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Post by chamm37 on Oct 29, 2008 22:53:40 GMT 10
Ken you amaze me on how much you know mate ;DJust amazing ;D
Regards,
Chris ;D
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Post by Hamburglar on Oct 30, 2008 3:28:23 GMT 10
The majority of my friends have girls. I always told them that it was just karma getting back at them for all the bad things they have done to the female population.
For a twisted pair to cancel out EMI they need to be balanced (equal and opposite signal on the two conductors). It'd be interesting to see if this works on a servo cabling because it doesn't have balanced conductors. I'm not even sure if it'd work on EMI which is induced into your +ve and -ve conductors because they are not balanced either. The return path for your servo position signal will be along the -ve conductor. Surely I must be wrong because you can buy twisted servo wire.
I've been toying with the idea of putting a capacitor across one of my spare Rx channels. Primarily to help out during times of high current draw and prevent a low Rx voltage. Theoretically this would help filter EMI on all of the power conductors too.
That's right Ken, distance from the tower is a huge factor in this. It's the good old inverse square law. Double your distance from the tower and the signal will be 4 times less, triple it and it'll be 9 times less, etc.
Thanks for the stimulating converstion.
Jordan
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Post by thevon on Oct 30, 2008 6:11:18 GMT 10
I don't think I've experienced the Bald Knob servo problems when flying at the gate, or at the Hump. We've spent hours DSing and you'd pick up on that pretty quickly! The problem occurs only in the middle of the field or flying off the SE knob itself. If I'm flying at the gate or the hump and go cruising up towards the towers I usually see control surfaces jiggling, and can experience temporary lockouts. Again I have to say that it doesn't seem to happen with the Multiplex IPD Rx in the Fazer.
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Post by skyboyken on Oct 30, 2008 19:37:24 GMT 10
Actually Jordan, not wishing to be a smart**se but with spherical spreading losses it's an inverse cube law. So double the distance get 1/8th the signal strength  . And yes, twisted servo cables do work to cancel induced current to an extent. This is well proven empirically, hence the availability of twisted servo cables as you say. What I don't have an understanding of, is why the twisting is less effective than using a ferrite choke. It should be equally effective according to the theory, but there is heaps of empirical evidence that it does not work as well. Ken. |
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Post by Hamburglar on Oct 31, 2008 7:11:49 GMT 10
Ken,
Interesting. Can you please explain or send my a link to why the inverse cube law would be used in this situation? All of the instruction I have received on this subject referred to the inverse square law for calculating signal strenth over distance. I understand that we do not live in a vacuum and there will be additional losses due absorbtion, scattering, etc.
The twisted wires and ferrites work on different principals. A ferrite bead creates an inductor which opposes rapid changes in current (RF). The twisted wires create a smaller effective antenna area and allow the EMF induced into one loop to cancel out the next. I've never worked with anything other than a twisted pair. Servo wires are the only things I have seen with 3 twisted wires. I've always thought that for a twisted pair to be effective it needs to have a balanced signal which a servo clearly doesn't have.
Jordan
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Post by Pij on Oct 31, 2008 14:13:18 GMT 10
I've been doing a train-of-thought on this. Probably the same reasoning my highschool physics teacher used, but I'm not sure I was listening at the time.
Imagine you have a point-sized transmission source, which transmits equally in all directions, 3-dimensionally. Just one part of that transmission concerns us, so if you like, you can imagine the transmission overall appearing as yellow light radiating from the point. The part that concerns us, can be imaged as red light, and this part is point-sized at the source, and is a fairly tight little beam, square in cross-section, that reaches a beam width of 1cmx1cm at a distance of 1m from the source.
This part of the overall transmission, coloured red for identification, would then reach a beam width of 2cmx2cm at 2metres, 3cm-square at 3m, and so on.
The INTENSITY of the beam at any distance could be measured by a receiving device, or we could simply imaging the intensity of the redness fading with distance.
The full power of the beam (or the full intensity of redness) radiates from the point-source, and is only slightly spread at 1m from the source. The intensity would be the mathematical inverse of the beam size (cross-sectional area) at any given distance.
So, at 1m, the intensity is 1/(1cmx1cm)=1 unit.
At 2m, intensity is 1/(2x2)=0.25 units.
At 3m, 1/(3x3)=0.11 units.
At 10m, 1/(10x10)=0.01 units.
At 100m, 1/(100x100)=0.0001 units.
I've never thought this through before, in terms of RC implications, but I find that to be pretty scary!
Anyway, as I reason things, that's inverse-square, not inverse-cube. This model should apply to anything radiating in 3-D, even sound intensity. I can't see any way to introduce a third dimension to the intensity issue, as it depends on cross-sectional area, not volume.
If we were talking about gas-dispersal, which was supposedly spreading in all directions equally, and was an ever expanding sphere of uniform concentration, then the dispersal would be through a volume, not an area, so then inverse cube would make sense.
If it was a nuclear explosion, the radiant energy intensity would be in terms of inverse-square, and the particulate contamination would be inverse-cube, but who would be around to care?
Still, this stuff is not my field, so if I've stuffed up the reasoning somewhere, please let me know.
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Post by skyboyken on Oct 31, 2008 18:37:45 GMT 10
Pij,
you are correct that the signal starts at point of origin with a given amount of energy. As it radiates outwards that same amount of energy is spread over more and more of the surface of either a cylinder (inverse square losses) or a sphere (inverse cube losses).
Hamburglar,
no need for a link, it's as simple as that. The signal from anything other than a beam-forming antenna spreads in a pattern which is closer to a sphere until it is constrained by the EM layers in the upper atmosphere, after which it spreads in a pattern closer to a cylinder.
The twisted wire pair effectively breaks a long antenna into many shorter ones adjacent to each other. The trick is that the current induced in each adjacent section is reversed (because of the twists) and so cancels out over the length of the wire(s). Thus each method should be equally effective but by different means. That's what I meant when I talked about the theory.
Ken.
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Post by shane on Nov 1, 2008 1:11:40 GMT 10
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Post by Hamburglar on Nov 1, 2008 3:38:25 GMT 10
Ken, I still can't understand where the inverse cube formula fits in to this. The ratio of the radius of a sphere to it's surface area is an inverse square ratio. Surface area= 4 x pi x radius squared (ie. double the radius and the surface area increases 4 fold) Using a cubic formula infers that the RF energy is being distributed throughout the volume of the sphere. Volume= 4/3 x pi x radius cubed. The energy is not lost though. It travels outwards from it's point of origin, reflecting, refracting, and diffracting and doing all sorts of crazy stuff until it is absorbed (by the atmosphere, earth, or even peoples servo leads  ) I can't see why a directional antenna would follow a different rule. They work on exacty the same principals as an omni antenna. It's still the same old RF. The energy is just reflected and directed to form a beam. Let me know what you think. It's great to get the old brain ticking over again. The course over here is rather dry. It's just page after page, manual after manual. Thanks, -Jordan |
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Post by Pij on Nov 1, 2008 7:14:12 GMT 10
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Post by thevon on Nov 1, 2008 8:23:09 GMT 10
Well I didn't know that twisted leads helped to reduce this problem at all! I've been avoiding them since they'd be heavier, and in the Nemesis I ran thin, very straight leads! So I've really learnt something here! Thanks.
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