First, take into account the propagation constant.? The line needs to be physically shorter than free space calculations.? So 400 MHz translates to 3/4 meter wave length.? One fourth of that is 3/16 meter, or .1875 meters.? Multiply by, say, 2/3 prop constant and you get about 1/8 meter.or 4.92 inches of coax.? You are operating at about 10% higher so knock off 10% length and you get about 4.4 inches.
Second, look closely at the quality of the short; it should be close to the cable, ideally no lead length.? Then, with the Smith chart display, sweep from about 300 to 600 MHz and look at the impedance around 450 MHz.? It should be near the right edge of the chart.? Use Port 0 display.
Bob On Tuesday, February 25, 2020, 02:46:40 PM PST, Robert Putala KE8UW <rcp@...> wrote:
I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to function.? A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an on-line calculator.? Cutting the coax to this dimension, shorting one end and measuring the other using the VNA calibrated at CW of 442 MHz does not show a high impedance.? There is a high impedance visible with "resistance" display and scanning frequencies from 100 MHz to 500 MHz, but it is far from 442 MHz.? I cut the length of the coax in increments and am now at a length of 2 1/2" (from the SNA connector to the shorted cable end).? This shows a high resistance at 375 MHz (peak at about 600 Ohm).? Admitted rookie, and doing this more to learn the nanovna functions than get a working antenna (dual band J-pole).? Suggestions on why 1/4 wavelength shorted coax at 7.4 inches doesn't reflect a high input impedance around 442 MHz?
