Topband: N0OQW Beverage Spreadsheet

[Tom Rauch]

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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