No subject

[Guy Olinger, K2AV]

Hi again,

I accidentally sent N7CL an unfinished version of this. Comments
interspersed below.

Eric N7CL wrote:

>w8ji wrote:
>>"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.

That assumes a broad front largely invariant signal being received. If it is
possible on H.F. to have multipath effects, then on *receive only* one can
have unequally excited antennas. This does not behave the same as a
transmitted pattern where each antenna is, by design, equally excited.

Presume that one antenna could be in a null, and the other antenna hearing
mostly full strength. Signal level X on Antenna A is already down 3db
compared to the combined feedline signal of both antennas equally excited.
Now the signal arrives at the feedline junction (assume a plain "T"
connector for worst case).

The unexcited antenna & feedline is now a 50 ohm (or so) terminated load.
The receiver probably less of a good match. The combination of the unexcited
antenna and the receiver input is now a considerable mismatch. Without the
receiver input impedance and feedline lengths, one can't calculate exactly,
but there will be at least 3db more loss, maybe more if the presented rx
impedance is high. Adding up all these losses does remind one of 10 dB
stated below.

But, note clearly that RX does not behave like TX. What we're hearing and
what they're hearing may not match.

A note for the case of treating RX like it is having H.F. multipath. One way
of looking at the ground reflections and their  cancellation (or fortunate
addition) IS multipath.  With an unchanging signal coming in at a specific
angle, there are heights where incident and ground reflected signals cancel.
These zones are planar and parallel to the surface of the earth. Exactly how
we mount yagis.

A yagi mounted in this cancellation zone is not excited. It is for the
moment a relatively efficient, relatively well-matched energy dissipator at
the frequency in question. It looks like a well-behaved 50 ohm resistor from
coax center to ground where the feedline combines.

So I think multipath applies on H.F.. Since we mostly deal with single
antennas, we have tended to think transmit=receive on patterns. With stacked
RX antennas and ground reflections this is no longer so.

This phenomenon is very straightforward if one is talking about diversity
microwave horns. We use separate receivers to prevent the unexcited antenna
from becoming an energy sink or from having out of phase signals from the
two horns cancel.  I wonder if appropriate one-way combining devices exist
on H.F. that can handle transmit power, and change that 10 dB down to 3 or 4
dB down. (That's a whole S-unit receive on fades).

>
>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.

Amen.

>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.

Perhaps the improvement has more to do with the lack of fades at their end
due to your use of a stack at your end. That's a completely different
discussion.

>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.

Amen again.

>
>73, Eric  N7CL
>

----------

Guy L. Olinger
k2av@qsl.net
Apex, NC, USA


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