At 03:41 PM 3/2/2005, Kelly Johnson wrote:
>I had a 40m vertical dipole up a couple of years ago. I took it down
>last year because of poor performance (ie. worse than my 40m dipole at
>40 feet). Can someone give me some possible reasons for why the
>performance would be so poor? I thought vertical dipoles lacked the
>"ground loss" problem of a ground mounted 1/4 wave vertical with a bad
>ground plane. Was I wrong? If so, what can be done to improve the
>performance of a vertical dipole? Elevate it? Add ground radials?
>Nothing?
Several things affect the perceived performance. All, some or none may
apply in your case:
1) You still have ground loss from the near fields interacting with the
ground (i.e. the electric and magnetic fields from the antenna induce
currents in the soil, and it heats up the ground). Granted, with the
vertical dipole, you don't have the losses from the "return currents" that
you do with a monopole.
mitigation: you can put down a conductive layer that "shields" the lossy
earth from your antenna (this is what WWVH does, for instance)
mitigation: sea water is a good conductor, so there's not as much loss in
the eddy currents.
2) You still have the far field reflection problem with vertical
polarization. At almost any incidence angle with almost any soil
properties, a horizontally polarized wave reflects fairly well. This is
not true for a vertically polarized wave. At shallow angles (depending on
the soil properties), part of the wave reflects, and part propagates into
the soil and is absorbed. The tedious thing is that for low angles (which
is often where you want decent performance for DX), the critical surface is
a long way from the antenna (at 6 degrees, the reflection point is about 10
times the antenna's center height away.. that's 100m if your vertical
feedpoint is 10 m high)
mitigation: pray for high angle multibounce propagation
mitigation: increase the surface reflectivity within several hundred meters
of the antenna (i.e. beach locations or, on top of a lake or ocean). Both
fresh and salt water make dandy reflectors.
3) Was the 40m vertical dipole up 40 feet at it's center?
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