Tam/WB2TT wrote:

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(*). 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.

(*)For a better general simulation of a *short* transmission line, use a
ladder network with all the reactances equal to Z0 except the end
components. For the end components, make the series L or shunt C half
the value of the rest. (For example, the LPF I showed would have input
and output shunt capacitors with reactance = 100 ohms, and remaining
components with reactance = 50 ohms. A five-component tee type network
would have input and output series inductors with reactance = 25 ohms,
and the remaining components with reactance = 50 ohms.) This model
improves -- in theory at least -- as more sections are added, being able
to imitate longer and longer lines. 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
model I proposed is better for simulating a half wavelength line while
providing filtering.

Roy Lewallen, W7EL