I am very interested; thanks for posting this.
Hi Target,
when looking at wires in front of me, I can see that on flat cable is insulation a bit thicker and there is also perhaps some micro layer which keeps stripes flat and joined together (as you noted).

BR
Roman
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Old Feb 02, 2011, 08:50 AM
agony sweetns the victory
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Any Flying Field Across America
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I'm not at all saying that I disagree with all the "well known facts" (which without data are really just opinions) that twisted wires can reduce (substantially or at a minuscule level) RF noise in certain electrical systems and applications. The physics supports that assertion (at least at some level of reduction), but still no one has shown any technical data that answers T's question that twisted wires will reduce RF noise in R/C sailplane F3X applications - a system which has multiple unique components. Take a minute to recall T's original post....

Quote:
Originally Posted by target View Post
Here's a question, and I'm looking for facts, not opinion!

Does it really matter if the servo wire is twisted or flat for the wings in our F3X planes, and the fuselages leading to the receiver?

Is a 72Mhz system more in need of this than a 2.4G system due to the longer Rx aerial??

It is much easier for me to deal with flat servo wires, especially in the fuse, than twisted. I've done it both ways, and haven't noticed any difference. Twisted is heavier and uses slightly more wire length, so is heavier.

Thanks for facts inputted here.

R,
Target
T want's facts, so where's the data to support this claim for F3X applications, or any R/C sailplane application?
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Old Feb 02, 2011, 08:59 AM
LSF V,LSF Secretary,AMA Lifer
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I used to twist wire, and go through other gyrations, when doing data aquisition and control work. I was looking for 10 -6 voltage levels in noisy rf environments, often with explosive events in the very near area. Twisting, shielding, and various other tricks all made this possible. You can get a cleaner signal with twisted pairs, no doubt.

In the past, using analog receivers and servos, especially around brushed motors or wet power with a noisy ignition system, twisting long servo wires helped a lot.

Since I've swapped over to 2.4 gHz and digital servos I haven't worried about twisting servo wires. It's not proved to be necessary. The digital amplifiers in newer servos don't accept much noise.
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Old Feb 02, 2011, 10:43 AM
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Here's my two cents:

"Twisted Pair" known from networking cables was mentioned. Those have not much to do with the servo cable situation and are working different(ial).

In Fast Ethernet for example, there's signaling rates of about 100 MHz. This is RF obviously, because it's well above your 72 MHz radio. A simple connection using one wire for the signal and the other wire as ground would result in an antenna in many cases. So what's actually used in networking is differential signalling, i.e. for each signal there's a pair of wires twisted together. One of the wires carries the signal and the second wire carries the same signal, but inverted.

As atjurhs already explained, a current running through a wire creates a magnetic field. Now, if you add a second wire that's very close to the first one during the whole length, that has the same current running in the opposite direction, the net effect is both fields cancelling out each other almost completely.

However, this doesn't apply to a servo cable for two reasons: (1) a twisted servo cable isn't a twisted pair. The current running through the signal wire is different to the current running through the other two wires most of the time (if you cut a network cable, you'll notice that there's several twisted pairs, it's not all wires twisted together). (2) Even with digital servos, the frequency of the signal is well below 200 Hz. The corresponding wavelength is > 1500 km, a lambda/4 antenna would have to be > 375 km long. Therefore, it's very unlikely for a servo signal wire in an RC plane to become a transmitter.

Note that this is about the servo signal wire being a possible _source_ of RF interference, while the main concern in RC flight is the servo being the _receiver_ of RF intereference.

Now I'm by no means an RF expert, my knowledge about twisted pair comes from an IT background and my knowledge about antennas is from school physics, personal interest in EE and successfully building a lambda/2 dipole antenna for CB radio years ago (2.7m wire hanging from the house's rain pipe, connected to the antenna signal of the radio in 1st story, another 2.7m wire hanging down, connected to ground of radio), but here's my simple guess, why twisting might indeed help: an untwisted cable is shielded by the ground wire from one side, but completely exposed during the full length to the other side. lambda/4 of 72 MHz is about 1m, which is in the order of magnitude of servo cables in a large wingspan RC plane. If you twist the cable, it gets broken down to spans of about 5-10mm, because on every turn there's a bit of shielding from the ground wire.
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Old Feb 02, 2011, 11:11 AM
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United States, VA, Falls Church
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It has always amazed me the technical knowledge of most of the guys in this hobby... and then... well... you've got guys like me.

