[TowerTalk] Tree attenuation

Jim Lux jimlux at earthlink.net
Mon Nov 7 19:49:16 EST 2005

At 04:20 PM 11/7/2005, Frederick M. Mott wrote:
>I operated a MARS station (AB8AH - 1st Bde 1st Inf Div) in Phouc Vinh than
>in Quan Loi, Viet Nam during 1967 and 1968.  There were no trees at both
>locations near our towers (100 ft Rohn 25G and a 70 ft commercial tower)
>except for 1 palm tree (about 20 ft and 1 10 ft banana tree). Our site
>worked very well, we ran approximately 1,100 to 1,200 phone patches a month.
>A sister station, AB8AAB, 2nd Bde of the 1st Inf Div was located in a rubber
>plantation (jungle environment)at Lai Khe.  Their wire beam was suspended
>from the rubber trees.  They were lucky to run 50 phone patches a month.

It is precisely the sort of thing you observed 40 years ago that prompted 
the research of George Hagn at SRI (and others, like Tamir) on propagation 
through or around forests and jungles.  They went out and did a bunch of 
testing, first in Eucalyptus forests around Palo Alto, CA, then in forests 
in Australia, and finally in actual jungle.  This test campaign also 
prompted the development of the Open Wire Line technique for measuring 
ground dielectric properties because their measurements that used the 
measured feedpoint impedance of  (low) dipoles were suspect.  SRI also 
developed the RELEDOP technique to measure 3D far field patterns of HF and 
VHF antennas.

There was some research after that, but really, all the work and analysis 
was done back then, and not much has changed since then, there being no 
commercial need anymore.  These days, HF propagation research foucuses more 
on Over The Horizon radars, typically for observation of ocean surfaces, 
and they manage to put their antennas in places without trees or forests. 
(and a bit of work on spectral monitoring in an OTH sense, to look for "bad 
guys" talking on their HF radios)

Given that lots of tactical radios work in the 30-88 VHF band, and the US 
military emphasis on being able to fight a land war in Europe, there was a 
moderate amount of research on propagation at those frequencies in 
temperate forests, which manifests itself in the papers from the late 70s 
and early 80s.

There's also a huge amount of work in the 800 MHz area in that time period, 
supporting cellular telephone applications.  Lots of guys driving around in 
monitoring vans making precision field strength and multipath measurments

The research these days is all on various microwave frequencies (think in 
terms of cellular, PCS, and WLAN applications), and more often than not, 
for urban environments.  There's also a fair amount of research on this 
(particularly about 10 years ago) to support various satellite to mobile 
applications (like Sirius and XM <grin>) which oddly, the customers expect 
to work when they're driving through the forest as well as through the city.

It's all about money, after all... If you want to do research, you've got 
to do something that will let you eat and pay the rent (there's only a 
limited amount of truly philanthropic money out there) and that means 
having someone with cash interested in the data.  McArthur grant screeners 
may contact me at my email address. I'll be happy to take this on for a few 
years, and half a million dollars would probably be enough to do a decent 
measurement and analysis campaign.

I suspect that if we were ever to be involved in a significant ground war 
in a forested environment, folks would start paying attention to HF and VHF 
propagation in forests again.  For now, though, the existing solutions 
(manpack and vehicular HF and VHF radios with ALE) work "well enough".  You 
too can own the current state of the art for about $20-25K.

>-----Original Message-----
>From: towertalk-bounces at contesting.com
>[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Lux
>Sent: Monday, November 07, 2005 5:13 PM
>To: Jim Brown
>Cc: towertalk at contesting.com

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