Re: [TowerTalk] Site Elevation and TOA

[David Gilbert <xdavid@cis-broadband.com>]

I concur with that, at least as far as HFTA is concerned.  I live on the 
eastern slope of a mountain range in southern Arizona, and the land 
slopes strongly down for about a half mile (about 15% slope), then 
slopes about 4% down for the next five or six miles.  According to HFTA, 
my peak takeoff angle on 20m and 40m is literally around 2 or 3 degrees 
with my antennas at 72 feet and 83 feet respectively. I can see a 
definite impact if I carry the terrain profile out far enough to capture 
the mountain range that exists 16 miles east of me, and which I can see 
visibly from my QTH.  In my opinion, anything between you and the 
visible horizon is fair game for HFTA.
As K4XS says, the easiest way to check is simply to create a dummy 
terrain file with a data point out at whatever distance you perceive 
your horizon to be.  HFTA will simply assume a linear slope from 
whatever real data point you have out to the dummy data point you add to 
the file.  The HFTA terrain file is a very simple text file containing 
two columns of paired numbers (one for distance and one for elevation) 
and is trivial to edit.
73,
Dave  AB7E




On 6/17/2014 11:29 PM, Grant Saviers wrote:
> Dean gave an HFTA talk at Seaside last week and I asked this question. 
> "How far away do I not need to worry about a mountain?" His answer:  
> "over the horizon".  He showed some patterns much further out than 
> 14000 feet.  To get those he changed the DEM baseline steps to 100m 
> from the default 30m.  The ray tracing matrix is of fixed size, 
> 150x150 as I recall, so a coarser horizontal step is needed to 
> calculate to further distances.  A limitation of a program written in 
> Fortran for a mainframe with less memory than your watch.
> OTOH, my 15 mile away mountain range is about 3 deg above horizon, so 
> while a purist might calculate the pattern, I think it is not 
> consequential (hopefully).
> One of the antenna books, I don't recall which, shows an example of a 
> DX station with a far mountain that significantly affects the pattern.
> Grant KZ1W
>
>
> On 6/17/2014 3:29 PM, Bill via TowerTalk wrote:
> >   According to N6BV, who knows his stuff...
> >   Beyond approximately14000 feet has very little effect on the TOA  
> > for HF.
> > Close in is far more important.  You can test this by making  up a 
> > file with
> > hypothetical elevations and putting it into N6BV's  program
> >   Bill K4XS
> >
> >     In a message dated 6/17/2014 9:47:04 P.M. Coordinated Universal 
> > Time,
> > w9ac@arrl.net writes:
> >
> > I'm  trying to locate land in south GA for a remote Internet 
> > station.  Two
> > self-supporting towers are ready for installation.  Tower #1 is 140 
> > ft  and
> > Tower #2 is 100 ft.  A full-size, 4L 40m monoband Yagi goes on the  
> > top of
> > tower #1.  A 30m-10m LPDA goes on tower #2.  Siting has  become a lot 
> > harder
> > than I imagined.  Here are my siting  constraints:
> >
> > 1) Low noise in the immediate area;
> > 2) Easy utility  power access;
> > 3) High speed data access over FTTH or CATV.  No DSL  unless I really 
> > get
> > desperate.  Too many future applications will need  the extra 
> > throughput;
> > 4) High land that either remains flat for the TOA  distance or slopes
> > downward.
> > 5) Land that fits within the project  budget.
> >
> > Sounds easy. Way harder than you think -- unless a home goes up on the
> > property and I move there where I have more options due to the 
> > higher  price of
> > properties.  Moreover, many counties won't allow a telecom shelter or
> > other structure as a primary use without first establishing a 
> > residence through
> > placement a house or manufactured home.  I don't want  that.  I want a
> > remote site only.  My main focus is Brantley County,  GA. There's no 
> > zoning in
> > the county.  There's also super-high-speed  fiber supplied to the entire
> > rural county by the local telco.  The telco  bet big and lost when 
> > they assumed
> > a housing market explosion in 2005 that  turned into an implosion.  
> > Along
> > the county highways are dozens of  started subdivisions that are now 
> > ghost
> > towns.  Cheap land, but the  developers recorded much of it early on 
> > with deed
> > restrictions.  Once  just a few owners take possession, changing the
> > covenants is a  nightmare.  It's one thing to take up the cause when 
> > you already
> > own the  land.  It
> > 's insane to consider restricted land when you're looking to buy from 
> > the
> > start.
> >
> > After looking at dozens of parcels,  I've found a few that might work.
> > Here's my question: In terms of  wavelength, at what distance is the 
> > TOA set
> > for elevated, horizontal  antennas?  I realize that the TOA is 
> > composed of
> > near, intermediate and  far fields above elevation, but there must be a
> > distance where say...90% of  the predicted TOA occurs.  What is that 
> > distance in
> > wavelengths from the  antenna?
> >
> > Paul,  W9AC
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> >
> >
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