On Fri, Jul 30, 2021 at 08:14 AM, <kk7xo@...> wrote:
There were a lot of interesting answers to other questions, but the question I
had was whether to use or not to use a balun, which roncraig1 did address.
Thank you roncraig1.
The answer to your question is that you need to do several things in order to accurately determine the length of a 1/4 wave length of balanced 450 ohm ladderline for operation at 2M (144 MHz.).
1. You need to keep the ladderline well away from any other objects
2. You need a proper test jig in order to establish a "reference plane" for the measurements. I use SMA connectors with a pin jack and SMD loads. Photo attached.
3. The NanoVNA must be running on battery power. The USB lead and PC will have a significant effect on measurements.
4. A balun and/or common mode choke (on the coax cable to the test jig) is required when testing a balance line.
Any common mode current on the coax cable will affect the measurement. If measuring at HF which has a longer wavelength you can probably get away without a balun or choke but at VHF this is not the case. A 1/4 wavelength of generic 450 ohm ladderline (VF=0.91) is 18.6 inches (472.5 mm) long at 144.3 MHz. If the length is 19 inches (482.6 mm) it is 141.3 MHz. So a 0.4 inch (10 mm) change in length results in a 3 MHz. difference.
A discussion of how to make a suitable balun or RF choke for VHF use is a detailed topic..
Once 1 to 4 above are in place I would make the measurement by connecting a 50 ohm load and the ladder line in parallel at the jig. The far end of the ladder line is short circuited. At 1/4 wavelength this results in a very high impedance in parallel with the 50 ohms resistor. By measuring Return Loss an accurate measurement of the 1/4 wavelength frequency can be made.
Roger
Roger Need
