Hi,
What you are seeing is perfectly normal and simply represents the fact that the antenna feed impedance (at the antenna feed point) does not exactly match the coax impedance. It's certainly pretty close to matched though because you have a nice dip still - it's just not perfect.? But no need to change anything here.
The coax is acting as an impedance transformer with the impedance transformation cycling every half wavelength of coax.? If you switch on the smith chart trace on the vna you will see a number of circles corresponding to the cycles on your swr trace. Each trip around the smith chart represents one half wavelength of change in the coax's length.
When you did the tests in the shop, the feeder length (from the point on the vna where you did the calibration to the antenna feed point) was very short, much less than a half wavelength - so you didn't see any cycles in the swr. But now that you have a longer coax, you see them.
As an aside, of course the antenna impedance is going to be a bit different in its outside location than it was in the shop just because of the different environment. If you want to see the actual impedance of the antenna then you could do the VNA calibration at the far end of the coax, in situ. That would eliminate the effect of the coax from the readings and you should get a smoother and more accurate reading. But what you have there looks fine for normal operation.
73,
Andrew
VK2EZF
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On 7/02/2025 09:40, jeffinwa via groups.io wrote:
When I am constructing an antenna, I have been using this device to plot the swr in my shop. I usually set a wide enough bandwidth to easily see the dip at resonance.
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. See attached
