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Author Topic: attenuation from trees  (Read 319 times)
K6CA
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Posts: 12




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« on: February 02, 2009, 12:36:48 PM »

The best location that I have for a tall vertical,  one of the new 43'  monobanders, is in a macadamia nut tree grove. My antenna would have a clear space about 7' in radius between trees. The grove avg height is 20'. So I'd be in the clear for 23' above the tops. Plenty of room for a proper radial system. And fairly invisable, but what are the losses?
tnx & 73,
K6CA
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N3OX
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« Reply #1 on: February 02, 2009, 12:42:06 PM »

"And fairly invisable, but what are the losses? "

No one really knows, but they're generally accepted to be small.  Depends on too many factors to hang any decent numbers on it though.

What's a 43' monobander?  Do you mean the 43' "multiband" antennas?  If you do and don't plan a tuner at the base, feedline losses will likely swamp anything the trees do ;-)

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N5LRZ
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Posts: 0




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« Reply #2 on: February 02, 2009, 02:44:13 PM »

Per the ARRL Handbook it takes 6 dbs of loss to see a mere one S unit decrease.  That is a 400% decrease of base amount of power.

Any loss to the trees is so minimal as to be not even worth considering.
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KC8VWM
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« Reply #3 on: February 02, 2009, 10:57:46 PM »

The losses from the proximity of the tree's are not the primary consideration. Ensuring the antenna is efficient, has low loss feedline and has enough ground radials would be the primary consideration.

73 de Charles - KC8VWM
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KE7YYB
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Posts: 41




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« Reply #4 on: February 03, 2009, 06:46:48 AM »

How about dipole 1/2 wavelength above ground?
Just wonder if it affect the performance (gain, pattern, take off angle etc) much?
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KB1NLW
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Posts: 69




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« Reply #5 on: February 03, 2009, 07:39:32 AM »

I asked the same question in septembeer (you may want to search on my call).  I got some useful references in the responses, the lossses are significant at 10M:

I found a presentation on the link provided by K8GU,

http://lists.contesting.com/archives//html/Towertalk/2005-11/

by Wolfhard Vogal, University of Texas showing
Average short path multi-tree attenuation at
– VHF (100 MHz) is 0.03 to 0.15 dB/m

So now I can figure out the approximate distance through the trees from my vertical for a take off angle of 7 degrees to be 162m (20m trees) It appears from his charts (extrapolating)that at 28 Mhz attenuation may be reduced by half. Thus the attenuation is in the range 2.5 dB to 12dB.

This may account for the relatively poor performance of my ground based vertical versus a G5RV mounted high in the trees or my vertical yagi.
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KT8K
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Posts: 1490




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« Reply #6 on: February 03, 2009, 11:36:53 AM »

I have mounted aluminum HF antennas in trees on more than one occasion (and have one now).  I noticed no loss except when the aluminum was in direct contact with a bunch of leaves.  Keep them a few feet away from the vertical and you won't notice the difference, I'd bet.
Best rx & 73 de kt8k - Tim
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N3OX
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« Reply #7 on: February 03, 2009, 12:11:39 PM »

"I noticed no loss except when the aluminum was in direct contact with a bunch of leaves"

What was  your control antenna ?

I think it'd be pretty easy to lose 75%/6dB of your power into tree losses without it being obvious.

I wish we had more good data available on this topic, because I think nearfield and propagation losses could be occasionally severe, and without setting up an equivalent installation well clear of the trees, no one will ever really know.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K9IUQ
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Posts: 1626




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« Reply #8 on: February 03, 2009, 12:24:02 PM »

My friend DAN says:>>" I wish we had more good data available on this topic, because I think nearfield and propagation losses could be occasionally severe, and without setting up an equivalent installation well clear of the trees, no one will ever really know."

There has been studies on this very subject. The best I have seen is the Book "HF Antennas for all Locations" by G6XN - L.A. Moxon. Next to "Reflections" this is my ALL time favorite Antenna Book.

There is a complete chapter on "The Antenna and its Environment". This chapter discusses the effect of trees, sloping ground, overhead powerlines, buildings, etc on the performance of antennas.

This book is a MUST read for any Ham interested in this subject...

Stan K9IUQ
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W5WSS
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Posts: 1651




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« Reply #9 on: February 03, 2009, 12:27:35 PM »

You can remove the foliage losses from the eqation by elevating the monopole base above the tree line and incorporating 4 tuned elevated radials. The vertical will push against the radials and the radials vertical contribution that adds to the feild strength having never passed thru the foliage, clutter in the first place. No near feild losses will attenuate the feild strength of the system when designed this way. The far feild effects of clutter occur to an unknown measure. Know that at 10m and up it can be a limiting factor best understood when compared to a comparable comparator...without which your skywave success is arbitrary. regards
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N3OX
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« Reply #10 on: February 03, 2009, 02:39:53 PM »

Stan,

Good to know...

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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