[Antennas] Altitude

[Eric Lemmon]

Jim,

Yours is an excellent question, and the answer is:  It depends.

The simple answer has to do with Earth curvature.  The higher your antenna,
the further away the horizon is, and thus your line-of-sight range
increases.  In reality, the frequency of operation has a great deal of
influence on range versus height, so greater height does not always result
in greater range.  On a very tall tower, feedline losses may negate any
advantage gained due to height.

Most mountaintop radio sites are not in the middle of a vast area of flat
land, so any hills around the radio site will greatly influence coverage and
range.  Besides free-space losses, there is attenuation due to Fresnel and
diffraction losses.  Although some very expensive propagation calculation
software like ComStudy can make some pretty good predictions, there are so
many variable to consider- tree and brush coverage, buildings, and ground
conductivity to name a few- that the only really accurate measuring tool is
a drive test with a Coyote or similar GPS-referenced signal plotter.

The General Electric Company developed some useful tools, available here:
<www.repeater-builder.com/ge/datafile-bulletin/df-10003-01.pdf>
<www.repeater-builder.com/ge/datafile-bulletin/df-10003-03.pdf>
<www.repeater-builder.com/ge/datafile-bulletin/ecp-159.pdf>

Don't forget to use the radiation center (RC) of the antennas, not the
overall height, when calculating propagation.

73, Eric Lemmon WB6FLY
 

-----Original Message-----
From: antennas-bounces at mailman.qth.net
[mailto:antennas-bounces at mailman.qth.net] On Behalf Of Jim Miller
Sent: Tuesday, May 13, 2008 9:22 PM
To: antennas at mailman.qth.net
Subject: [Antennas] Altitude

OK (embarrassed) what's the deal?

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).

WHY ????

Thanks es 73, de Jim KG0KP
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