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.
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