On 8/25/14, 10:49 AM, RLVZ--- via TowerTalk wrote:
Hi Guys,
FWIW, I will share an actual experience where a Ground Plane Vertical
located in dense trees worked very poorly. One of my favorite Field Day
antennas over the years has been a simple Hustler 5BTV vertical elevated 10-20'
above ground level, and operated with 2 tuned radials per band. I've made
thousands of FD Qso's on 40-10 meters with this arrangement and often get
real nice pileups going. However, a few years back, my son-in-law asked me
to do FD from his QTH in Central Illinois, which was surrounded by hundreds
of trees. It was like operating from within a dense forest. The GP
Vertical performed very poorly and operating was a miserable experience as I
found it very difficult to makes Qso's, in fact it was more difficult to make
Qso's than many of my QRP experiences. That said, my daughters cooking made
up for it!
My personal conclusion is that I love Verticals and won't be without one
or more regardless how many Yagi's I have in the air. But now I'm a firm
believer that they need to be located "in the clear". (just as many antenna
books advise)
73, Dick- K9OM
Pin
Hi, Guys:
My understanding is that you'd have to put a vertical radiator very close
to
a tree for sap/no-sap to have any impact on the antenna's performance --
perhaps within a foot of the trunk. Having said that, I know of several
hams
who did very well with "disguised, stealth" vertical wires run right up
alongside the trunks of substantial pine trees. These gents worked lots of
DX with such setups. Of course, YMMV!
73 and HNY,
Dean, N6BV
Senior Assistant Technical Editor, ARRL
Editor, The ARRL Antenna Book
-----Original Message-----
From: Hallas, Joel W1ZR [mailto:W1zr@arrl.org]
Sent: Tuesday, January 02, 2007 12:24 PM
To: Larry Banks
Cc: Straw, Dean, N6BV
Subject: RE: Dear Doctor: Antennas and Trees
-----------------------------------
Larry,
I haven’t seen definitive words on the topic, however I believe trees have
more of an effect for HF signals on vertically polarized signals then on
horizontally polarized ones. Floyd Koontz, in his Horiz Ewe article in Dec
06 QST asserts that sap flow makes a difference and that if the sap drains
in winter there is less of an effect on signals. This makes some sense,
although I’m not sure why trees with wide branches wouldn’t have similar
effect on a horiz component.
I am copying ARRL Antenna Book editor Dean Straw, N6BV, in case he has any
thoughts,
those of you with a bent for modeling can model the trees by making a
"wire" that is the height of the tree with the right conductivity and
epsilon.
Most of the tables give an epsilon of 2-3 for wood, but that's for dry
wood, not a green tree. Sure, most of the heartwood is dry and only the
cambium is noticeably sap filled, but that makes it like a big
conductive tube. You might use something like 4 or 5 for epsilon and
see if it makes a difference.
Finding a handy reference in the journal "Tree Physiology" (GIMF)
http://treephys.oxfordjournals.org/content/26/9/1105.full.pdf is the
theory (no actual data)
http://treephys.oxfordjournals.org/content/26/9/1113.full.pdf has
buried deep inside a resistivity of 85.3 ohm-cm
Then there's this paper which attempts to use electrical methods to
measure water stress in trees
http://www.agri.gov.il/download/files/ArieNadlerEtAl2008_1.pdf
They give epsilon of 2-6 for stems, and conductivity of somwhere between
15 mS/m for olive trees up to 70-80 for banana and mango
These measurements are all at DC or low AC frequencies and probably not
as relevant for RF, but they'd be a starting point.
NEC doesn't really allow for dielectric constant of wires (at least NEC
2 doesn't.. you could fake it in NEC4 by making an insulated wire), so
looking just at the resistivity/conductivity. the LD card, type 5,
specifies the conductivity of the wire in Siemens/meter, so you could
just plug in the 0.050 and see what happens.
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