[sdiy] filter cap needed?

[Electronic Battle]

> Is there some Rule of Thumb frequency to be handled by the circuit,
> when rails needs to be analyzed as transmission lines rather than
> ideal power?


Yes - when the length of the rails approaches the wavelength of the 
frequency of interest in the dielectric of interest ...

so at a frequency of (say) 40kHz, wavelength in air = 7500m , wavelength in 
dielectrics like PVC or polyethylene or fibreglass = maybe half of this 
value ...(1/root (epsilon)) if your power supply feeds down cables that are 
maybe a couple of kilometres long then it becomes an issue. Epsilon = 
relative permitivvity of 
dielctric:http://www.microwaves101.com/encyclopedia/coax.cfm

At frequencies of (say) 100MHz, wavelength is 3m ...400MHz = about 70cm, 
1GHz = 30cm: these lfrequencies "see" lengths of transmission line now and 
can be treated as such.

Just short out the lines with a low ESR (ceramic or polyester) capacitor of 
a few hundred nF in shunt to a larger smoothing cap of (say) 1 or 10uF.

EB