Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx> Hi Bob,
Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx> > 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.
Yes, I am very much aware of this. The real question is what is the transient time of the cable. The context for the above statement is one round trip thru the cable (down and back). Cables with no termination at either end and no cable losses can bounce forever. What I believe we have in this lower frequency range is something between what you say and the ideal worse case senario that I mention.
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.
Yes, I agree for a single trip down the cable to the scope , but I think there is more to it than this.
I think Pspice had a transmission line model so it could be simulated relatively easily.
Yes and I have done this simulation and got an artifact that I can't explain any other way. The steady state response was seemingly correct but the result makes me question the transient response.
Gerry R.