Now that I know what gauge wire I have a real newbie question. When installing the bus do I do a complete loop like a snake eating his tail or can I stop any where on the layout?
Billy21
Stop anywhere. The ideal situation is to have the booster connect dead inthe middle to keep the bus runs as short as possible: ============<booster>===========
rather than: <booster>==================================
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I've been told not to make a loop in the track bus. The logic makes sense but I've never seen trouble doing so. Remember that a simple oval with two track feeds creates a loop via the rails.
I also can't see any advantage in a loop if the booster is located in the center.
Martin Myers
mfm37I've been told not to make a loop in the track bus. The logic makes sense but I've never seen trouble doing so.
I've been told not to make a loop in the track bus. The logic makes sense but I've never seen trouble doing so.
That's because it's an old wife's tale that keeps getting repeated. On the other hand, the control/throttle buss should never be looped back on itself. But it's fine to loop the track buss.
mfm37 Remember that a simple oval with two track feeds creates a loop via the rails.
Remember that a simple oval with two track feeds creates a loop via the rails.
Exactly!
mfm37 I also can't see any advantage in a loop if the booster is located in the center.Martin Myers
There may be a slight advantage in that the current will (literally!) take the path of least resistance, which would be the shortest leg of the loop from the booster to the load.
But loops with a single booster would generally tend to be on smallish layouts with shorter buss lengths. Assuming the buss wire was sized properly, the resistance would be so low anyway that the effect would be minimal.
Steve
billy21 Now that I know what gauge wire I have a real newbie question. When installing the bus do I do a complete loop like a snake eating his tail or can I stop any where on the layout?
When you ask about the "tail eating", you aren't really asking if the loop ends are connected, are you?
Stevert mfm37 I've been told not to make a loop in the track bus. The logic makes sense but I've never seen trouble doing so. That's because it's an old wife's tale that keeps getting repeated. On the other hand, the control/throttle buss should never be looped back on itself. But it's fine to loop the track buss.
mfm37 I've been told not to make a loop in the track bus. The logic makes sense but I've never seen trouble doing so.
One of many that continaully get repeated.
Engineer Jeff NS Nut Visit my layout at: http://www.thebinks.com/trains/
On my layout my bus wires are looped together...I use an NCE PowerCab and have no problems.
Dennis Blank Jr.
CEO,COO,CFO,CMO,Bossman,Slavedriver,Engineer,Trackforeman,Grunt. Birdsboro & Reading Railroad
Stevertere may be a slight advantage in that the current will (literally!) take the path of least resistance, which would be the shortest leg of the loop from the booster to the load.
It will ALSO take the other path. the current will split over all paths in a ratio inversely proportional to the impedance of those paths.
So, a loop will act like two feeds in parallel and provide less voltage "drop" for a given wire size. Most of the current will flow over the easier path and the rest over the other path(s).
The mind is like a parachute. It works better when it's open. www.stremy.net
Stevertmfm37I've been told not to make a loop in the track bus. The logic makes sense but I've never seen trouble doing so. That's because it's an old wife's tale that keeps getting repeated. On the other hand, the control/throttle buss should never be looped back on itself. But it's fine to loop the track buss.
It's not a old wife's tale. It is generally not recommended to loop the bus wire. This is how a UHF or shortwave antenna works. if you know an electrical engineer ask them about it. This loop of wire picks up radio waves (which there are huge amounts of these days) which interferes with the DCC signal. The bigger the loop the more radio waves that are picked up and more interference on the wire. By not looping the ends you eliminate this antenna. While the track creates a loop and thus another antenna you can reduce signal lose by not looping the ends of the bus wire. if you do then you will have multiple antennas.
Again most of this is done to reduce signal loss, which for larger layouts becomes a concern. Signal Loss causes runaway trains and other operating problems.
I hope this clarifies why loops in track busses are generally not recommended.
Colorado Front Range Railroad: http://www.coloradofrontrangerr.com/
And by having two arms go out but NOT loop together you have a simple dipole antenna. The whole thing is negated if you are talkign about an oval track plan, the TRACK is two conductors forming a complete loop.
The bus wires on my 8x12 double track oval were NOT looped, but that's because I had plans to make more than one power district out of it so I planned ahead and ended the bus wires without connecting them. The track was not gapped so i had two concentric continuous loops and there were no odd control issues. Large, really large, layouts sometimes have signal degradation issues but that's not usually because of induced RF. Incomign signals from broadcast antennas, unless you build you layout directly on one, and in the millivolt range, far smaller than the 12-15 volts of the DCC signal. The problem with really long runs becomes one of capacitance and inductive loss from having a long run of parallel wires. Once again, how you run the bus has little to do with it because no matter what it's connected to track which is always (we hope) a fixed distance apart and (again, we hope) perfectly parallel. Over a long distance this can result in distortion of the DCC signal and possibly cause control issues. In those cases, people have found it useful to put a terminator at the far end, if it's not practical to break the long bus run into smaller sections.
What Randy said.
Also look up long wire antenna.
