Hi Sinisa,
See the last paragraph in this email for a very specific answer to
Ray's original question...
Your comments about applying this 57 year old classic publication
are exactly correct.
First determine the desired elevation angle(s), t hen select the antenna
height that optimally illuminates the Fresnel zone.
HFTA does a good job of performing this basic analysis, although it
doesn't perform Fresnel zone analysis. Instead HFTA leaves it up to
the user to analyze several azimuth offsets to adequately analyze the
full width of the Fresnel zone over which the antenna's elevation
pattern is formed.
The width of the Fresnel zone is 5.66 times the height of the antenna
at a distance of approximately the height of the antenna (for antennas
at the optimal height of the desired elevation angle) divided by the
tangent of the elevation angle.
For an elevation angle of ten degrees the Fresnel zone reaches its
maximum width approximately six wavelengths in front of the
antenna, the near edge of the Fresnel zone is approximately one
wavelength in front of the antenna and the outer edge of the
Fresnel zone is approximately 30 wavelengths in front of the
antenna.
Detailed Fresnel zone analysis of vertical antennas is impractical
because of the variability of soil conductivity within the large
Fresnel zone. Fortunately s oil conductivity isn' t a significant factor
for horizontal polarization except for very dry, low conductivity soils.
The location of far edge of the Fresnel zone for vertically polarized
antennas (related to Ray's original question) is highly dependent on
soil conductivity in approximately the outer 75% of the Fresnel zone.
The outer 75% of the Fresnel zone can add 3 dB or more to the
low angle performance of vertical antennas if most of the soil
(or water) within the outer 75% of the Fresnel zone is highly
conductive.
The specific answer to Ray's original question regarding a
160 meter 1/4 wavelength vertical is:
For a 10 degree elevation angle -- f or each specified azimuth angle --
more than half of the low angle radiation is formed in an area
approximately 500 feet wide located 3000 to 15,000 feet in front
of the vertical.
73
Frank
W3LPL
----- Original Message -----
From: "shristov" <shristov@ptt.rs>
To: topband@contesting.com
Sent: Saturday, April 20, 2019 8:16:37 AM
Subject: Re: Topband: Fresnel Zone
> donovanf@starpower.net wrote:
> The classic source for Fresnel zones as they apply to HF site
> selection is NBS Technical Note 139:
> https://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote139.pdf
This indeed is a classical source, but it should be noted that the derivation
assumes that the elevation angle actually used for communication
is equal to the elevation angle of the peak of lowest ground reflection lobe.
This may or may not be the case.
In amateur operation, communication is often done using elevation angles
well below the peak of the lowest lobe, because of inability to erect very high
antennas.
In such cases, the first Fresnel zones extends much further than the
equation 3.4-4 predicts. Correct formulas may be derived using the same
procedure, while separating communication angle from lobe peak angle.
Using vertical antennas additionally complicates matters.
I am not aware of any work on this topic.
73,
Sinisa YT1NT, VE3EA
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|