Re: Antenna Fundamentals including Nano VNA survey of a few HF antennas at K3EUI


Barry K3EUI
 

Hello Nano VNA folks
Again, thank you for the feedback. Yes, this is what I asked for.

I try to be careful with language, but the slide on antenna "efficiency" was just sloppy.

My main point of putting this together was to look at how to interpret the various graphs that a VNA shows.
I am not an Electrical Engineer, and had no prior experience with the VNA vocabulary.
Six months ago I had no idea what a '"return loss" was, let alone explain why it is positive or negative.

Hams talk about SWR all the time. On nets, I often hear folks say they are worried since their SWR is 1.8:1 at some frequency, and they WANT 1:1.

The book by Walt Maxwell W2DU was fantastic reading for me and taught me a lot about SWR and feedlines and impedance.

I wanted to start my presentation with some basic antenna characteristics, like resistance, reactance, impedance and phase. Then branch into the VNA world to discuss how these parameters can be measured at least with reasonable accurately at HF with a $50 gadget and free software.

So that was my goal. I am adding some slides on "reflection coefficient" (rho) in tems of what various values of rho look like on a Smith Chart. Rho of zero is just one point on that graph: R = 50 ohms and zero reactance (with 50 ohm coax). So if you visualize the Smith Chart as a Dart Board, and the center is where you are aiming, then a SWR of 1:1 is only at the exact center of the dart board (Smith Chart).

So if ou miss the Bull's Eye, your reflection coefficient is bigger than 0. And how much you miss will indirectly tell you LOTS of information. The perimeter of the Smith Chart is a reflection coefficient of 1. Of course, that is just awful. Reflection Coefficient (0-1) leads to RETURN LOSS (dB) and that leads to SWR.
I think I now have an 8th grader understanding of all of these terms now.

What puzzled me was how to interpret the PHASE curves on simple one-band dipoles.
At resonance, reactance is zero (I get that) and PHASE crosses over from neg to pos.
WHY? If a ham (non engineer) can explain that in a few sentences, then that is a pretty good understanding of the concept of resonance. Of course, these are measurements made in my shack, about 100-150 ft of RG213 from the antenna being measured.

I find most hams (I talk with) think the SWR changes with feed line length (other than attenuation).
Many hams (I talk with) do not see why impedance changes with line length if Z is other than 50 ohms.
These can be easily confused by most of us in our (casual) discussions.
Running around 180 degrees in a Smith Chart was a real "eye-opener" for me, personally.

I noticed that QST article last May had a quick "review" of the Nano VNA but did not go into any detail explaining the relationships of the various graphs. Perhaps a second article for QST would help us non-engineer types see the value of a $50 VNA so we can adjust our antennas to work better.

Again, from a rookie at VNA language (but a ham for over 60 years)
Thank you. This IO group obviously has some very talented hams.

Barry k3eui

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