The AM Forum

Been looking at simple dipoles placed at extraordinary
low heights, 18 inches. Motivation easy to erect and measure
before it is raised. Using a portable vna found the SWR was
1:1 flat for a 40 meter dipole, 7-7.4 MHz. Simple. No need to place into NEC for any tweaking. This is a precursor to trying a 40 meter
extended double zepp (EDZ) at about 180 feet in length. However, the
match for the EDZ involves some added stuff so I was curious how it would play at a few feet above ground. Placing the EDZ into NEC at the
usual height of 60 to 70 feet showed the nominal 300, 450 or
600 ohm window line would do the job along with a simple
transformer/balun. However, when dropping this monster to a
couple of feet, the driving point Z crashed. No easy match was
possible. Huh? The simple dipole in the field did not show this
issue. So, placing the dipole into NEC at 18 inches showed the same
disconnect! The driving point Z was horrific! Turns out, the definition
for the ground composite, conductivity and dielectric K has a major impact on antenna Z at these low heights. Once raised, the affect is somewhat minimal. After adjustment of the ground definition, the EDZ at low height played out in the same manner as the simple dipole and an easy match was found. The simple dipole pretty well matched the actual test case.

Lesson learned, take care in definition of the surrounding ground material.

Good point! I had to read that a couple times to understand.

Thank you Pat for forcing me to clarify better!

Here are two pictures and this drives the point home. The center insulator (a hacked toothbrush handle) is less than a foot above ground. The vna clipped to the dipole. The sweep 6-8 MHz. Centered up at 7.040 MHz. Again, in NEC, if you simulate with an inappropriate ground definition, you will not get agreement with this configuration. SWR will be significantly higher and attempts to match a more complex configuration from NEC data will be frustrating. Take care in defining ground. It may be prudent to construct such an arrangement on ones property, then fiddle with the ground definition to force closer agreement with measurements.

And the other picture...