Hi Eric,
> I should not have used the term capture area. Especially since
> it has been so tainted and confused with effective aperture as
> you point out. But I think we need a term to describe the effect
> of having multiple physically separated structures which extract
> RF energy from the incident field(s) at different locations and
> deliver it all to a single load (the receiver).
here is a reason no one named that effect, it already has a name.
It is not actually from diversity, but rather
from increased directivity (skywave only).
If the fading is caused by Faraday rotation where polarity is shifting
increased directivity caused by stacking elements will do nothing at all.
Neither will mixing a horizontal and
vertical antenna in fixed phase. The odds of receiving a signal null are
the same no matter what we do, unless the antenna mixing system is
"intelligent" and sorts out the best combo.
If the fading comes from multipath, where two different propagation paths
arriving at different angles or from different directions, then the "stack"
makes a big difference. It focuses better on one path
(hopefully the best one) and attenuates the undesired path that causes
phase related fading.
On 160 meters, I combine two separate Beverage systems spaced 500 feet or
so apart. After I combine them, I combine them with a 4 square array I use
for transmitting. Day after day, I can go back to the same settings for
certain arrival directions and angles. This works only because the system
is
focused in one direction better. It especially has much less fading on days
when I hear signals from multiple directions, because it reduces multipath
signals by focusing much better on one path (hopefully the best path).
For example, some days the VK's come from both northwest and southwest.
Pointing due west (where signals from both paths mix in the antenna )
produces
lots of fading, while focusing southwest or northwest alone will greatly
reduce QSB. The same effect occurs with different wave angles, like a
higher angle more hop signal on twenty meters mixing with a lower angle
fewer hop signal from the same source.
Focus better on one path, or null the other, and fading can drop a large
amount. Even a few dB of increased directivity can greatly reduce depth of
fades.
I have a two element phased array with horizontal elements at 250 feet near
the four square, and it is totally useless in combining with the vertically
polarized Beverages and 4 square. That's because the cross polarity
prevents a gain increase (better focusing). I can adjust it for better
signal at any one instant of time, but a moment or so later it is all out
of whack.
Broadcasters know this well, and make every effort to NOT transmit multiple
lobes or dual polarity over sky wave paths. That's why a 5/8 wave vertical
is never used for long range AM BC, and why a .525 wl or so vertical is
called a "non-fading vertical" by clear channel high power AM broadcasters.
> However, it is also clear that during times when QSB due to
> multipath is causing the output from one antenna to be reduced to
> nearly zero, the nonzero vector sums at the other antenna are
> filling in very nicely. This is not what one would expect from
> considering the array as a single higher gain antenna with a
> single phase center located somewhere central to the structure.
I would expect it to be better. The more focused the array the less fading
occurs. My horizontal antenna fades at much different times than the
Beverages or 4 square, yet combining it into the system actually makes
things
worse! It's a wild card of random phase signals that are out of step with
the verticals.
73 Tom
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