[TowerTalk] Stacks (optimizing gain at takeoff angle)

[Guy Olinger, K2AV]

The model analysis is correct, particularly with the assumptions of
H.F., or uniform uncluttered ground. Aim it at the horizon.  What goes
wrong and is usually overlooked in the "intuitive" step of raising the
boom, is that increased power is transmitted to higher angles of no use.
You have taken power from a bounce that adds to the desired angle and
placed it at a useless angle.

But at some point on the way to VHF the raw absorption by the minutia of
ground cover (manmade and otherwise) begins to reduce the level of the
ground reflection. At that point, elevating the boom slightly makes a
difference. I would be careful as to the angle, particularly with a long
boom yagi. 10 or 15 seems too high for all but rather poor sites.

I have been at poorer sites where raising the angle simply concentrated
the power at the first takeoff angle that had ANY chance of going
anywhere except absorption or miscellaneous reflection. I suspect this
use of the technique is the source of this oft-repeated, highly
situational "principle".

HOWEVER, that trick does no good at excellent sites which have a horizon
view. At large distances to the "first bounce" where the incident angle
is a degree or less, other factors take hold, and the boom needs to be
level.

I understand this particular problem from first-hand experience in
Putnam county, NY. I was on the south-side of a hill that had a good
shot from 130 degrees through 270. The hill destroyed the rest of the
compass. I later discovered in the VHF tests if that I kept JFK
(airport) control on the scanner, that for some number of minutes after
a 747 took off  I could work louder New England stations by beaming 175
degrees (at the airport on Long Island).

If I espoused a "principle" that all VHF contesters should have a beam
fixed on the local airport, should anyone take it seriously? Only if
they are in a situation like mine in Putnam County, and the airport
serviced 747's.

The vertical two meter yagi 4 stacks used by the PEARL 145.13 repeater
up on Mt. Ninham to enhance reception of mobiles and handhelds in
Westchester and New York City were NOT aimed 10+ degrees. They were dead
on the visual horizon and worked extremely well.

- - . . .   . . . - -     .   . . .     - - .   . - . .

73, Guy
k2av@contesting.com
Apex, NC, USA



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