The transformer does not decouple the nanovna and PC/laptop and, therefore,
anything conductive within the house from the measurement. If you used
only the NANOVNA connected to NOTHING, not even holding the VNA, you might
not see as much difference. When you reverse the connections to the
transformer, the two sides of the OCF set of wires certainly interacts
differently with the house wiring and anything within the house/ceiling.
You need to decouple the feedline/NANOVNA/PC/laptop from the transformer!
Also realize the difference in coupling to anything in the house which is
conductive.
Dave - W?LEV
<>
Virus-free.www.avg.com
<>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
On Mon, Mar 17, 2025 at 6:13?PM Bruce KX4AZ via groups.io <bruce=
[email protected]> wrote:
A ham friend has an unusual indoor antenna that he uses with a Zachtek
WSPR transmitter. He gets lots of spots from the 200mW transmitter (>500
unique band/spotter combinations in 24 hours), and the beauty of the
Zachtek is its tolerance of essentially any antenna load (from nothing
attached to fully shorted). His antenna is effectively an off center fed
dipole, where the two wire legs wrap around the inside bedroom walls, and
are of random/different lengths..i.e .just whatever would fit onto the four
walls. The two dipole legs are connected to a simple 6:4 turn ratio
transformer (approx. 2:25:1 impedance ratio), with the primary side of the
transformer attached to a 4 ft coax feedline line to the Zachtek
transmitter.
I decided to make some measurements of the SWR with the nanoVNA attached
to this "creative" antenna setup. The nanoVNA was OSL-calibrated, followed
by attaching the antenna feedline to collect data in nanovnasaver
software. After collecting the data, I decided to reverse the two dipole
legs connected to the transformer secondary, just to make sure I was
getting reproducible data. What confused me were significant differences
in the SWR/return loss/Smith charts for the two configurations. I am
wondering if this simply reflects how non-balanced the dipole is, such that
the coax feedline shield (about 4 ft long) from the transformer plays a
different role in each of the two configurations. While making the
measurements the nanoVNA was dangling hands free while connected to the USB
port on my laptop...so the coax shield side would be the same in both
cases...and the only electrical difference was in reversing the dipole legs
(of differing lengths) connected to the transformer secondary.
--
*Dave - W?LEV*
--
Dave - W?LEV
