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Date: Sat, 6 Dec 2008 11:04:43 +0000
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: ERP calculations / measurements
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A perfect lossless gound and a lossless (superconducting :-) radiator.

Real reductions in gain which appear as increase in antenna resistance
come about when the lossy ground terminates the local E field, and to
a lesser extent tle H field.  Not-too-much modelling software can cope
with that.

It is DIRECTIVITY that is exacly 3 for a short dipole over an infinite
perfect groundplane.   The normal directivity of 1.5 calculated for a
short dipole, doubled due to its reflection's contribution.

Gain is  [directivity - loss].

Andy  G4JNT
www.scrbg.org/g4jnt



2008/12/6 Brian Rogerson <brian@esoterica.pt>:
> Rik and LF
>
> Your email below prompted me to carry out another afternoon of measurements
> from higher ground for the station FS/ERP survey.  The results seem
> spectacular though I am unsure of what they mean.  I hope to place the data
> on my web site later.
>
> 73, Brian CT1DRP
>
>
>
> At 09:36 20/11/2008, you wrote:
>
> Hello group,
>
> it seems to be generally accepted that a small vertical antenna has a gain
> of 3 (4.77dBi / 2.62dBd), due to the radiation pattern in the vertical
> plane.
> But in very few cases this gain seems to be measured when doing ERP
> measurements.
> As far as I understand this gain of 3 is based on a perfect ground (endless
> conductivity) that acts as a (perfect) mirror and thus creates the "mirror
> image" needed for radiation.
> But what about a short vertical antenna above a not-so-perfect ground?
> I have done some simulations with MMANA-GAL (15m high vertical at 500kHz)
> and over a perfect ground I get the expected 4.77dBi and a take-off angle of
> 0 degrees.
> If the conductivity is not endless the gain drops and the take-off angle
> rises:
> 1000mS/m (salt water) = 4.5dBi / 9 degrees
> 500mS/m (clay) = 4.4dBi / 10 degrees
> 50mS/m (marsh) = 3.9dBi / 14 degrees
> 15mS/m (loam) = 3.5dBi / 17 degrees
> 5mS/m (wet sand) = 2.9dBi / 21 degrees
> 0.05mS/m (dry sand) = -1.5dBi / 32 degrees
>
> Could this explain (a part of) the missing dB's ?
>
> 73, Rik  ON7YD - OR7T
>
>
> Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm
>
>
>
>