Eric: I just wanted to add a little emphasis to what you said about the
sound of the stack. When I first put my system on the air I was
astounded by the number of people that asked if I was using a different
microphone when I made the antenna change tests because the audio was
NOTICEABLY better with the stack. I can only surmise that there is less
selective fading and resultant distortion. In busting piles it is also
apparent because because of propagation often I am getting through on the
first call but getting no better report than the next guy in line.
73/Mike, N7ML
Eric Gustafson wrote:
> 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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