Re: Terry's New Plane Wave Antenna & DC's capacitive antenna

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi,
----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, December 26, 2005 11:23 PM
Subject: Re: Terry's New Plane Wave Antenna & DC's capacitive antenna


> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
>
> >Original poster: Steve Conner <steve@xxxxxxxxxxxx>
> >
> >>One question I have been pondering is the transient response for
> >>Terry's antenna.  At frequencies below the pole formed by the 20 nf
> >>base capacitance and the 100 ohm impedance (50 ohm coax + 50 ohm in
> >>series) or about 80KHz, neither end of the scope will be
> >>effectively terminated.
> >
> >There is no 50 ohm terminating resistor to ground in Terry's design.
> >It goes into the 1 Meg input impedance of the scope, and the coax
> >just looks like a capacitance which adds to the 20nF loading
> >capacitor. So the pole is not at 80kHz like you say, in fact it is about
10Hz.
>
> Yes, there is not a 50 ohm termination to ground at the scope end
> only a 1Meg ohm input impedance of the scope.  There is, however, a
> 50 ohm resister at the antenna end in series with the coax to
> terminate anything reflected off the scope end that is high enough in
> frequency.  It is those frequencies that are too low for this type of
> termination that I worry about.

At frequencies whose wave length is significantly longer than the transient
time of the cable, the open circuit cable will look like a capacitive load.
So it will form a zero with the 50 ohm driven end termination.
So the effect on frequencies that you describe as "too low for this type of
termination" is just a little phase shift and attenuation caused by that
zero.

At frequencies approaching and beyond the 1/4 wave length frequency things
are complicated.
1. The cable will look like 50ohms so the input amplitude will be halved.
2. The scope end is effectively un-terminated so the signal will double at
that end and be reflected.

I think the effect of 1 and 2 cancel and produce a flat response and note
the reflected signal will be absorbed by the antenna termination.

3. At various resonances (poles) lengths i.e. 1/4, 3/4 etc the cable will
resonate. I don't think (guesstimate) there would be any zeroes.
I think (guesstimate) the effect of this at those resonant frequencies is
probably doubling in amplitude at the scope end.
Depending on the cable length this could occure as low as 15MHz for the 1/4
wave mode and be very broard due to its low Q of say 2.

I think Pspice had a transmission line model so it could be simulated
relatively easily.
>
> There are two poles.  One is with the [coax impedance + the 50 ohms
> in series with it] and the 20nf thevenin source capacitance - this
> determines where the source series termination ceases to be
> effective.  The other is with this capacitance and the 1 Mohm input
> impedance of the scope (your ~10Hz pole) and determines the low
> frequency response..
>
> Gerry R
>

Robert (R. A.) Jones
A1 Accounting, Inc., Fl
407 649 6400