Message-ID: <38E2530F.622F@xtra.co.nz>
Date: Thu, 30 Mar 2000 07:01:35 +1200
From: "vernall" <vernall@xtra.co.nz>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Amp metres
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g3kev wrote:
snip snip
> At this frequency with both antennas resonant on 136 khz on the same site, the
> induced rf from the small vertical into the larger one would distort any
> meaningful tests. The antennas would need to be sited several kilometres apart,
> when co located one would need to be dismantled while the other one is tested.
> I have already tested this by starting off with a 60 ft vertical and progressing
> to 120 ft and it certainly made a big difference for the same given rf power
> output. Signal reports all over europe confirm this.
> G3KEV

One fairly basic approach is that the far field is a function of antenna
"amp metres".  If the amps are the same, and the effective height is
increased, then there is an obvious gain.  It is far easier to arrive at
a given product of amps and metres in a tall antenna!  Changing from 60
to 120 feet, with the same pattern of current distribution, would give a
gain of 6 dB for the same applied current.  This is for one antenna in
the environment, with no mutual coupling to something large nearby.

An amateur regulatory constraint of 1 watt eirp limit starts to bite in
to the transmitter power budget for taller (more efficient) antenna
systems.

Bob ZL2CA