To wrap up, here is a list of my saucer antenna project discoveries. This is a collection of useful things I learned along the way.
First, the funny bit. My 3D printed plastic saucer form worked well. But when a hailstorm struck, it was damaged. You can see the fractures from half-inch hail shown above right.
Second, it was interesting to see what performance you get from a plain 8 foot pipe antenna, without resonators. I used the NanoVNA to look at the bare antenna fed with coax and attached to two 24′ radials on ground. See the picture above left. I expected to see a resonance around 27 MHz, which represents the wavelength where an eight foot pipe is ¼λ. As you can see above, this short pipe provides good performance on the old Citizens’ Band.
But wait, there’s more. My first resonance at 4.9 MHz turns out to arise from my 50′ coax feedline from house to antenna. Apparently, the outside of a fifty foot length of coax also acts as a tuned radial at this frequency. A common mode choke at the antenna base will get rid of this artifact.
My second bare metal resonance comes in at 9.8 MHz and arises from the 24′ radials laid on the ground. The third resonance at around 15 MHz results from the third harmonic of the coax cable. And so on.
When I attached the NanoVNA directly to the antenna base and substituted a simple ground for the radials, all of these other resonances disappeared except for 27 MHz, the natural resonance of an 8′ ground plane.
Saucer Antenna Project Discoveries – Odds and Ends
You will find that a NanoVNA is a wonderful tool for this type of project. Easy measurements and so much data. However, for portable use, its tiny TFT display is hard to read in sunlight. I found myself taking some measurements at night!
When you build the saucer resonators, make sure to use lock washers on the capacity hat connections. Tooth lock washers probably work best. Otherwise the spring steel wire tends to move in the wind and wiggle the connection loose. Also, even with Teflon coated wire, accumulation of water from rain on the saucer changes resonance.
My overall conclusion is that this short top-loaded vertical might make a great portable antenna for 40-10 meters, but I think not so great for a main antenna at home. My guess is that a magnetic small transmitting loop (STL) would perform better in the absence of a great ground plane.