On Oct 3, 11:32*pm, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
On 10/03/2010 07:31 PM, Owen Duffy wrote:





I have explored what you have said in an NEC4 model of a quarter wave
monopole with three quarter wave radials at varying heights over 'average
ground'. The results are summarised at
http://www.vk1od.net/lost/Clip053a.png. The reference for the graph is
the efficiency of the same antenna with 120 buried radials in the same
soil type.

If the models are correct, laying just a few radials on or very close to
the ground (eg the popular method of pinned into the turf) would appear
to be a very poor option. The model indicates efficiency improves with a
very small increase in height above the dirt, just 30mm is a 6dB
improvement of lying on the dirt, just half a metre achieves 90% of the
available efficiency.

Owen

Thanks Owen for the Clip053a.png
The results are just amazing.

But I just cann't understand why the current in the radials, don't
induce a significant current in the earth, that ends as an important
lost power.
--
Alejandro Lieber *LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:http://1fcr.com.ar

because the currents in the radials cancel out below them which
reduces the current in the ground. as long as there are enough of
them and relatively symmetric. look at a radial field from below, lay
on the ground and look up at the radials all branching out from the
center... now visualize the current in each one, note that they are
all in phase, the current flows out from the middle to the ends all at
the same time. now if you start adding up the vector representations
of the fields from each little piece you will see that ideally they
all cancel out. in the middle that is likely pretty good, but as you
move out from the middle you get farther away from the radials on one
side than the other so cancellation isn't as good, but the current is
also lower so not as much gets coupled to the ground.