"Reg Edwards" <g4fgq.regp@ZZZbtinternet.com> wrote=20 in message news:d9h6q9\$iss\$1@nwrdmz01.dmz.ncs.ea.ibs-infra.bt.com...
>
> "Ian White GM3SEK" =20 wrote
>
>> That was an administrative policy decision = rather=20 than a technical
> one.
>>  From the technical = viewpoint,=20 everybody agrees that 120*0.25wl is
> more
>> than enough = to=20 override the local ground conditions under the tower
>>=20 irrelevant.
>>
> = =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>=20
> - - - -  and since soil resistivity decreases with=20 increasing
> frequency, and the impedance due to soil capacitance = also=20 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=20 above, 'soil resistivity decreases with increasing frequency'? Are = you sure=20 you didn't mean soil conductivity decreases with increasing = frequency? In=20 my experience with AM BC antennas I've found that conductivity = decreases, not=20 resistivity.

The FCC charts showing signal level vs = conductivity=20 and frequency overwhelmingly show conductivity decreasing with = frequency. So you=20 ask, what proof is there that the FCC charts are correct? Well, Reg, = soil=20 conductivity measurements of thousands of AM antenna systems world wide = have=20 proved them correct.

As an example that I posted a few days = ago,=20 consider the coverage area from afforded by a single 1/4wl vertical=20 radiating 250 watts at 550 KHz with a signal strength of 1 mv/meter = at=20 one mile and a conductivity of 8. If the frequency were raised = to 1500=20 KHz with a 1/4wl vertical at that frequency, the power required to cover = the=20 same area is 47 KW.

Does this example indicate a decreasing = soil=20 resitivity with increasing frequency or a decreasing=20 soil conductivity?

Walt, W2DU