Understanding VNAs - Antenna Measurements

[@w2aew]

Nice video Paul. A couple of minor points that often draw questions: 1) The impedance seen at the input of the feedline is NOT the same as the antenna impedance (except for the case where the feedline is an odd-quarter wavelength long). 2) The length of the feedline does NOT affect SWR, provided the feedline is not acting as part of the antenna, and is sufficiently low loss. And 3) Strictly speaking, Return LOSS is a positive value (so large values mean a LOT of loss, thus little energy returned). The Reflection Coefficient is typically expressed as a negative value (just the opposite sign of the RL value). I know you know these things, but I figured your viewers would benefit... 73!

[@pauldenisowski]

Thanks Alan! In the interest of time, I didn't include any information about the feedline - that deserves an entire video (or several) of its own. And the sign of return loss is always a problematic topic. :) Personally, I also consider return loss to be a positive value (as mentioned at 9:50 in the video), but I often see it expressed as a negative value when used in antenna measurements. Incidentally, the editor-in-chief of IEEE Transactions on Antennas and Propagation published a short article describing the origins and proper use of the term "return loss" because a close to a third of the people submitting papers to this journal were using the term incorrectly. Note that he too considers return loss to be positive. T. S. Bird, "Definition and Misuse of Return Loss [Report of the Transactions Editor-in-Chief]," in IEEE Antennas and Propagation Magazine, vol. 51, no. 2, pp. 166-167, April 2009. Perhaps the best solution is the one I saw recently in a NanoVNA software package: it lets you choose if you want return loss shown as a positive or negative value :)

[@rik7373]

Great presentation! Not too long and to the point!

[@pauldenisowski]

Thank you! It's a lot harder to make a 14 minute video than a 60 minute video :)

[@aturegano87]

Thanks for the video! 👍

[@pauldenisowski]

Thanks for watching!

[@mohamadhamoudy8232]

Please Mr. Paul , could you kindly explain how to measure FM Antenna mounted on the top of a 200 meters tower , because when connected to the VNA , the measurements are not clear because the Antenna receives RF signal from space , regards

[@pauldenisowski]

There's no easy solution to working around problems due to external signals being picked up by the antenna. Ideally, antenna measurements would always be made in a shielded / anechoic chamber or in a OATS (open air test site) far away from other sources of RF in the measurement range. Assuming the antenna must be measured in place, one possibility is to use filters to attenuate the external signals. Another option is to try to make measurements during "quiet times" Since the sweeps usually only take seconds, hopefully it's possible to find a time when the RF environment is quiet for a short period of time. Hope that helps!

[@mohamadhamoudy8232]

@@pauldenisowski , Thanks a lot for the detailed explainantion Paul , please how to use filters , suppose I', receiveing some FM carriers during measuing the FM Antenna , how I conect the suggested filters ? , regards