"Reg Edwards" wrote in message ...

"Ian White GM3SEK" wrote

That was an administrative policy decision rather than a technical
one.
From the technical viewpoint, everybody agrees that 120*0.25wl is
more
than enough to override the local ground conditions under the tower
irrelevant.

====================================

- - - - and since soil resistivity decreases with increasing
frequency, and the impedance due to soil capacitance also decreases
with increasing frequency, everybody agrees that 1/8th wavelength or
less is more than long enough.

Reg, do you really mean what you said above, 'soil resistivity decreases with increasing frequency'? Are you sure you didn't mean soil conductivity decreases with increasing frequency? In my experience with AM BC antennas I've found that conductivity decreases, not resistivity.

The FCC charts showing signal level vs conductivity and frequency overwhelmingly show conductivity decreasing with frequency. So you ask, what proof is there that the FCC charts are correct? Well, Reg, soil conductivity measurements of thousands of AM antenna systems world wide have proved them correct.

As an example that I posted a few days ago, consider the coverage area from afforded by a single 1/4wl vertical radiating 250 watts at 550 KHz with a signal strength of 1 mv/meter at one mile and a conductivity of 8. If the frequency were raised to 1500 KHz with a 1/4wl vertical at that frequency, the power required to cover the same area is 47 KW.

Does this example indicate a decreasing soil resitivity with increasing frequency or a decreasing soil conductivity?

Walt, W2DU