RadioBanter - View Single Post

October 14th 03, 05:06 PM

On 14 Oct 2003 08:15:58 -0700, (Jeffdeham) wrote:

Cecil Moore wrote in message ...

Zero current. A voltage estimate would be V^2/600=100w, or V=245V RMS.
The net RMS voltage would be double that value so peak voltage might
be around 700 volts.

V^2/600=100w

Where did the value of V and 600 come from in this formula?

I'd like to be able to calculate the voltages also for let's say my 5 watt QRP rig.

73!

Jeff

Hi Jeff,

Reg already explained it. You know the applied voltage to a tuned
circuit whose (presumed for this discussion) opposite leads are at the
far end. You know the characteristic Z and the radiative loss R. You
can guesstimate the Q of this circuit as the multiplier. If you reach
out and touch that element's end (or bring it suitably close to a
conductor or other lossy element) you would necessarily depress that
same Q - by being the load. -Zap!- as the comix used to say. So for
your 5W you can expect that as your hand approaches the tip (how'd you
get up there anyway?) both your body and the proximity of everything
around you (like a ladder, or simply the ground you stand on) has done
far more to depress that same calculated Q to diminish the multiplying
effect, but not without some prospect of surprise. Many shock
injuries come from that alone (falling off the ladder is what kills
you, not the 50V 5mA buzz that rattled your nerves).

When I was in the Navy, we had all sorts of exposed radiators around
the ship. Common lore (certainly untested by me) was that if you
found yourself holding one when transmission was begun, to not let go
because you would draw an arc where formerly the voltage was depressed
by your contact. Now, as to how you could distinguish when the QSO
started or ended to time your unfortunate hand-hold was never offered;
and so I was loath to test this lore. Further, no one asked about why
you would stand there holding it while simmering. I generally carried
an NE-2 neon bulb with me when I went aloft so that I had at least
some method to detect hot surfaces (and scramble back down to find out
what jerk had thrown a tagged breaker).

73's
Richard Clark, KB7QHC