Tam/WB2TT wrote:
"Roy Lewallen" wrote in message
...

Tam/WB2TT wrote:

Roy,
I wasn't too clear, but I have 2 T networks back/back. That makes the
center cap C/2. I am going to run SWCad on the Pi configuration later,
and see what that does.

If you've cascaded two sections, you have two 1000 pF capacitors in
parallel at the center. That makes a total value of 2000 pF at that point.

Roy Lewallen, W7EL


It's a high pass filter to reject the AM broadcast band. So, the two 1000 PF
caps are in series. Am I missing something?

Sorry, I missed that you had made a T network rather than pi.

In general, a tee network substituted for a pi will have the same
characteristics only at one frequency, but will have different transfer
and/or impedance characteristics at other frequencies. So the
substitution should be done with care if characteristics are important
at more than one frequency.

In this case, though, if you make a tee network which has the same "half
wave" characteristic as the pi at the design frequency, it'll have
identical transfer characteristics (it's got the same filter response)
and complementary impedance characteristics. That is, at frequencies
where one network has an input impedance greater than 50 ohms, the other
will have an impedance that's less, and the phase angles are the
negatives of each other. And, luckily, the transformation is simple for
this particular special case -- the T network reactances are also all
the same and also equal to the Z0 of the "transmission line". So one is
just as good as the other.

The HPF equivalent doesn't of course simulate a transmission line,
although the impedance transformation though the filter is unity at the
design frequency. Otherwise, it works in pretty much an opposite way
from the LPF.

I need to correct and clarify a couple of points I made in my earlier
posting.

The "half wave" lowpass filter simulates a half wavelength transmission
line only at and near the design frequency (where the reactances are all
the same). It doesn't do a very good job either above or below that
frequency. For a better general simulation of a *short* transmission
line, reduce the end pi or T network components to half their values.
This model improves -- in theory at least -- as more sections are added.
In practice, imperfection in the components limits the quality of the
approximation. But I don't think this is of particular interest in
making analyzer measurements. The 7 MHz example terminated with 50 ohms
will show an input impedance within 2 ohms magnitude and 2 degrees phase
of 50 ohms between about 6.2 and 7.4 MHz, so it's good for the entire 40
meter band. But it will disturb measurements on lower bands. You should
construct one for each band and, preferably, one for each general
impedance level you expect to measure. A single one won't do for
multiple bands as I implied.

Roy Lewallen, W7EL