开云体育

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.