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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