Hi Tom,
To: <towertalk@contesting.com>
>Date: Mon, 19 Oct 1998 13:59:05 -0400
>From: "w8ji.tom" <w8ji.tom@MCIONE.com>
>
>Hi all..
>
Snip... (long section of cannonically correct discussion of
antenna gain)
>>way to predict what he might actually get. Also I was not
>>addressing any possible benefits due to space diversity in the
>>elevation plane or increased capture area, etc.
>
>"Space diversity" does nothing but establish a new pattern,
>unless there is some voting system that automatically selects
>the optimum phasing or antenna. During times of slow QSB, the
>operator might do that manually.
Diversity available by switching is exactly what I had in mind.
>Capture area is one of amateur radio's premier myths. Capture
>area is more correctly called "effective aperture" and relates
>only to antenna gain. It has nothing to do with antenna size
>except how that size affects gain. A simple dipole can have more
>capture area than a large antenna one hundred times its size, if
>the large antenna has less gain.
>
>Any antenna with more gain has more "capture area" than an
>antenna with lower gain, no matter what physical size the
>antennas are.
>
>73 Tom
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).
I and others have repeatedly observed the following positive
effect of having phased two physically separated antennas to
produce maximum gain in a particular direction:
1. Peak signal level from desired station on either antenna
alone S9+5 ish dB
2. Peak signal level from desired station with both antennas
phased for maximum signal difficult to accurately determine
but more than S9+5 and Less than S9+10 dB. Could be 2 or 3
dB better - but who knows?
3. QSB depth on either antenna alone 30 to 35 dB (fades take
other station down to S2 or so sometimes farther)
4. QSB depth with both antennas phased as described above,
probably less than 10 dB (no fades to less than S8 - actually
to less than halfway between S8 and S9)
5. Remote station always reports the combination as a tremendous
improvement even though when pressed for peak strength
comparisons, there is usually a less than 1 s-unit reported
difference. W7DD uses this technique to beat up on N7DD in
shootouts to europe.
I have observed this effect both with Yagis stacked vertically on
the same tower and with Yagis on different towers pointed on the
same azimuth and phased together in the shack.
It is clear that the forward gain was not increased beyond the 2
to 3 ish dB expected. So the effective aperture is not larger
than one would expect from the gain figure (whatever that turns
out to actually be).
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 think we need a term for this effect.
73, Eric N7CL
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