SEC: UNCLASS: NEC-LIST: Why coil-load?

One reason that vehicles are probably overlooked is that it takes time and
effort to produce and perhaps validate a good vehicle model. NEC can also
have some difficulty in modelling too close to the ground, particularly if
wheels are implemented as loops. I generally leave them off and hard-wire
the equivalent capacitance. A Sommerfeld ground is essential. As Roy
notes, the vehicle is a large part of the antenna structure. Consequently
it can have a significant impact on the antenna parameters including the
radiation pattern. The expected donut pattern from a vehicle whip can
develop significant vertical radiation when parts of the vehicle (eg canopy
bows) become resonant. For the Land Rover, this occurs upwards of 20 MHz,
and robs gain from the lower angles. If you could tune this structure to
below the critical frequency, and you would have a useful NVIS antenna.

  All whips are really dipoles in disguise - there is always a second
element. You might be be standing on it, sitting in it, or in the case of a
manpack radio, it is you are the second element.

Regards,

Alan Nott.

-----Original Message-----
From: Roy Lewallen [mailto:w7el_at_eznec.com]
Sent: Thursday, 17 February 2005 7:57
To: nec-list_at_robomod.net
Subject: Re: NEC-LIST: Why coil-load?

I'd be surprised to see any significant difference between NEC-2 and
NEC-4 for an above-ground model like this.

I concur wholeheartedly with the approach. For reasons that are
mysterious to me, the vehicle is generally overlooked in the discussion
of mobile antennas. The vehicle is the other half of the asymmetrical
dipole being fed, and its radiation is at least as significant as that
of the "antenna".

Roy Lewallen

drcuthbert_at_micron.com wrote:
> Doug,
>
> A good question on the current distribution in the ground. I don't have
> a definitive answer- perhaps someone else does.
>
> I would approach the problem of the ground loss like this: Build a wire
> grid model of the vehicle and place it over Sommerfield-Norton ground.
> Then experiment with the loading coil Q and placement. I only have NEC-2
> and that will give optimistic results and will not have absolute
> accuracy. But it might be good enough for relative comparisons. If you
> have NEC-4 then you are all set. The vehicle model will be good for
> exploring the effect of nearby structures. For example, the center
> loaded whip mounted on the fender will have a higher E-field than the
> base loaded whip and will couple more current into the roof. This then
> travels down the roof pillars thereby lowering the radiation resistance.
>
>
> Dave Cuthbert

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