[TowerTalk] Ground Resistance
dezrat at copper.net
Wed Aug 9 12:49:37 EDT 2006
>I have followed the posts on radials and ground resistance for some time. Is there any simple way to measure the impedance of the ground radial system as you install them to know when you have enough of them in the ground?
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Here's the method I use with an SWR analyzer such as the MFJ-259 or
Understand that a 1/4 wave vertical over a "perfect" ground will
measure a feedpoint impedance of about 36 ohms. This impedance is
purely of the vertical part; the ground, if "perfect", has zero
impedance. If you have additional ground impedance due to insufficient
radials, the 36 ohm figure will go up and so will losses by a
If, like many of us, your vertical part is shorter than 1/4 wave, you
must make an approximation of what it actually is. The ARRL handbook
and antenna handbook have graphs which will help. For example, if you
determine that your shortened vertical is only 10 ohms, and you have a
"perfect" ground, 10 ohms is what your SWR analyzer will measure. More
likely, it will be higher than 10 ohms, and that difference is the
additional loss caused by insufficient radials.
Hope this makes sense. It's simple Ohm's law.
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