On Sat, 04 Dec 2004 21:51:09 GMT, Richard Clark
wrote:
On Sat, 04 Dec 2004 18:40:20 GMT, Richard Clark
wrote:
At 30 MHz, this 1.5pF capacitance represents a reactance of 3.5K Ohm.
This rather sweeps aside your specification for the load and replaces
it with 4700 -j3537 Ohms.
Actually 1700 -j2258 Ohms
As you can see, this is the reason why open stubs are avoided.
"Knowing" the parasitic capacitance is problematic. The fringing
effect at the end is difficult to manage whereas a short is much
simpler to define and implement.
73's
Richard Clark, KB7QHC
Dear Richard,
Each and every one of your concerns is deeply appreciated. I had
seriously about measuring that 4700 ohm resistor by itself for just
the reasons that you mention. The reason that I did not is quite
simple. The Resistance dial on the General Radio Model 1606B RF Bridge
only goes to 1000 ohms. C'est la Vie!
I could have chosen a much lower resistance to start with, but then I
would not have had the desired high SWR, which is critical to the
experiment. You might well ask why I made the switch from a 1/4 wave
open ended stub (at 10.6 Mhz) to the 30 MHz test using the same piece
of line. That was to get a high SWR and still have reasonable
confidence in the value without actually measuring it. Perhaps I
should use a capacitor instead. That's still a possibility. I am
thinking of doing it over with a measured 1 k resistor, and just
accepting the SWR of only 20:1. I don't think I have any low ohm
resistors that would give me something around 1 to 3 ohms and still be
low inductance - not from my parts box. Actually, I doubt seriously
that the capacitance effects are as bad as you are suggesting, but I
won't argue that without some additional measurements.
My reason for not going with a short circuit was even simpler. I
wanted a concrete value for a non-zero load power.
Regarding the artificial adjustment of the attenuation loss from
around 0.9 up to about 1.72 did not bother me at the time that I did
it, but I can see why it bothers an independant observer. I will have
to do something about that, and perhaps doing it over with a 1 K
resistor is the answer to all of that - so, here goes, and I will get
back to you in a few hours.
BTW - this is becoming more and more academic, since I think the real
mystery has been resolved, but there's nothing like getting it wrapped
up with a proper ribbon.
Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk