Topband: Current, current, here's the current??

[Tom Rauch]

> It was asked if you measure the voltage at any point
> on the coil, how do you know the current? I assume the
> power is constant at any point on the coil so if you
> calculate the voltage, guess what? You can also
> calculate the current assuming you know the power.

Greg,

That is a common perception but unfortunately it is not correct.

We can NOT multiply voltage times current to obtain true or effective power
in a system with reactance, unless we also know the phase angle and consider
the vectors when making calculations. If we ignore phase, we simply get VAR
reactive power, and that can be hundreds of kilowatts with only a 100-watt
transmitter as the source!!

I've placed a small article on my website at:

http://www.w8ji.com/mobile_and_loaded_antenna.htm

It explains how voltage and current changes, and differences in systems.
Unfortunately we have to accept the fact in reactive systems like small
loaded antennas scalar voltage and current doesn't tell us much about power.
Otherwise you may unintentionally invent "perpetual motion".

There are some very good AC circuit tutorials available on the Web that will
explain AC circuit basics if anyone doubts rules governing reactive systems.
One is at:

http://www.ibiblio.org/obp/electricCircuits/AC/index.html


73 Tom