Effect of tree canopies at VHF frequencies...

[Steve Olney - VK2XV]

G'day - I am in the process of positioning an antenna and I am wondering what effect a path of about 5m - 10m through the canopy of a line of trees would have on signals in the upper VHF/lower UHF frequencies (300MHz to 400MHz).    The minimum elevation is about 65 degrees.   The best I have been able to determine is of the order of about 1dB attenuation at 300MHz.

Does anyone have any further information on this ?

Steve VK2XV

[Marcus D. Leech]

Trees with foliage will attenuate signals -- more so above 300Mhz.
Keep in mind that 1dB is roughly the magnitude of signal you'd expect
for a passage of the galactic plane at 400Mhz.




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

[Hans Michlmayr]

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Hi Steve ,

Can't really put a dB figure on it , but in any case there couldn't even
be an approx. figure calculable in advance, depending on foliage type ,
depth , moist.content , etc .
Here I have operated between 350 - 375MHz a few years ago, my
restriction is to the south , affecting neg.declination beyond 70 deg.
The problem is much greater at 1400MHz , however the amount of
attenuation is less of an issue than the random variations in "sky
noise" caused by foliage moving to and fro in the wind !
With a stable and high gain Rx this will probably cause you some grief
even at 400MHz , in particular when operating with a nice, narrow
antenna beam width.....

Regards,

Hans Michlmayr

[Marko Cebokli]

The trees will also increase your noise temperature, especially on 400.
And as Hans said, the worst thing is that it will be time variable on all
kinds of timescales (0.1 sec to 1 year...), making any serious work
impossible.


Now, I have that nice chainsaw, that I don't use much lately, and was thinking
about selling.... :-)

Marko Cebokli

On Tuesday, January 08, 2013 09:11:41 PM Hans Michlmayr wrote:
> On 8/01/2013 5:42 PM, Steve Olney - VK2XV wrote:
> > G'day - I am in the process of positioning an antenna and I am
> > wondering what effect a path of about 5m - 10m through the canopy of a
> > line of trees would have on signals in the upper VHF/lower UHF
> > frequencies (300MHz to 400MHz). The minimum elevation is about 65
> > degrees. The best I have been able to determine is of the order of
> > about 1dB attenuation at 300MHz.
> >
> > Does anyone have any further information on this ?
> >
> > Steve VK2XV
>

[Marcus Leech]

Indeed, it's the variability that's the real killer.  Over unpredictable timescales.

I had a weird one on my emerging C-band system.  The street lights, once again.  These things are RF-pumped fluorescent lamps.  At lower frequencies (well into VHF), the RF induction transmitter produces all kinds of undesirable and unstable harmonics.

At C-band, I noticed that one of the street lights was likely on the edge of my beam, so the feed was "seeing" this hot black-body source suddenly turn on in its field of view.  Nice G.P. transit in progress, then *blam*, the street lights came on :(

[Jan Lustrup]

I have allready cut down all trees on my property last year, so you may have my chainsaw for free!

 

Jan

[Whitham Reeve]

Steve - Here is a link to information on foliage losses at VHF and UHF: http://www.reeve.com/Documents/Articles%20Papers/Reeve_9_Foliage-BuildingLoss.pdf

This is chapter 9 from a book I wrote in 1994, BETRS Radio Engineering Guide. The whole book eventually will be on my website.

Whit

[David Ocame]

I cut down my trees and my neighbors (don't him I was the one that did it).

 

This message was NOT sent from my Blackberry
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*****I'm not dead, yet!************
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Dave Ocame, WS1ETI

Stony Creek Observatory
FN31og
-72.834 longitude
41.272 latitude

Member: The SETILeague, Inc. (Awards Chair),
The Society for Amateur Radio Astronomy (SARA),
The International Trumpet Guild, Shoreline Jazz Collective, and
The Connecticut Symphonic Winds

---

From: Jan Lustrup <j-lu...@online.no>
To: sara...@googlegroups.com
Sent: Tuesday, January 8, 2013 1:26 PM

[Steve Olney - VK2XV]

Thanks to all those that replied.

