KN4LF wrote:
"I've been busy lately modeling a variety of 160 meter antenna's on EZNEC
3.0. For ground type I used real minimec and ground description
(conductivity) medium, frequency 1850 kc. From what I understand real
minimec approximates 32 1/4 wave radials."
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The real Mininec ground option in EZNEC assumes a ground system with zero
resistance. 32 quarter-wave radials has a ground resistance/loss of
about 12 ohms, so you should insert a 12-ohm resistor ("load") in the
model between the antenna feedpoint and ground to approximate 32
quarter-wave radials.
A quarter-wave vertical with a "perfect" ground system has a feedpoint
impedance of about 36 +j0 ohms at resonance. KN4LF's 32 quarter-wave
radials and their 12-ohms of ground resistance would be a close match (36
+ 12 = 48 ohms) to 50-ohm coax. This is why those with mediocre ground
systems have a low SWR on their vertical or inverted-L, and why they see
the SWR go up when they lower the ground resistance by adding more
radials.
Dont fret, however -- even though the SWR is a bit higher, the antenna
efficiency is greater. With the 32-radial system, the SWR on a 50-ohm
line will be about 1.04:1, but about 25% of the power will be lost in the
ground. If the radial system is improved to where the ground resistance
is 4 ohms, the SWR will be about 1.25:1 but only about 10% of the power
is lost into the ground.
73, de Earl, K6SE
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