Re: NEC-LIST: Monopole above soft water

Alexander,

This is more of a vertical dipole, as any vertical conductor (the buoy)
under the monopole feedpoint will act as the bottom half of a vertical
dipole (unless the monopole has radials). The quickest method with NEC-2
is to build a vertical dipole from just above the surface of the water
to the top of the monopole. Feed it with a current source. Then place
the receive antenna at the appropriate height and distance. Place a
suitable termination at the receive antenna and measure the receive
current. You can also build a model of the buoy from simple to complex
and place the monopole atop it.

I have been doing some ocean water simulations and they are simpler than
fresh water in that the skin depth is only 2 cm at 150 MHz and so I can
treat it as perfect ground. The spreading resistance from a wire
inserted into the water is about 10 ohms. Fresh water is quite
different, with the skin depth being 1.3 meters at 150 MHz. What happens
with RF current flowing along the submerged portion of the buoy? There
is certainly dielectric loss, which I think we can roughly estimate with
pencil and paper. I don't believe the RF radiated from the submerged
portion will propagate very far- but I'm not certain of this. If one was
designing the buoy antenna from the beginning, two or three radials
could be placed at the base of the monopole to decouple the antenna from
the buoy and avoid the issue of RF current along the buoy.

    Dave Cuthbert
    Micron Technology

-----Original Message-----
From: nec-list-bounces+drcuthbert=micron.com_at_robomod.net
[mailto:nec-list-bounces+drcuthbert=micron.com_at_robomod.net] On Behalf Of
Alexandre Kampouris
Sent: Monday, June 06, 2005 11:06 AM
To: nec-list_at_robomod.net
Subject: NEC-LIST: Monopole above soft water

Hello,

A relative of mine complains of poor results while attempting to track a

small buoy floating on softwater lake at VHF.

As far as I can gather, the antenna is probably a monopole, and the
lower
part of the antenna is formed by the immersed buoy.

I've given her already a couple of tips on how to improve reception, but

I'd also like to have a look at the situation with NEC2, to see if I
should
expect shadow for a distant observer close to ground.

Qualitatively, the soft water's surface should be something of a
magnetic
wall, forcing the tangential H field to zero (if my brain hasn't turned
to
mush). So I suppose that a vertical monopole may behave quite
differently
than one would expect on say, salt water.

So here are my questions :

1) Is NEC's ground model applicable to something like softwater
(epsilon~=80, relatively low losses)? I've never used SOMNEC before, so
this is something new for me.

2) Modelling the immersed part is impossible with NEC2, but I consider
testing two things :

a) Model a monopole just in contact with the water's surface, like NEC2
allows (I'm only interested in the radiation pattern, not in calculating

the input impedance).

b) Instead of a discrete contact, have a network of short radials
converging to the base of the monopole.

What do think above approaches? Can one neglect the underwater component
of
the buoy, or is my approach pointless?

3) Do you know of any literature on this or a comparable problem? (eg:
propagation studies on inland navigation).

Thanks in advance,

Alexandre

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