Antenna Modeling, Radiation Patterns, Geography…and the Caboose

Antenna modeling for the amateur radio hobbyist has always struck me as uniquely esoteric. I mean sure, hams build plenty of antennas, but we have all seen the predicted idealized radiation blobs, donuts, and figure-eights associated with the usual dipoles and end-feds. We know that for typical HF wire antennas, the closer to the ground, the more vertical the gain. And contrarily, antennas approaching a half-wave length in height have better take-off angles for distant transmissions. We know in what directions the radiowaves eject from the radiator. What is the point of modeling the same concepts over and over again?

Until one night in a radio club lecture, I learned about EZNEC software and I realized it could help me understand why we did not get many contacts on Winter Field Day 2021. Yup, the one on the train in Chester, Massachusetts. The pink line on the satellite map shows the approximate orientation of the antenna shown below.

The 40M Off-Center Fed (OCF) Dipole (Frequency Devices, Inc.) was erected in an inverted V configuration. A mast held the 4:1 balun approximately 22 feet in the air. The 45.5′ 14-gauge copper wire long arm of the OCF extended westward until it reached a height of approximately 3′ above the ground. The 23′ short arm extended eastward, stopping at 10′ above the ground. The pink line on the satellite map above shows the approximate orientation of the antenna on Winter Field Day.

Plugging this data into EZNEC demonstrated a fairly predictable radiation blob, with the most significant gain of nearly 5dBi in the vertical direction (i.e. considered 90 degrees). The 3dBi beamwidth spanned from 38 to 142 degrees.

If we were sitting in a cornfield in Kansas, this radiation pattern may have been very successful. But when you combine the antenna model with a relief of the geography of downtown Chester, you begin to see why we probably had trouble with propagation. Below are the satellite and topography maps. Our antenna was located within the heart of the populated area near the center of the maps.

I used the tilting 3D function of Google Earth to create a relief map of downtown Chester looking from the South toward the North. I then edited the predicted radiation pattern for the OCF dipole into the relief map.

Well, that is my foray into ham radio antenna modeling. Overall, I recognize this analysis is quite crude, and probably does not totally explain the poor propagation we had at Winter Field Day. I am certainly not an RF engineer, and realize I may be way off base with this. But, this geographically enhanced radiation pattern does indeed validate my feeling that I was operating from the bottom of a salad bowl. Because, well, in not-quite-so-technical terms…I was.