[TowerTalk] TowerTalk Digest, Vol 33, Issue 32

[Jim Lux]

WA1X wrote:

>Ian,
>    Same answer as before.  The 0.25 wl spacing with 0.18 radials all around
>is clearly superior to your alternative.  Using 0.18 wl radials is a very
>small compromise such that you would probably not be able to measure the
>difference between that and 0.20 wl radials.  It is not a linear
>relationship.

I'll agree with the preceding, but not the following:

>Since the current density in the ground, going radially
>outwards from the base of the vertical element is very similar to the
>current density on the vertical element itself.  As you go outwards, the
>current drops sinusoidally to zero at 0.25 wl.

This is not true.  The current does get very small as you get "far" away, 
however it is not even close to sinusoidal.
Chapter 3 of the ARRL antenna book as some pictures and graphs that 
illustrated this.

The current is larger near the base, but the earth current at a given 
distance is a combination of the currents induced all along the antenna. 
Close to the base, the current in the base dominates, both because it's 
close, and because it's big.

Go out some distance (say, half the height of the antenna), and the center 
of the antenna isn't much farther away than the base, so it makes a larger 
fractional contribution to the current.

Go out several antenna heights, and the distance is basically the same to 
all parts of the antenna.

For an ideal monopole over a uniform ground, the current density actually 
goes as 1/r.


>  Since the current is already
>low at 0.18 wl, extending the radials to 0.20 wl buys you very little.

This is certainly true.  Except perhaps if you had a huge number of radials.


>There is a rule of thumb that says for a given length of radials, you reach
>a point of diminishing returns when the tip to tip chordal distance between
>adjacent radials is 0.025 wavelengths.  Adding more radials beyond that buys
>you very little.

This has more to do with optimizing the length of radials for a given 
length of wire (or mass of copper).


Jim, W6RMK