Topband: RE: skewed paths

[Tom Rauch]

> "The first couple of days of full operation on 160 Meters we knew the
> signals were coming from the SSE but we were having problems copying
> them on a two wavelength long Beverage oriented specifically in that
> direction.  Reports via the pilot stations indicated our transmit
> signal was being heard quite well."
> 
> It seems to me that this is a perfect example of back scatter, where
> the geometry of the skip along one path is impossible in reverse.

It is impossible to have any idea of what was going on because we 
have no idea how any of the antennas at that site actually work. 

How do we know the 2WL Beverage didn't have a lobe oriented into 
a major source of noise, while the dipole just luckily had a null 
towards that noise?  We don't, unless they had dozens of receiving 
antennas.  

One fundamental problem is we have often been misled when trying 
to understand reciprocity. We somehow think an antenna has the 
same figure of merit on transmit as receive. This is often implied in 
material we read, for example we incorrectly hear that a yagi gives 
us "6dB gain" on transmit and receive, so it helps us 6dB "both 
ways". That is true so far as absolute signal level, but it is total 
nonsense when considering S/N ratio. 

As an example, would you EVER consider transmitting on a 
Beverage rather than an even modest vertical? The Beverage 
greatly improves S/N ratio because it has deep nulls in the 
direction of noise compared to response to the signal.

Since we have no idea what the direction and angle of noise was at 
XZ, we can't possibly form any accurate opinion of what it means. 
All it means is they had better luck with the location of nulls on the 
dipole reducing noise than with the Beverage. Changing the 
beverage heading 20 degrees, or moving it 1000 feet, might had 
produced entirely different results.

Not only that, 2WL Beverages are often "dogs". They look OK on a 
model, but often work worse than shorter antennas for many 
reasons. Bigger is often worse. I just installed a ~2wl  west 
Beverage pair spaced 350 feet, and despite a good pattern on 
EZNEC it is a dog compared to an identical array with 780 foot 
elements.  

> Milt continues regarding a low dipole:
> 
> "The proof is in the results because the high angle arrival signals
> from the SSE skew path received by the low dipole accounted for
> approximately 80% of the 400+ NA stations that were worked on 160
> Meters from XZ0A."
> 
> Again, I submit that this is consistant with what would be expected
> from backscatter. 

Unfortunately none of us have the faintest idea what the wave angle 
was. It is only a guess, based on no real evidence because nothing 
they had allowed wave angle measurements. Keep in mind the 
ONLY thing a better S/N ratio tells us is the antenna responds 
better in the direction of signal than in the direction of noise. We 
haven't any idea at all what that means so far as how the signal 
gets there, what polarization it is, or what wave angle it is. 

It could be that the dipole had a null in the direction of noise 10dB 
deeper than the Beverage, and the signal direction gain was the 
same. It could mean almost anything. 

> I'm no expert by a long shot but Milt's empirical evidence seems
> consistant with the back scatter explanation.

Unfortunately it only means for them with the dominant noise 
source they had the dipole had a deeper null compared to 
response in the direction of the signal.

Now if no one over hear could hear them until they used the dipole 
for transmit, that would indicate the wave angle was high or the 
vertical had poor efficiency.
73, Tom W8JI
W8JI@contesting.com