Chain-sawing is not an option... :-)     Instead I might have to get a 'licence' from the 'boss' to place an antenna on the roof at the front of the house - on the side visible from the road.

I love this block with all the ~100 trees on 0.76 acres, but they can be a handicap for radio.

Steve VK2XV

[Steve Olney - VK2XV]

G'day Whitham - thanks for the link.   Very useful information.

I look forward to seeing the rest of the book (btw - probably not much use to you at this stage, but there is a typo in the first sentence "trees that loose their leaves in the fall").

Your site is very interesting with lots of useful and interesting information.    Thanks for taking the time to put it up.    Wish there were more sites like this.

Steve VK2XV

[Bruce Rout]

Any chance of getting south of the trees?

[Steve Olney - VK2XV]

Bruce - not really.

The target is at DEC -45.    This site is at LAT 34S.   So the elevation angle goes from 65 deg to 79 deg and back to 65 deg over a 4 hour period.    I have a roof with a section at 13 deg slope at the rear of the house which would be perfect for a base except for being blocked overhead by tree canopies at the extremes of the 4 hour period.

A position at the front of the house would require a much more complicated mounting structure - and be visible from the kerb.

Steve VK2XV

[Jan Lustrup]

Getting north of the trees would be better in VK land!
Jan

-----Opprinnelig melding-----
From: Bruce Rout
Sent: Tuesday, January 08, 2013 10:26 PM
To: sara...@googlegroups.com
Subject: Re: [SARA] Re: Effect of tree canopies at VHF frequencies...

[Bruce Rout]

Trade houses with someone south of you?

[Steve - VK2XV]

Nobody south for a couple of miles... :-) We are north of a cliff
over-looking a river plain.

I did a quick Google Earth estimate and about 95% of the 3/4 acre block is
under tree canopy - which leaves (excuse the pun) little area to choose
from...

Steve

----- Original Message -----
From: "Bruce Rout" <bbr...@rasl.ca>
To: <sara...@googlegroups.com>
Sent: Wednesday, January 09, 2013 9:12 AM
Subject: Re: [SARA] Re: Effect of tree canopies at VHF frequencies...

[Bruce Rout]

The trees are on a cliff? Is the cliff running east-west? Is it steep
and high? Is it south of the trees? Can you set antennae into the side
of the cliff?

[Steve - VK2XV]

G'day Bruce,

Imagine a block 30m x 100m with the long dimension N-S. Cliff face at the
southern end. Imagine it with 95% canopy cover.

I have found a spot on the roof of the house (the only place where there is
an absence of trees...) by lying on my back and looking up with gives about
a 3 hour arc unobstructed for DEC -45.

Ideally from a take-off point-of-view an antenna down the south end of the
block would be best. Unfortunately the house is at the north end of the
block (80m away). Maybe one day I could go to the expense of installing an
observatory there - but not at the moment. I would want to get at least
some results before investing.

Cheers.

[Paul Oxley]

Steve   Have you thought rasing the antenna to get above the trees? A hundred feet of tower is cheaper than moving to a different location.   Paul Oxley --- On Tue, 1/8/13, Steve - VK2XV <pul...@vk2xv.net> wrote:

[bbr...@rasl.ca]

Ah yes, but we are starting to think outside the box. Actually, we're thinking on the side of a cliff. Would any neighbors be interested in joining your little enterprise?
Sent from my BlackBerry device on the Rogers Wireless Network

-----Original Message-----
From: "Steve - VK2XV" <pul...@vk2xv.net>

[Steve Olney - VK2XV]

Bruce,

On Wednesday, January 9, 2013 12:10:16 PM UTC+11, bbrout wrote:

Ah yes, but we are starting to think outside the box. Actually, we're thinking on the side of a cliff. Would any neighbors be interested in joining your little enterprise?

Ah - No... :-) 

[Steve Olney - VK2XV]

Thanks to all the responses.   I now have a solution.

I have found a spot on the roof which gives a unobstructed view.    While not as easy as the 'through the canopies' position it eliminates the uncertainty.

I have learnt from your feedback.

Thanks.

Cheers

Steve