The only way to really know is to run the numbers for the location. I am
blessed with a sloping foreground in most directions: some out to about 4
miles, some out to 14 miles, and the resulting radiation patterns are
different, but not radically so. The difference between the 2 mile and 4
mile calculations are significant. My rule of thumb when planning new
antennas is to go out at least 4 miles for this location, and even then
there are still differences from tower to tower and direction to direction.
I am not blessed with high ground conductivity, though. I am still looking
for that gently sloping hill of salt water!
73 John N5CQ
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Grant
Saviers
Sent: Wednesday, June 18, 2014 1:29 AM
To: Cqtestk4xs@aol.com; TOWERTALK@contesting.com; w9ac@arrl.net
Subject: Re: [TowerTalk] Site Elevation and TOA
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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