Hi Ford,
I suspect almost everyone is keenly interested in receiving
antennas.
> John reports a range of lengths. Frankly, you probably wouldn't be able
> to "hear" any difference from different beverages within the lengths
> reported.
That's been my experience.
Certain lengths can create extra useful nulls if you have undesired
signals (that includes the signals we call noise) coming from
specific areas, but you sure better have some idea where those
areas are when you plan the antenna!
> spreadsheet finds an "optimum" within range of all of the so-called "cone
> of silence" lengths.
That's great if the noise primarily comes from cone of silence
areas! The key is the null has to be aligned with the direction of the
most unwanted signal energy.
> ground or at different heights? Anybody actually "tuned" their beverage
> for optimum F/B or output that can report their measurements?
Unless multiple systems are up at the same time for a fairly long
period of time, a person could never determine if he optimized or
improved an antenna!
This isn't something we can look at an S meter and measure. We
can't make a change and rely on what that change immediately
does, unless it is a stable groundwave noise we are nulling.
>From day-to-day, let alone hour-to-hour, I can see my antennas
drastically change F/B ratio and "effectiveness" on sky-wave
signals without any change in the system itself! F/B ratio on my
conventional Beverages range from a few dB at times to over 20
dB, for sky wave signals. F/B ratio of my vertical arrays change
from maybe 10 dB to as much as 50 dB.
Most important, the antenna with best F/B or lowest overall noise
often isn't the antenna with best ability to dig out weak signals.
Even a few minutes later, that can all change.
The problem I observe is that phase difference of arriving signals,
even in a space of only two or three wavelengths, is so unreliable
that there is now way a physically large antenna or array of
antennas can be "designed" to be optimum.
The largest practical separation that offers reasonably stable
combining of antennas is about 1 to 1-1/2 wavelengths of total
spatial area. On stable nights, away from sunrise or sunset, it's
possible to make the array larger (I can vary the number of
elements phased together) but still it requires adjustment of phase
between the various antennas.
Now imagine how likely you are to "hit" a magic combination that
works with a system you can't adjust, like a 1500 foot Beverage!
Sure, you'll find a time and distance where the antenna works
better than a shorter antenna...but you'll also find just as many
times when it won't work as well as a shorter (1 to 1-1/2 wl area)
antenna!
A very large array won't work as well as predicted because phase
of the signal won't be stable over the area of the array. Phase can
be as much as 180 degrees out one time and back in phase at
other times, and it can change from moment to moment on some
nights!
We need to be able to determine a directivity factor, and
understand that if the array is physically large (more than 1 or 2
wavelengths) the signal arriving at all parts of the array won't allow
us to achieve the performance we estimate.
73, Tom W8JI
w8ji@contesting.com
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