[Antennas] Altitude

[Jim Miller]

Thanks Guys,

The question was meant to be in reference to HF only and the spec of a
vertical verses a directional antenna was meant to negate any directional
properties that may have been implied.

Reiterating the altitude question "another location that is identical".

It appears to me that the answer to my altitude question (within reason at
least) is that IN ITSELF, altitude has virtually NO effect on propagation.
And thanks JAY, you said it straight out.  All the other factors that are
the determining factors on performance at ANY location are really the "only"
ones that apply.

Thanks es 73, de Jim KG0KP


----- Original Message ----- 
From: "Jay Eimer" <ad5pe at sbcglobal.net>
To: <antennas at mailman.qth.net>
Sent: Wednesday, May 14, 2008 11:16 AM
Subject: RE: [Antennas] Altitude


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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