Antenna theory is pretty complex (I used to teach it) but loop or no loop makes little practical difference if the lengths and placement match some broadcast frequency. In the unlikely event that happens, simply add or subtract a bit of length or ground through a small capacitor. Most common radio broadcast frequencys are far above DCC signals in the spectrum.
rrinker And by having two arms go out but NOT loop together you have a simple dipole antenna. The whole thing is negated if you are talkign about an oval track plan, the TRACK is two conductors forming a complete loop. Large, really large, layouts sometimes have signal degradation issues but that's not usually because of induced RF. Incomign signals from broadcast antennas, unless you build you layout directly on one, and in the millivolt range, far smaller than the 12-15 volts of the DCC signal.
Large, really large, layouts sometimes have signal degradation issues but that's not usually because of induced RF. Incomign signals from broadcast antennas, unless you build you layout directly on one, and in the millivolt range, far smaller than the 12-15 volts of the DCC signal.
Dipole antenna's generally pick up lower frequencies whereas closed-loop antenna's pick up higher frequencies. DCC uses a fairly high frequency signal and is much more susceptible high frequency interference than to low frequency interference generated by a dipole antenna. High frequency causes more problems than low frequency noices. interference is a signal and when added to the original DCC signal it produces a new signal. only the amplitude of the signal is voltage dependent. the DCC signal is frequency dependent. When you make a change on a throttle it makes a change in the frequency of the signal.
rrinkerThe problem with really long runs becomes one of capacitance and inductive loss from having a long run of parallel wires.
The problem with really long runs becomes one of capacitance and inductive loss from having a long run of parallel wires.
Generally signal losses in a wire are lost more to the natural resistivity and magnetic field generated by putting a current thru the wire. By twisting the wires together you can eliminate alot of the interference between wires. This is because the magnetic fields generated by each wire are generally equal and oppisite. This cancels out the magnetics fields. By putting a current thru a wire you creat induction. you can not lose induction.
How are you saying that there is a capacitive loss? once a capacitor becomes charged it acts a wire.
Inductive loss. The capacitor effect of parallel conductors can alter the waveform - mainly by rounding off the square waves. Rememebr the DCC signal is alternating, so any capacitor created by parallel wires (more likely the rails) would not just charge up and stay charged. A capacitor on AC rounds off the waveform.
As for the whole antenna idea - the actually ability to pick up anything is extrememly limited. The antenna won;t resonate unless it's an even multiple (or fraction) of the frequency being received, and the chances of anyone having a bus length that is exactly a multiple of a common frequency is very slight. Plus for there to be any significant amount of induced current you'd have to be an even wavelength away on the same plane as the transmitting antenna. You would get far more induced voltage inthe DCC bus if you cabled it with a 12V AC power bus for running structure lights. What's REALLY a bad idea is cabling the bus with 120V AC wiring. ANd if you have remote sensors for track occupancy, do not cable those wires with the DCC bus, and if they cross the DCC bus they should do so at 90 degree angles to avoid inducing any voltage crosstalk from one system to the other.
Seriously, I have NEVER heard of someone having their DCC system go nuts because their bus wiring acted as an antenna and coupled a radio broadcast to the decoders. The extremely long runs probably has shown up once or twice on REALLY huge layouts - although I don't think I've heard it being mentioned by any large modualr groups, and when they set up at shows they typically make a layout far larger than anyone has at home. The main issues those guys have is when there are multipel groups in close proximity all using wireless DCC systems, which all tend to be on the same frequency - but that's not coming through the track or bus wiring, that's right through the radio receivers.
We need to keep in mind that we aren't dealing with radio waves here and that the minimum pulse width of a DCC signal is 58usec and a total duration of 116usec (from NMRA standard S-9.1). This calculates to a frequency (for those who still demand that DCC is an AC signal) of 8620 cps. That is in the audio bandwidth and nowhere close to UHF. Of course if we are discussing induced currents, which is really what a radio wave is over extreme distances, then we can talk about the distance of the bus from the track itself but in doing so we will need to look at the load impeadance of the track, the locomotives and the output impeadances of the boosters to determine how much of an induced current the bus can have on the track. Now if we want to have a discussion on radio terminology like SWR (standing wave ratio), which is a consideration in antenna design on how much of the power is transmitted into the air vs. how much is reflected back to the source and lost, then we'd need to calaulate to wavelength of a 8620 cps signal. I'll do the calaulation for you, it is 34,778m . I don't know of any layout that is this big. Now I cheated slightly here and used the speed of light for the EMF propagation speed. The speed in copper will be slightly less but not enough to skew this calaulation.
Now where the discussion of long buses, looping and such have any merit is in the waveshape of the DCC signal. This is a function of the inherent induction and capacitance of the bus conductors and their length. Termination RC networks are used to cleanup the leading edge of the signal so that it is still within the rise time specifications for the signal. Twisting the bus conductors can reduce the distortion but only on very long buses.
Most of us don't have layouts where any of this really matters but not looping the bus is perfectly fine. I recommend folks use more blocks, which means shorter bus runs, smaller fault domains, smaller bus wire and improved expandability.
Yes, this has gone WAY into the realm of esoteric and far from the everyday reality. As I mentioned, I've only heard of signal issues on HUGE HUGE layouts and have NEVER EVER heard of any system picking up radio signals and making the locos run out of control because their bus wire was in the form of an antenna.