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  1. fmradio

    How to Calulate Received Field Strength

    MoM software such as NEC can be used to calculate e-field intensities including the near field for the conditions set in the NEC model, which then can be plotted. The graphic below is one example, showing that the transition from near-field to far-field conditions exists at an h-distance of...
  2. fmradio

    Effects of Frequency & Earth Conductivity on the Groundwave Propagation of AM Broadcast Stations

    Not always. I have answered or commented on the posts of others, also. My goal in this case was to post information that perhaps was not obvious, or well-known -- a tip, which seemed appropriate for posting on ENG-TIPS.COM. Thank you for your interest and evaluation.
  3. fmradio

    Effects of Frequency & Earth Conductivity on the Groundwave Propagation of AM Broadcast Stations

    Below is a link to a paper showing the effects of frequency and Earth conductivity on the propagation of groundwaves in the AM broadcast band. The study confirms the physical reality that for long propagation paths and some sets of conditions, an AM broadcast station radiating 1 kW can produce...
  4. fmradio

    Monopole Low-Angle Radiation

    On 25 Aug 2013 VE1BLL posted: "...zero in the horizontal plane..., as some expect..." I've not met anyone that actually believed what is so obviously untrue, based on the real world fact that radio communications do actually work (as you pointed out). ____________________ This is a very common...
  5. fmradio

    Monopole Low-Angle Radiation

    VE1BLL wrote (in part): "The limitations of NEC2 w.r.t. imperfect ground planes is well known; ... I had assumed that it was well known that NEC2 users need to be cautious when applying NEC2 to cases with complex ground conditions. The same caution should be applied to NEC4 results; e.g. the...
  6. fmradio

    Monopole Low-Angle Radiation

    That last NEC4 graphic was posted to show that fields from a monopole are not zero in the horizontal plane even at 10 km downrange over a real earth path, as some expect by looking only at a NEC far-field analysis. For a NEC4 analysis and discussion showing elevation angles and skywave...
  7. fmradio

    Monopole Low-Angle Radiation

    Yes, the Y axis is the height above ground in meters, 10 km downrange (for a flat ground plane).
  8. fmradio

    Monopole Low-Angle Radiation

    VE1BLL wrote: "The 30 ohms case in your latest link; I suspect that's just a virtual series R in the feed point (within the simulation)." __________ NEC4 calculates the r-f loss resistance in whatever set of buried wires is defined for a monopole antenna in the model. That loss depends on...
  9. fmradio

    Monopole Low-Angle Radiation

    VE1BLL wrote: "It's been my understanding that with a very good ground plane (120 radials), one would expect coverage right down to zero degrees elevation." __________ Yes, that is true. But it is also true for monopoles using short/sparse buried radials, or just a few ground rods. The link...
  10. fmradio

    Monopole Low-Angle Radiation

    The point that fields produced/launched over a real earth path from monopoles of up to 5/8-wave height are not really zero in the horizontal plane, and not much more than zero at low angles above it -- as shown in NEC far-field elevation patterns. If they were, then AM broadcast band stations...
  11. fmradio

    Monopole Low-Angle Radiation

    Yes. All three plots were made using 120 x 1/4-wave buried radials, and the same Z-matched power (100 watts) at the feedpoint terminals. Using a web browser on a PC or Mac to open the URL in my original post will show the graphic with better resolution.
  12. fmradio

    Monopole Low-Angle Radiation

    For consideration / comment... Radiation launched by vertical monopoles of 5/8-wavelength and less in height always is maximum in the horizontal plane -- regardless of earth conductivity and the number/length of buried radials they use. This point is illustrated in the NEC4 analysis...
  13. fmradio

    Dipole Questions

    Previously... "- what does half-wave, full-wave, 3/2 wave... resonance mean?" ______________ "Resonance" of a radiator simply means that the impedance of that radiator has no reactance at its input terminals, at the operating frequency. For example, a self-resonant, center-fed dipole a bit...
  14. fmradio

    Axial Ratio (CBR Antenna)

    True, even though I wrote "about" the lowest. However in my 40+ years of broadcast industry experience the statement is true for FM broadcast transmit arrays installed on large cross-section towers. The most common types of side-mounted, "c-pol" FM broadcast transmit antennas can have axial...
  15. fmradio

    Axial Ratio (CBR Antenna)

    thread247-93327 The following statement was posted on 6 Oct 2004 in the referenced thread: "In FM and TV broadcast transmit antennas, the cavity-backed radiator has about the lowest (true) c-pol axial ratio, as installed." In support of that statement, the attached gif shows the measured...
  16. fmradio

    base loaded whip

    The intrinsic directivity of an infinitesimally short dipole is over 91% that of a 1/2-wave dipole (so says Kraus). The problem is that the radiation resistance of the short dipole or monopole is so small, and its input reactance so large that most of the available r-f power is dissipated in...
  17. fmradio

    Elevation Patterns of Ground Mounted Vertical Monopoles

    "Then again, I'm not sure that a guy with a slide rule in the 1930s could have done much better either, no matter how smart he was." __________ BL&E's investigations were not a slide rule study, they used real hardware in the real world. And THAT is why their conclusions have proven valid in...
  18. fmradio

    Elevation Patterns of Ground Mounted Vertical Monopoles

    Yes, they are. A benchmark, 1937 IRE paper by Brown, Lewis & Epstein of RCA Labs first investigated the effectiveness of such buried ground systems using radials of various numbers and lengths. They found that a vertical monopole using about 120 radials each 0.41-lambda in length radiates an...
  19. fmradio

    Elevation Patterns of Ground Mounted Vertical Monopoles

    "Getting back to the primary point: I suspect that there isn't as much of a gap between Terman and NEC as there might sometimes appear." _________ Agree. NEC will give ~ the same output result as developed by Terman and other early authors as long as its operation is understood, and it is used...
  20. fmradio

    Elevation Patterns of Ground Mounted Vertical Monopoles

    This was Terman's method to show the data in a manner he wanted.

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