Affect of TV Receive Antenna Height on Signal Strength It Receives

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by W9XMT, Jul 22, 2019.

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  1. AI3V

    AI3V Ham Member QRZ Page

    Also keep in mind that this is very much a theoretical ideal.

    In the practical world, it will typically be completly masked by the many "multipath" effects.

    The most common multipath observed is probably "picket fencing", the rapid in and out fading when listening to a vhf/uhf signal while mobile.

    We have all stopped at a light and noticed the radio getting noisy or even dropping out completly, and observed moving a quarter wavelength that the signal came back.

    Rege
     
  2. W9XMT

    W9XMT Ham Member QRZ Page

    The comments and graphic below show that this theory commonly is realized in physical fact.

    FM stations with antenna heights greater than the HAAT permitted for their Class of station must reduce their ERP to provide a radius to their 1 mV/m field contours equal to that of a station exactly meeting the maximum ERP and HAAT authorized for that Class.

    As an illustration of this, below is a comparison of the FCC F50,50 coverage radius to the 1 mV/m field intensity contours for Class B FM stations on the Willis (Sears) Tower in Chicago and a 50 kW FM at 500 feet HAAT.

    The reason for the parity of the two signals is the lower propagation loss resulting from better Fresnel zone clearance for the higher transmit antenna.

    [​IMG]
     
    Last edited: Jul 24, 2019
  3. AI3V

    AI3V Ham Member QRZ Page

    Except this is a computer calculated guess at theoretical signals.

    Based on a perfectly spherical earth- that is a surface completly smooth.

    No hills, no valleys, no buildings, no trees, no power lines, nothing.

    The results to 2 decimal places !!! give it away.

    I am not saying higher isn't better, it usually is.

    I am saying, putting a transmitter of power x at height y and going to distance z will almost certainly see a variation of signal strength from theoretical, and by variation it might be anywhere from +6 to - infinity.

    And you will not be able to tell if it is from "pure" fresnel effects, or absolutely random multipath.

    Rege
     
  4. W9XMT

    W9XMT Ham Member QRZ Page

    The FCC F50,50 FM Propagation Curves are based on historical and measured field values statistically occurring at 50% of receive locations 50% of the time, for a terrain roughness factor of 50 meters. They are not guesses based on a flat Earth model.

    Longley-Rice propagation analyses add and consider the effects of terrain database elevations in calculating field intensities at various ranges. Its output is more specific to the actual installation, but for reasonably flat terrain, the FCC F50,50 FM curves produce about the same average contour distance results as L-R, and include Earth curvature.
     
    Last edited: Jul 24, 2019
    KB0MNM likes this.
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    So it is only half correct. :rolleyes:
     
  6. KB0MNM

    KB0MNM Ham Member QRZ Page

    I like a lot of what is written here by many authors, yet would also like to add: The sound carrier for old-fashioned NTSC analog TV, still broadcast by a few amateur television stations is FM, yet the video modulation is ( was re: modern Television ) Vestigial Sideband Modulation ( VSB ) which is closer to an A.M. signal. Now the propagation effects are similar and in theory- one would apply Fresnel zones and/or Rayleigh fading to FM signals. As to a Fresnel antenna, there are circular polarized microwave antennas which are sometimes known as Fresnel antennas because they use a lens ( or lense, depending on the dictionary in use ) that has been designed based upon Fresnel-based mathematics. I would imagine it might be possible to scale these to UHF frequencies, yet my experience does not include seeing any UHF nor VHF during maintenance. I did own an ATV 'fast-scan' ( NTSC standard ) transmitter until approximately 2008, and worked worked it as a transmitter when the Brainerd Amateur Radio Club opened their station approx. 2006. A mobile ATV station also participated in the grand opening, and we were able to monitor each other using agile conventional 'old' analog battery-operated receivers. So the other question about anyone using 'ATV' 'anymore' can be partially answered. In approximately 2005, I was advised that much of the ATV 'fast-scan' action surrounding Minneapolis/St.Paul had moved to the 1.2 Ghz. band. This might refer to video carried by D-Star ( data ) signals, or to a higher frequency VSB/FM NTSC signal. I hope that some of this illuminates the answer to the original post, and that of 'in-use?' questions. There have been other non-amateur radio uses for the UHF NTSC/VSB video modulation which were discussed on QRZ forums in 2007 and 2008. That may fall outside this discussion for now.
     
  7. KB0MNM

    KB0MNM Ham Member QRZ Page

    The answer about Digital TV is different than for analog T.V.- There is a continued 'repack' ( reallocation of channels ) for the same frequencies that were used for analog (NTSC VSB/FM) which favors larger networks, often because longer antennas are required. It is cheaper to make an antenna electrically shorter, thus some are likely to cease broadcasting the new digital signal T.V. where others are actually incorporating higher-resolution in 'repacking' their available bandwidth. As to digital either working or not, that depends on whether or not one is able to tolerate the 'Max Headroom' ( From an MTV video ) effect of loss of synchronization and temporary 'pixelation'. While there are other DTV receivers, I believe the "iKonvert" product sometimes has a slight advantage due to the chipset. This is not a formal endorsement of that product, just an FYI of the one that I could afford when working at TBN. In a similar vein, I used the 'Leaf Metro' antenna and received approximately 30 useable channels with time-of-day an important factor in a major metropolis. Of these, 10 were worth watching. I received local news, TBN, oldies, international news from asia, and a few others that my XYL cared to watch. We considered dropping the cable/dish based video... that is another story.
     
  8. W9XMT

    W9XMT Ham Member QRZ Page

    That is true, however for most terrestrial point-point paths. the effect of Fresnel zone clearance is (nearly) fixed at a given receive location, while other multipath effects are highly variable for that same length of propagation path.

    The graphic below shows the effect of the physical clearance of Fresnel zones above a reflecting surface on the free space, LOS field that would arrive for that end-end path length.

    It can be seen that the received field has several nulls related to its height above the reflecting surface, and also that the field change can be very small for some changes in Fresnel zone clearance (centered on the odd-numbered Fresnel zones).

    It is distinctly possible that a higher mounting elevation of a receive antenna above a reflecting surface reduces its output voltage compared to a lower elevation for that antenna.

    The effect of marginal Fresnel zone clearance is especially troublesome when the reflection occurs from a sea-water surface. When the elevation of that surface changes due to tidal activity, received field might drop periodically below the minimum needed for acceptable performance.
    [​IMG]
     
  9. W9XMT

    W9XMT Ham Member QRZ Page

    A reference was made in Reply #24 of this thread to the use of Longley-Rice algorithms to predict received field intensity. Below is an example of a L-R analysis.

    Note that:
    • a line-of-sight path exists between the transmit and receive antennas, however
    • the terrain profile for this path obstructs the 0.6 Fresnel zone, which adds about 12 dB to the free-space, LOS loss for this path length and frequency

    [​IMG]
     

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