So what I'm gleaning from all of this - twisted wires, probably not necessary, but might be better and probably lighter. So no downside...

I got my answer... I'll still run whatever is in my shop...

D
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Old Feb 02, 2011, 11:54 AM
Don't take your guns to town
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Joined Mar 2004
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Quote:
so where's the data to support this claim for F3X applications, or any R/C sailplane application?
A bunch of people have posted their experience with twisting resulting in improvement. A bunch of other people have explained the physics behind it. Hitec has stated that they provide twisted wiring on certain servos for this reason. What exactly are you looking for?

RFI is one of the most elusive, unpredictable and difficult issues to address in the field of electrical engineering. You will not find any absolute answers on this subject. Every single system is completely different and even just moving a wire an inch to the left may cause or cure RFI problems.

Quote:
I got my answer... I'll still run whatever is in my shop...
Yep. For F3X with modern equipment it's likely doesn't matter what you use. But for F5B or FPV it may be worth the trouble.

On the subject of weight, I measured and weighed all the wing servo wires of my last 3.7m using Tefzel AWG28 weighing 5.38g/m for 3 twisted conductors. The total wing wiring weight not including connectors was 23g:
- RX-to-MPX: 4 3-wire sets .48m (19") long: 10.3g
- MPX-to-flap: 2 sets .25m (10"): 2.7g
- MPX-to-ail: 2 sets .96m (38"): 10.3g
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Old Feb 02, 2011, 12:19 PM
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Vespa - Do you have a good source for Tefzel? Please let me know. Dean has had very good success, and my MXC's need to go on a diet.
Now correct me if I'm wrong, but these have sheilding correct?
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Old Feb 02, 2011, 12:20 PM
Play loud, Fly high
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Vincennes,IN USA
Joined Apr 2007
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Testing

As I stated earlier, I am willing, and have started, to do some bench testing. As many have stated, this is a very situation specific issue. My background includes process control instrumentation, data cabling, audio systems, power quality, grounding/bonding, and I am a licensed master electrician who majored in physics at an engineering college. I have no formal training, and very limited knowledge of RF circuitry. I will focus my testing on the servo pulse signal leaving the receiver and entering the servo. Local club members have agreed to participate by making accessible as many different brand name systems as possible. I have 20MHz scopes in both DSO and analog and I will look for noise upsetting the pulse train to the servo at both ends of the wire. Please don't expect this to be a thesis level research project, don't have the time or tools, let alone the desire to take it that far. Just take the challenge of providing REAL DATA as an educational opportunity at the school and maybe offering a little payback for favors I have received here on this forum. Results when/if available attached to this thread, or a new one started if anything major develops..........Gil
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Last edited by Gil Gauger; Feb 02, 2011 at 12:21 PM. Reason: typo
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Old Feb 02, 2011, 01:41 PM
Don't take your guns to town
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Thousand Oaks, CA
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Cool Gil! That would be interesting to see! What I have found in most cases with strong GHz radiation is an offset in the servo position.

Sorry jgs, I don't know an easy way to get nice wire. Tefzel is just a slightly improved version of the more common Teflon (PTFE) and there are many other insulation types that are as good or better. Search Digikey for "hook up wire" to find some examples but it gets pretty expensive by the time you get a few colors. You can sometimes buy scraps (hundreds of feet) direct from the mfg (RDS Wire, Cooner Wire, etc.) but they have a limited selection of scrap in stock and high minimum order prices. All the insulation materials are around the same density so you really just want something thin. High strength materials like teflon can be very thin and still be rugged and high dielectric but are usually pretty stiff and not well suited for loose cables that are repeatedly handled.

If you really want something light, use magnet wire, use a very small signal, twist them and wrap the bundle in kapton for abrasion resistance inside the wing. Pretty extreme though.
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Old Feb 02, 2011, 04:02 PM
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Quote:
Originally Posted by schrederman View Post
Sorry... the analogy for network structured cabling and our servo wiring doesn't work. Cat 5 - 5E - 6 cable pairs are twisted so their signals don't interfere with the other pairs in the same sheath... and it has not much to do with RFI.
Dude - you're missing the forest for the trees. Maxwells equations tell you wire carrying current emits a field, and the same equations tell you reversing the current reverses the field. The wires are twisted to minimise radiated field - exactly how is that different from RFI?
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Originally Posted by schrederman View Post
In fact, if you look real close, you'll see that each of the pairs is twisted differently... i.e. some are twisted with more turns per inch than others.
I know that. Do you know why that's done? It's to vary the periodicity of the emitted field from each pair, so they don't add up along the length of the wire (destructive interference rather than constructive).

