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" <shris...@ptt.rs> To: topband@contesting.com Sent: Saturday, April 20, 2019 8:16:37 AM Subject: Re: Topband: Fresnel Zone > donov...@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