At 10:38 AM 1/3/2006, Dennis OConnor wrote:
> Hmmm, the supposition that driving the array will make up for an
> inadequate ground system does not stand up to critical thought... Any
> 1/4 wave vertical array requires that the ground mirror the other half of
> the wave front...
>In fact, there are studies that suggest that parasitic elements in a
>vertical array suffer less ground losses than driven elements... The
>suggestion is that the driven element in a parasitic vertical array will
>benefit from elevated radials whilst the parasitic elements can use ground
>radials with little penalty...
I wouldn't expect this... There's two factors you might consider:
1) The loss resistance of the element (which is independent of whether it's
parasitically excited or not). For all intents and purposes, this is a sort
of constant percentage loss, so the power lost is proportional fed to the
element. I suppose that if powers are seriously uneven, then it would pay
to reduce the loss of the most heavily excited element.
2) The loss reduces the maximum excitation you can get in the parasitic
element, which might make it hard to form a good null. Forward gain is a
lot less sensitive to the precise excitation.
So, I suppose that if the parasitic elements are not very tightly coupled,
so they have a small fraction of the power it might be worth doing elevated
radials on the single driven element.
So.. from an outright efficiency standpoint, an all driven array *might*
suffer in comparison to a parasitic array. However, the all driven array
has a LOT more control over the excitations and the resulting pattern. On
receive, this is probably more important than efficiency (since atmospheric
noise dominates over receiver noise).
>In pursuit of that theory my current 160 arrays use an elevated, single,
>radial for the driven element with a grounded parasitic element... It is
>either the best or the second best 160 array I have put up... They jury is
>still out on that... The best was my 4 element, all elevated array that I
>used to work VK0 on 160 a few years back, but keeping all those elevated
>radials running through the woods in the air during a Michigan winter
>became a never ending chore... Also, keeping them clear of limbs that
>caused arcing was another headache... I finally gave up, but probably
>should have hung in there...
>Obviously, I am a parasitic kinda guy... I don't like anything that dumps
>part of my transmitted power into a dummy load...
Either it winds up in the load, or absorbed in the ground, or in the
element losses. And there are lots of ways to make an all driven array
where there's no load. The load is just a hack to allow use of a simple 90
degree "collins" lumped L/C hybrid.
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