This is very interesting Guy! I have wondered about the receive mode the way
you have and concluded something similar only I looked at it from a different
view in that I thought about the fact that some antennas might be in a null
while others might be in a (QSB?) peak. The antennas in a null would only
dissipate maybe 3 dB compared to the large contribution of the antennas in the
peak.
Another interesting point is that I have consistently been used to getting
bigger reports than I can give out. I run an honest 1500 watts and the stations
on the other end using that level give me about 2 s-units better reports often.
I have always written it off to a stingy Yaesu FT-1000D or MP s-meter that I
use. Maybe there is more to it!
As you can tell, I am very afraid to formulate any technical conclusions, it is
all very complex, but I am learning a lot from this discussion, thanks to all.
73/Mike,N7ML
Guy Olinger, K2AV wrote:
> 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
>
> --
> FAQ on WWW: http://www.contesting.com/towertalkfaq.html
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
> Search: http://www.contesting.com/km9p/search.htm
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|