[Antennas] Altitude

[Jay Eimer]

True.
   
  Fresnel zones and knife-edge refraction might make some signals stronger or weaker at heights that don't make sense to the conventional wisdom, those are factors unique to that spot.  The original question was "flat terrain" (for miles in all directions).  Assuming such a place exists (western KS, anyone?) then HAAT is what will matter for VHF and up and take-off angle will matter for HF.  There's no real difference between 3000' msl in western KS and 700' MSL in central OK for HF propagation, nor is there any real difference for V/UHF unless you happen to find the odd terrain feature that could invoke knife-edge, or you find the rare hill (and put your antenna on top of it).
   
  Even on HF, terrain can play both negative and positive roles.  The theory says if you're near a ridge, the land sloping away from the antenna lowers the take-off angle.  But if the land's slope is a cliff, does it lower the take-off angle "too much"?  That depends - what take-off angle do you "need" (varies with ionosphere and where you're trying to reach) vs what take-off angle would you get without the terrain in question (a dipole at 10' probably doesn't HAVE a take-off angle - but if it's broadside with a cliff, does the height of the cliff count as height above ground in the direction towards the cliff - and how close to the cliff do you have to be to see the effect?)

James Duffer <dufferjames at hotmail.com> wrote:
  
Seems that I've heard that there are things called Freznel zones. These zones cause a signal that is being received from a fixed transmitting source to vary with height. That the signal strength will be stronger at some points and weaker at other points in height. Of course this probably would be significant in fixed point to point communications, and not too important in ham radio. I have seen demonstrated where signal strength was greater at a lower height for microwave antennas. There is also the "knife edge" effect where a signal is actually bent by a mountain ridge and results in a stronger signal at a lower height.

73, Jim, wd4air


> 
> The higher location, will be better on the VHF and UHF bands. Line of 
> sight is line of sight, and any height advantage pushes that radio 
> horizon that much further out.
> 
> BUT
> 
> On HF, there would be minimal at best differences of these two 
> locations. Performance on HF is determined mostly by ground 
> conductivity, and the very near field area around the antenna. Here the 
> height of the tower and the surrounding immediate area make 99% of the 
> difference.

> >
> >Say I am surrounded by level ground for miles in all directions and there are no obstructions. I have a tower with a vertical on top and elevated radials at whatever the magical length and number is the greatest. My altitude is 600 feet above some standard reference (mean sea level if they haven't changed the name for that too).
> >
> >I have another location that is identical but the Altitude if 1600 feet.
> >
> >WHY is it that the one that is at the higher altitude is supposed to be so much greater of a location than the other as far as performance is concerned? (I just heard it again - First guy says - I am at 8xx ft and the next guy pipes up proudly and BOASTS that he is at 12xx ft like 800 is a horrible place to be).

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