[TowerTalk] Is "The Truth about Trees and Antenna Gain" the whole truth?

[jimlux]

On 2/7/18 2:06 PM, Shawn Donley wrote:
> I believe the article certainly is accurate with respect to trends.  The NEC tree models track well with the infinite lossy cylinder closed form solution, which is a good double check.
> 
> 
> But the real world, at least the one around my QTH, doesn't have a single tree to consider but a forest of trees.   One of my wire antennas is a full wave loop in a vertical plane fed from a lower corner with ladder line.   The top and bottom runs are 100 feet and the vertical sections are 40 ft.  The top is about at 65 feet.   Running EZNEC, I saw that I could get a lower takeoff angle on 40M if I fed the middle of one of the vertical sides.  But then the polarization is primarily vertical, and those trees (hardwoods) are about 15 ft away.   I may try it anyway just to see if I can notice any difference.
> 

This is where looking at "forest as dielectric slab" model might help. I 
was just discussing this with Jan WB6VRN at work, where his observation 
was that a large vertical loop in a forest worked terribly, even though 
clear of immediate trees.

There's sort of two things to model in this situation.  The first one is 
"near field loss/detuning" - it would be easy to add a few more trees to 
the QST model (NEC even makes it easy with the GM card).  What I would 
do is look at the loss (total power radiated in all directions before 
and after the trees, and adjusting a matching network).

Then, I'd assume my antenna is a infinitesimal dipole and look at a slab 
model with dielectric boundaries between soil, trees, and air - this is 
more like what HFTA does, or even better, what ionosphere propagation 
models do.
I've not looked at Tamir, et al's or Cavalcante, et al's papers 
recently, but they might have a simple analytical model for this.

Otherwise, it's actually a straight forward ray tracing 
(straightforward, not trivial, or I'd just write it out here). You might 
be able to adapt a program for looking at UHF/VHF ducting, but most of 
those seem only to deal with the change in refractive index, not loss.










> 
> For the Yagi-in-the-trees, I have that situation too.   My Optibeam OB9-5 is at 70 ft but the trees are 80-100 ft high.  I have 20+ feet clearance to the nearest branch, but some of those branches approach a horizontal orientation coming off the trunk.  So like the vertical wire near the tree trunk, I suspect there may be some tree losses.

Add those branches to the QST model - clever use of the GM card to 
rotate, shift, and scale the trunk model is one way.



> 
> 
> This is interesting stuff and not easy at all to model completely.   Another reference for measurements is at
> 
> 
> http://nvlpubs.nist.gov/nistpubs/jres/68D/jresv68Dn8p903_A1b.pdf
> 
> 
> But the measurements were in a jungle environment (most of this stuff was back in the Vietnam days).

Yes, but the propagation model still works, if you plug in the 
dielectric properties of temperate or boreal forest as needed.

Ultimately, in the far field, you model it as a mixed media dielectric 
(i.e. ignore scattering and reradation) -> if you've got 10% by volume 
of something with epsilon 80 and sigma 1 S/m, and the rest is air, it's 
basically epsilon 8 and sigma 0.1 S/m


If someone's interested, I think I have some Matlab code to generate a 
random forest based on some published distributions of tree diameters 
and heights.  I was going to use that to generate lots of lossy wires in 
NEC.


There might also be some actual measurements made in the last 20 years - 
there's a sort of perpetual low level interest in propagation through 
forests and vegetation.  yeah, the Foliage Penetration (FOPEN) radars 
were part of it, but comm is also of interest.

Or rope some grad student into doing it for a Masters thesis