On 10/27/2013 2:26 AM, Jim Thomson wrote:
# Lemme rephrase that slightly. IF the ground is relatively flat....higher
is STILL better. The 1st 45 feet doesn’t count. You need that just to clear
junk +
clutter in the surrounding neighbourhood.
I won't dispute that.
## While 70 ft may seem high....on 40m + 80m, its not. 70 ft on 40m is like
35 ft up
on 20m.
Gee -- from your earlier post, to which I strongly objected, I got the
distinct impression you were talking about tribanders. But that is is
exactly my point -- in addition to clearing the clutter, the most
important contribution of height is forming the vertical radiation
pattern of the antenna, and for that, height must be thought on in
ELECTRICAL DEGREES, not feet or meters. Up to a point, additional height
boosts low angle radiation, but eventually a height will be reached
where it produces nulls in the vertical pattern. At lower heights these
nulls are at higher angles, but when you go higher, they are at lower
angles.
I'm currently in the midst of a project to raise my 80/40 fan dipole
from around 120 ft (155 degrees on 80M) to around 140 ft (182 degrees)
to improve its low angle radiation on 80M, and to add a reflector to
increase the gain to the east coast and EU. If it were flat land, NEC
predicts 2 dB for the increased height and 3.5 - 4.5 dB for the
reflector (the lower gain number is because the available trees don't
allow the reflector to be parallel to the driven. We'll see.
Take one of these 89 ft crank ups....and run it from 26 ft nested..up to
89 ft extended..and it’s a real eye opener. I have a slight uphill rise from
N-S...all facing east. And slightly downhill.... from N-S..all facing west. I need
all the height I can get when pointed at EU, AF, SA, etc.
Right, but you said flat land. I have similar topography here to the
north and east, and on all bands, higher IS better in those directions,
even on the higher HF bands. But it is NOT better to anywhere in the
Pacific, because I have five miles of down-slope in that direction.
The beauty of N6BV's HFTA software is that it first models the
contribution of terrain to the vertical pattern along any azimuth that
you choose, then allows you to superimpose statistical modeled data for
the vertical propagation to a range of destinations for the non-WARC
bands. This allows the user to see when those vertical nulls are likely
to get us in trouble, and when they don't matter. I have two towers, one
holding a 3-el SteppIR (no trombones) at 120 ft, another placing
monobanders for 20 and15 at about 45 and 35 ft respectively, and a
third push-up holding a 10M Yagi at 20 ft. In all cases, HFTA accurately
predicts the actual on-air performance of these antennas.
As to stacking Yagis -- I've heard W3LPL do an excellent presentation on
this topic at part of K3LR's Contest University. I don't know whether
this material is available on line, but it's well worth seeking out.
Someone objected to my use of "profanity" to make a point. The late
Paul Klipsch, was one of the most highly respected of engineers working
in pro audio as well as some other disciplines (explosives and firearms,
as I recall), and founder of the loudspeaker company bearing his name.
At annual meetings of the Audio Engineering Society, Paul made a
practice of sitting at the back of technical papers sessions wearing a
plaid sport shirt (and a bola tie, as I recall). When the technical
content of a given paper strayed from the laws of physics, Paul would
stand in the aisle at the back of the room and open his shirt to reveal
a tee-shirt containing the single epithet "BULLSHIT" in large letters.
73, Jim K9YC
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