And that's also done to minimise RF emission, and RF pickup.
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Originally Posted by schrederman View Post
The data rates are much greater than we're dealing with while sending signals from the decoder section of the Rx to the servo amps.
True, and also irrelevant. The point is that the twisted wires minimise both emission and pick up of RF fields. As I made very clear in my original post, the degree of importance of that depends on your application. But there is not the shadow of a doubt that twisting matching wires carrying an outbound and return signal reduces interference.
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Originally Posted by schrederman View Post
Jack Womack - RCDD - I do networking on the physical level as part of my living...
Been there, done that...I taught networking classes for a while. And spent quite a few years studying electromagnetism at graduate level before that.
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Originally Posted by schrederman View Post
Not trying to diss anyone... but that's the real poop on that one.
Your facts are entirely accurate, but you seem to have missed the bigger implications beyond the narrow field of computer networking and specific data transmission rates.

-Flieslikeabeagle
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Old Feb 02, 2011, 04:22 PM
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Originally Posted by wuselfuzz View Post
So what's actually used in networking is differential signalling, i.e. for each signal there's a pair of wires twisted together. One of the wires carries the signal and the second wire carries the same signal, but inverted.
<snip>
However, this doesn't apply to a servo cable for two reasons: (1) a twisted servo cable isn't a twisted pair. The current running through the signal wire is different to the current running through the other two wires most of the time
Think about it - you're sending a signal - moving electrons back and forth - to the servo. It takes a complete circuit to send a signal, so there *HAVE* to be two wires involved, one carrying current out to the servo, one bringing it back. Without that, you don't have a complete circuit, and without a complete circuit, you don't have a working servo!

If you think about it, when you raise the signal wire high (+5V), the current returns through the ground wire. When you drop the signal wire low (0V) the current flows out through the positive wire, and returns through the signal wire.

So twisting the +5V and 0V wires round the signal lead makes very good sense. It's not quite as good as a differential signal carried by a floating wire pair, sure, but its a lot better than no shielding.
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Originally Posted by wuselfuzz View Post
(if you cut a network cable, you'll notice that there's several twisted pairs, it's not all wires twisted together).
Four pairs, actually. Used for full-bandwidth simultaneous data transmission in both directions - one pair for each direction - at 10 MHz and 100 MHz network speeds. I believe all four pairs are used in Gigabit Ethernet.
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Originally Posted by wuselfuzz View Post
(2) Even with digital servos, the frequency of the signal is well below 200 Hz. The corresponding wavelength is > 1500 km, a lambda/4 antenna would have to be > 375 km long. Therefore, it's very unlikely for a servo signal wire in an RC plane to become a transmitter.
Square waves have a huge array of harmonics at higher frequencies, you know. Back in the days when I was building my own radios, I used to use a few inches of wire connected to the ouput of a 1 KHz multivibrator (square wave oscillator) as an RF signal - and I could pick up harmonics of that square wave AUDIO signal all over the AM band (up to 1.6 MHz) and clear through most of the shortwave bands (up to tens of MHz).
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Originally Posted by wuselfuzz View Post
Note that this is about the servo signal wire being a possible _source_ of RF interference, while the main concern in RC flight is the servo being the _receiver_ of RF intereference.
Note that the Maxwell equations are symmetrical under time reversal, or equivalently, reversal of the direction of motion of an electromagnetic wave - what works well as a transmitting antenna also works well as a receiving antenna!

This is why a parabolic reflector works equally well to focus the light from a flashlight bulb into a beam (transmitter), or to pick up a beam of sunlight and focus it on a thermal concentrator (receiver). Same for a convex lens (magnifying lens) - it will collimate the light from a lamp in a lighthouse (transmitter), or fry an ant when sunlight hits it (receiver).

Twisting the servo wires reduces the amount of RF emitted by the signal they're carrying, yes, and equally reduces their tendency to pick up RF signals in the vicinity.
Quote:
Originally Posted by wuselfuzz View Post
successfully building a lambda/2 dipole antenna for CB radio years ago
CB is a perfect example - same antenna used for receiving and transmitting, right? Because what works well as a transmitting antenna also works well as a receiving antenna!

-Flieslikeabeagle
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