Topband: Tree losses....

[Bob Kupps]

Very nice study and IMO neither small nor inelegant. Thanks


From: Roger Parsons <ve3zi at yahoo.com>
To: Topband <topband at contesting.com> 
Sent: Friday, August 9, 2013 7:29 AM
Subject: Re: Topband: Tree losses....


I just did a small and inelegant piece of modelling with EZNEC.

I took a wire 128' vertical, and it showed a gain of about 1.7dBi over a particular ground. 


Keeping everything else the same, I introduced a 'tree' 3ft away from it, with no branches, exactly parallel, also 128' high and initially with zero resistance. This changed the gain to 2.0 dBi with a 0.7 dB front to back ratio.

I then introduced series resistances at 20 equally spaced points in the 'tree', and looked at the effect of varying these.

With 1R resistances (20R total) gain was about 0.6dBi and 0.6dB f/b.
With 2R resistances (40R total) gain was about 0dBi and 0.5 dB f/b.
With 3R resistances (60R total) gain was about -0.1dBi and 0.3 dB f/b (the minimum gain modelled)


and so on, until with 10R resistances (200R total) gain was about 0.7dBi and 0.1 dB f/b

and so on again, until with 100R resistances (2000R total) gain was back to 1.7dBi and 0dB f/b.

Of course this is highly unrealistic in many respects, but I would be amazed if any 128' high tree under any conditions of sap would have a total end to end resistance of only 2000R.  And bear in mind that this is a self resonant tree selected to couple very strongly indeed to the main radiator.

I then repeated the process with a non-resonant tree only 64' in height. No value of series resistances produced even 0.01 dB change in gain. (Of course at this point the wire vertical was being supported by an invisible sky hook.) 

I do believe that trees can affect things in at least two ways - as others have said, high voltage points adjacent to foliages can definitely cause losses - and these are very hard to quantify. My own past experience with tree supported inverted L and T antennas has been that quite small changes in the position of the element can cause big changes in feed impedances - but that is not quite the same thing at all.

73 Roger
VE3ZI
_________________
Topband Reflector