LC ladder network measurements

[digital1955]

Greetings,

I asked this question before, and I didn't get an responses, so I'll try to re-ask and be more clear.

I want to build this "artificial transmission line" network:



I used AADE to model/verify my particular design - which needs to have a characteristic impedance of 313 Ohms and a phase shift of 120 degrees.

Two simple questions:

1. Once I build the circuit, is it possible to accurately measure/verify the phase shift using the 4170, or do I need the 2180?
2. How can I verify the Zo? I've used "measure Zo" before on transmission lines, but will it work on this circuit?

Thanks!

73, Scott, WU2X

[Bob]

Hi Scott,

Any of the AIM's can be used to measure the parameters of your transmission line.
Put a resistor of known value at the far end, set Zref using the Setup Menu to this resistor value, do a scan and look at the SWR. At low frequencies, the Zo of your line will be the SWR times the value of the resistor. As the frequency goes up, the SWR usually goes down due to loss in the line, even for coax.

With the line open, note the frequency where the phase shift passes through zero (this is displayed at the upper left corner of the graph = Resonant freq). This is the frequency, Fo, where the one-way phase shift is 90 degrees. If you want 120 degrees, that frequency is (120/90)*Fo.

A few years ago I needed a transmission line with a length of about 500 feet. That much coax is expensive and very bulky. I made an LC line like yours on a small board, very compact, very light weight and low cost.

A very popular application for the AIM is to trim a section of coax transmission line for a certain phase shift at a specified wavelength. A function called "Stub Tuning" in the Function Menu helps with this.


-73/Bob

[digital1955]

Hi Bob,

Thanks. I've built one cell out of the planned six for testing. Could you explain your emphasis on "one-way". I did make a measurement with the source first passing through the inductor. Everything looked as expected. Since you mentioned one-way, I thought you were implying that if I put the source on the other end that the result would be different. And it was - very different. The phase was flat at -90 from 1 to 30Mhz. There was small bump around 15Mhz, but it didn't approach zero (no resonance points anywhere from 1 - 30Mhz).

The circuit will be used for both transmit and receive, so it needs to work in both directions. I modeled the circuit in reverse in AADE (I can't move the source end), but in AADE, the phase shift still looked correct.

So is this a measurement error, or does this circuit only function correctly in one direction?

Thanks and 73,
Scott, WU2X

[Bob]

Hi Scott,
I'm sorry I didn't get back to you sooner. I recently moved to a new qth.

This website has details on designing an LC Filter of the type that I used:
www.n6rk.com/Lumped_LC_Delay_Networks.gif

This design is symmetrical, so it will work the same way for transmit and receive.

--73/Bob