[BC] RF absorption by foliage

[DHultsman5@aol.com]

There will be alot of opinions on this.  Florida has usually a very  low AM 
conductivity except for Salt Water between NW FL and the west coast of  FL.  
Due to the high sand content I prseume.  I don't recall exactly  what the M3 
shows but probably 3 or less.
 
In the Pensacola area,  I was involved with a station old Class  IV  1 
kW./0.250kW. nite, non-directional.  The manager and sales  manager had all been 
there 10-15 years and claimed that the signal was down from  where it used to be. 
 When I first arrived at the station,  I found  that the tower area was all 
grown up with trees in clumps some as high as 40  feet  most being sycamore and 
willows.  In checking around the  residential neighborhood,  some of the 
adjacent land where the ground  radials were buried was sold off by a previous 
owner and a church built on  the properity.  Also there was an undedicated dirt 
road on the north  perimeter of the site which some time in the past the city 
got the properity and  installed a paved road with no concern or about 65 feet 
ofradials.
 
After using a field set and looking for radials I could not any past about  
40 feet from the tower and could only see about 15.  After 30 years or  more I 
recommended a new ground screen and radials.  We could not get full  lenth 
quarter wave on radials due to the land restrictions.
 
First,  I had the manager arrange for all the trees to be cut down  and 
stumps removed, carefully around the tower.
 
I made some field strength measurments out 8 miles from the  array on 8 
radials.  I was unable to make 8 miles in several  directions due to the Gulf and 
coastal bays.   I marked the  measurmeents on maps and plotted the radials.  
Conductivity was plotted at  between one and 2.   Plan was to make additional 
measurments after the  installation of the new ground system.
 
Unfortunately the manager got the trees and vines all cut before I made the  
measurments. When I arrived to make the measurments they all were elated at 
how  much better the 250 Watts at night was doing on the local channel after the 
 removal of the trees.  
 
We proceeded on the ground system replacement on the unipole  radiator,  I 
installed a 24 by 24 ground screens with perimeter 4 inch  ground strap since 
all radials were too short as the city and church would allow  us to go on their 
properity I did 240 radials as far as I could to the perimeter  of the 
properity and to our building.
 
After completing the ground installation I made measurments at the same  
points and found that the effect of the new ground system made a change of  
approximately 10% in the measured field based on the averaging of the  measurments.
 
It has been my experience that near field absorption of the ground wave  
signal by trees, bushes and high vegetation always makes a difference in the  
radiated signal.  I also have seen comments from other consultants about  keeping 
the area of the towers cleared to the perimeter of the radials.
 
In another case in central Alabama,  I helped a guy get his old  transmitter 
on the air again after he repossessed the station.  In 10 years  the 
vegetation had grown up to the 30-40 ft height.  He even had a tree  limb going thru 
the tower.  Since much of it was pine he got a pulpwood guy  to clear it free 
for the pulpwood.  He did not put in a new ground system  but cleared back about 
200 feet from the tower base.  He was supprised how  good his 20V-2 sounded 
with new tubes, a new Modul-Limiter and no near field  attenuation.  He claimed 
the signal was better than when he sold the  station.
 
I believe Palmer Green wrote a paper on gorund systems that talked about  
absorption in the near field.
 
Dave Hultsman