The null-filling idea depends on one of two concepts:
1) ground clutter or irregularity filling nulls because the ground is
not a smooth reflecting plane. Effects of this can be seen with
programs like K6STI's TA (terrain analysis) program. The narrower the
null vertically, the easier it will be for terrain irregularity to
fill it in with miscellaneous bounces. In the terrain I have run, the
large nulls from lower heights don't get filled in because they are so
far across angle-wise.
2) the normal radio reflecting properties of the atmospheric layers
are not that smooth, and to a lesser degree than terrain, have
irregularities that fill in smaller nulls. If it was as smooth as a
mirror, 4-6 wavelength antenna heights would still be QSB horror.
This is of course something very hard to layout and prove. Scattered
anecdotal evidence seems to support it. Problem is that most folks
can't get anywhere near the heights needed to check this out at HF.
And regardless, folks with 10m antennas at 200 feet report times when
lower is better. If that antenna at 200 feet really behaved like 3db
over free space, it shouldn't be possible to stomp it. Go figure.
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <towertalk@contesting.com>
Sent: Saturday, February 02, 2002 7:53 AM
Subject: RE: [TowerTalk] Super high antennas on 10 to 20
At 09:13 PM 2/1/02 -0500, Chuck wrote:
>
>In getting back to the basics, I submit that at heights over 4 to 6
>wavelengths above ground, the antenna is in "free space" and
>that's why the "nulls"are filled in. Also, at these heights, in free
space
>means you have no gain from ground reflection, so they are 5-6 db
>down from lower antennas. How does one define free space? How
>high above ground would you have to be to be considered in this
category?
A lot higher than 4-6 wavelengths. I ran a 10m yagi in EZNEC at 200
feet
above ground, and both the deep nulls and reflection gain were still
fully
evident.
73, Pete N4ZR
Sometimes a tower is
just a tower
______________________________________________________________________
__
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
----------------------------------------------------------------------
-
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
-----------------------------------------------------------------------
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|