Re: NEC-LIST: Modeling a living White Pine tree at HF

Chuck:

I suggest that you model it as a grounded vertical mast with tapered
diameter. I can send you some data on epsilon r and sigma, but have to
dig it out.

I successfully used a tree as a grounded vertical mast to send HF
traffic over a 300 km path in thailand in the 1960s. I fed it as a
shunt fed mast, like half of a delta feed with the "image" providing
the other half of the feed. I drove nails at 6-inch intervals and
measured the input impedance till I found the "50-ohm nail." I then
loaded a KWM2A Collins transceiver into it and sent my traffic. I did
a 16-ft grounded vertical monopole for a comparison test and was about
15 dB down with the tree (cant recall the species).

For now, you could use sigma of about 10-6 S/m for the tree
conductivity, which derives from the cambium layer just under the
bark. Try epsilon r of about 40 for the active layer (sap). You can do
parametric sensitivity on the electrical parameters, so vary sigma
from 10-7 to 10-4, and epsilon r from about 20 to 50.

You can measure these parameters indirectly (and check your model) by
using a feed like I did and computing the input impedance as a
function of frequency and then measuring it. You can vary the
parameters till you get a best fit. If you dont want to punch a hole
in the tree for a feed, use the tree as a parasitic element and do
your measuring on a "driven" element that is a vertical that is
grounded but in proximity to the tree trunk, analgous to a two-element
Yagi-Uda array.

Hope this helps. Let me know how you make out.

Regards,

George Hagn
Engineering Consultant
4208 Sleepy Hollow Road
Annandale, VA 22003-2046
(703) 941-7663 voice/fax
 

Original Message:
-----------------
From: Chuck Counselman ccc_at_space.mit.edu
Date: Sun, 12 Nov 2000 13:38:54 -0500
Subject: NEC-LIST: Modeling a living White Pine tree at HF

How should a living White Pine tree be modeled at HF?

Can anyone tell me the bulk conductivity (sigma) and permittivity
ratio (epsilon/epsilonsubzero) of a living tree trunk, at HF?

Or suggest where I can look it up?

Thank you. -Chuck
Received on Fri Nov 17 2000 - 05:32:57 EST