10 meter stack

Chuck Van Hoorn cvanhoo at heartland.bradley.edu
Tue Aug 16 14:51:50 EDT 1994

This summer I put up 2 identical 6 element 10 meter yagis at 60 and 80 feet.
Those heights were used because they could be rotated at those heights 
without hitting guys wires. They are fed using a T connector to 1/4 wave
of 72 ohm coax to equal lenghts of 50 ohm coax to the antennas. The SWR 
looks fine, but on many test my Mosley Pro 57 at 110 feet out preforms
the stack. When these antennas were fed seperatly they were usally about 
the same as the Pro 57. It seems that since I have fed them together 
the performance is slightly degraded. Most of the openings this summer
have been Sporadic E, but I have tested the antennas on some openings
to LU and for the most part the pro 57 has been better by an s unit.

What did I do wrong? Any thoughts or suggestions including reading material
ideas would be greatly appreciated.


cvanhoo at heartland.bradley.edu

>From tree at cmicro.com (Larry Tyree)  Tue Aug 16 20:57:44 1994
From: tree at cmicro.com (Larry Tyree) (Larry Tyree)
Date: Tue, 16 Aug 94 12:57:44 PDT
Subject: Non concrete anchors
Message-ID: <9408161957.AA25266 at cmicro.com>

There is another solution that the utility companies use.  You can buy a 
plate that you can drop into a 4 foot deep hole and then attach an 8
foot rod that you hammer into the ground so it mates up with the plate (it
is threaded and has a big nut).  I go the extra mile and put a bag or two
worth of concrete in with the plate.

I buy these from a place that sells to utility companies, but they are willing
to sell to hams.  This place was also a great place to buy guy grips (at
about $1 each) and egg insulators.

I think these would be easier to install then the screw anchor and the 
power company uses them!!.

Tree N6TR
tree at cmicro.com

PS: I think I paid about $120 for enough hardware for six anchor points.

>From Michael Owen <MOWE at SLUMUS.STLAWU.EDU>  Tue Aug 16 22:12:47 1994
From: Michael Owen <MOWE at SLUMUS.STLAWU.EDU> (Michael Owen)
Date: Tue, 16 Aug 94 17:12:47 EDT
Message-ID: <16AUG94.18590347.0024.MUSIC at SLUMUS>

Basically, screw-in anchors work like this:  Imagine the anchor in the
ground, screwed in all the way, say 4'.  Picture the cone of dirt
above the anchor.  The cone's interior angle is approximately 70
degrees (35 deg each side of the anchor... this is called the angle
of internal friction).  If the soil is cohesionless, like sand,
or perhaps *very* wet mud,
the screw-in anchor will stay put until it it pulled by a force
equal to the weight of that cone of dirt.  If the soil has some
cohesion, then the strength is increased *slightly*.  The main
result of cohesion is that failure will be sudden rather than gradual.
The limiting case is something like concrete where the anchor would
"go ballistic" upon failure.

You can figure the strength of the screw-in anchor by calculating the
volume of the cone (ask your kids to help); multiply by the density of
soil : 1.4g/cm^3 for wet silt, up to 1.9g/cm^3 for wet sand.  Dry
stuff will be LESS.  In case you're still reading, rock's density
is about 2.5 g/cm^3 - so screw your anchors into solid rock if you
can :)

Homework : calculate the holding strength of a 4' screw-in anchor
in "normal soil" (moderate cohesion, density = 1.5 g/cm^3).  Show
your work.

Vol of a cone = 1/3 * pi * r^2 * h
  where h = height (here 4' = 122 cm)
        r = radius of the top of the cone = h tan(35 deg)

so : r = 122 * 0.7 = 85.5 cm
     Vol = 1/3 * 3.14 * (85.5)^2 * 122 = 925,000 cm^3 (more or less)
     Weight = 925,000 cm^3 * 1.5 g/cm^3 = 1,387,500 g = 1,388 kg
     which is about 3,000 lbs.  [ notice that this figure drops
     *rapidly* if you don't screw the thing in all the way ]

Class dismissed.


Michael R. Owen, Ph.D.                        a.k.a.: W9IP
Department of Geology                         Northern Lights Software
St. Lawrence University                       Star Route, Box 60
Canton, NY  13617                             Canton, NY  13617
(315) 379-5975             -  voice  -        (315) 379-0161 (6-9pm)
e-mail: MOWE at SLUMUS            FAX   -        (315) 379-5804

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