[TowerTalk] Trap Resonance

[Jim Lux]

At 07:57 PM 11/21/2003 -0500, Joe Subich, K4IK wrote:

>Get off your high horse.  In a series circuit, which a loading
>coil most certainly is, the current into and the current out of
>a device must be the same.  In the RF circuit, if the current
>into and out of the coil is not the same only three things can
>be happening:
>
>1) there is radiation
>
>2) there is loss
>
>3) the measurement is disturbing the system being measured.
>
>I suppose radiation is fine if one has a helical antenna (a
>coil designed radiate) but that's not what the debate is about.
>
>If you can't understand those concepts, go back to the very
>first circuits (and E&M) courses.
>
>ANY measured difference in I(in) vs. I(out) is either due to
>one of those three effects and to argue otherwise is to argue
>for perpetual motion.

Based on experience with tesla coils which, while not designed to radiate, 
do, and which have significant capacitance to ground, I'd add a fourth 
effect: parasitic capacitance from the coil to ground (which is, of course, 
different for each part of the coil).  While not exactly a transmission 
line (at least a TL isn't a very good model), the coil does look like a 
series of inductors that are coupled to adjacent inductors, and also 
connected to ground through a capacitor (much like a ladder 
network).  Significant current can and does flow from through that capacitance.

Just as a quick ballpark of magnitudes, to check reasonableness:
  A thin wire far above ground has a capacitance of about .3 pF/cm.  Let's 
assume we've got 10cm of wire, for 3 pF.  At, say, 10 MHz, the impedance of 
that capacitor is about 5K.  That's close enough (i.e. within an order of 
magnitude) to the impedances you'll find in a monopole that I'd think that 
calculating the real effect is worthwhile.

One might also approach it another way... Assume the loading coil is, say, 
10 cm in diameter and 30 cm long.  Consider a 10 cm long chunk, which would 
have a surface area of about 300 square cm.  Assume further that this chunk 
is about 1 meter above the ground.  A 300 square cm capacitor spaced 1 
meter away would have a capacitance of 0.03*epsilon Farad. No dielectric 
here to speak of, so epsilon = 8.85 pF/m for a C of 0.3 pF. Interestingly, 
the capacitance of one part of the coil to another is probably more 
significant.

In any case, parasitic C is a very important part of such a circuit, in 
addition to the radiation from the coil itself.

Jim W6RMK