On Thu, Feb 6, 2025 at 03:46 PM, jeffinwa wrote:
In the shop, connecting this device directly to the SO239 connector, the sweep
on the screen is perfect. But in testing with the antenna installed with coax,
and testing at the end of the feed line, the sweep seems strange.
The ripples you are seeing are not unusual. It is not due to the antenna - it is the transmission line and/or adapters being used. . When the impedance of the transmission line is slightly different than the 50 ohm impedance of the NanoVNA you get ripples which give a higher SWR at odd multiples of 1/4 wavelength (1/4, 3/4. 5/4 etc.) and lower at multiples of half wavelength (1/2, 1, 3/2 etc).
Attached are two screenshots. 50 feet of RG213 with 50 ohms at the end was measured. SWR should be close to 1. In one plot a SMA to SO239 adapter was used and in the second a SMA to PL259 adapter with a barrel connector. Those female barrel connectors have about 35 ohms of characteristic impedance. Notice how much more ripple there is with the barrel inline.
Roger Need
