HFTA's problems with close spacing begin *much* closer than that -
IIRC, <10 ft.
From the HFTA Help file:
The internal Yagi model in HFTA is a very simple mathematical
model. It does not compute interactions between individual Yagis
in a stack — HFTA assumes that each antenna is a “point source.”
For antennas stacked more than about a half wavelength apart this
is not a problem. For example, you should be cautious specifying
spacings less than about 20 feet on 20 meters (and proportionately
scaled on other bands) because of mutual-coupling effects between
real antennas. Spacings less about 20 feet on 20 meters will show a
false increased gain in HFTA, even though the real effects of
interaction between the beams will actually be to decrease the
gain.
On any band HFTA seems to increase the gain from stacking as the
antennas get closer but, as Dean says, that does not factor in the
interaction between antennas. Dean's "rule of thumb" is 20 feet on
20 meters (0.28 wavelength) but inflated gain begins to appear at
any separation less than about 1/2 wavelength if one compares the
vertical pattern generated by HFTA over flat ground with similar
patterns generated by EZNEC using the high accuracy ground.
73,
... Joe, W4TV
On 2015-03-10 9:19 AM, Pete Smith N4ZR wrote:
The *magic* of 1X/2X spacing is that it tends to cancel the second
vertical lobe. Depending on your takeoff angle needs, this might not be
a good thing, but on the other hand if you can switch to use just the
lower antenna you'll find that it emphasizes the same angles that the
stack cancels. You might look at 50/100 ft spacing too. HFTA's problems
with close spacing begin *much* closer than that - IIRC, <10 ft.
73, Pete N4ZR
Check out the Reverse Beacon Network at
http://reversebeacon.net,
blog at reversebeacon.blogspot.com.
For spots, please go to your favorite
ARC V6 or VE7CC DX cluster node.
On 3/9/2015 9:11 PM, W0MU Mike Fatchett wrote:
What values did you get using HFTA. HFTA has some issues with very
close spaced stacks. The numbers are wrong.
Mike W0MU
On 3/9/2015 7:09 PM, Jorge Diez - CX6VM wrote:
Thanks Jim
Sorry for the delay, just returning from along trip after ARRLDX SSB
Is curious HFTA. It stopped my project for a two stack for 20 mts yagís.
Since many contesters told me to put it at 69/139 ft, HFTA told me
that this
is not the best distances.
OK about your document, will read it, but here we don´t have such
facilities
than in USA to know ho´w the ground conductivity, the terrain data,
etc, etc
73,
Jorge
-----Mensaje original-----
De: TowerTalk [mailto:towertalk-bounces@contesting.com] En nombre de Jim
Brown
Enviado el: domingo, 08 de marzo de 2015 02:44 p.m.
Para: towertalk@contesting.com
Asunto: Re: [TowerTalk] 80 mts four square or yagi?
On Sun,3/8/2015 10:06 AM, Jorge Diez CX6VM wrote:
Wish someone have some analysis or field test to compare a 4SQ with
sixty 1/4 WL radials each vertical and a 3 element wire yagi with boom
at 115ft
This is not exactly the answer, but it should help you think about it.
http://k9yc.com/VertOrHorizontal-Slides.pdf
Also, if the land around you is not flat, you also need to study and use
HFTA extensively. It comes on the CD with the ARRL Antenna Book. HFTA
tells
only about horizontally polarized antennas, NOT verticals, because they
interact with the earth very differently. The slides tell a big part
of that
story.
73, Jim K9YC
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