[TowerTalk] More on 75/80M Antennas

[Chuck Counselman]

Dennis wrote:
>I noticed you said you ran your vertical up the trunk of
>a tree.  How did it work?? what was the input "z"
>like.  Sounds to good to be true to me .  :o)


It worked like every other vertical: poorly in comparison with a 
horizontal antenna at the same height, except for local-area 
ground-wave propagation.

I researched the topic of tree conductivity a few years ago, and 
received much useful info from subscribers of the PROP-LIST (for 
info, send an email to <propagation-list-request at gweep.bc.ca> with 
just the word 'help' in the message body).  One of them, I believe 
George Hagn, had measured the conductance of living tree trunks, and 
had even used them as vertical antennas -- _without_ running wires up 
their trunks.  No, I'm not joking.

He did this with very large, very tall trees in Southeast Asia -- 
Thailand, IIRC.  Depending on where you live, YMMV.

He used two methods of matching 50-ohm coax to a tree trunk.  The 
simpler method was to drive large nails into the trunk, one nail at 
ground level and another nail vertically above it by some distance. 
The coax is connected directly to the nails.  The load impedance that 
you see increases with increasing distance between the nails.  I 
guess this is a form of shunt feeding, or beta-matching.  I imagine 
that you'd have to cancel the inductive part of the load impedance 
with a series capacitor, as you do when you feed a metal tower.

The other method was nicer because it was non-invasive.  He 
constructed a very large toroidal coil of wire, like the winding on a 
ferrite toroid, except with no ferrite (just air), and much larger. 
I believe he constructed a skeletal coil form of lightweight wood or 
bamboo.  The axis of rotation of this toroid was vertical and 
coincided with the axis of the tree trunk.  In other words the toroid 
formed a ring around the tree.

The toroidal wire winding was the primary of a step-down transformer. 
The tree trunk was the secondary.  I imagine that this transformer 
would have a large leakage inductance that you'd have to tune out 
with a series capacitor.

If you have NEC-4 you could simulate this system pretty well, so you 
could get it to work and to present a 50-ohm resistive load to your 
feedline relatively easily without much cut-and-try.

Although I have NEC-4 I have not simulated this system, so I can't 
supply design data and I can't tell you how lossy the antenna would 
be.  I did simulate the 55-foot White Pine trees that support the 
ends of my present station antenna, which is a wire doublet, mostly 
horizontal but with 5 m hanging vertically at each end.  I wanted to 
know how how much power would be dissipated in the trees by currents 
induced in them by the nearby wires, especially by the vertical 
wires, which carry substantial currents when the antenna is excited 
on the high HF bands.  I forget the answer, but I recall that I 
decided that just a few feet of horizontal separation of the vertical 
wire from the tree trunk was enough to reduce the dissipation to a 
negligible level.

If you can get a wire high enough in the tree, then the wire will 
work better as an antenna than the tree by itself; and the antenna 
will work better if you can get the wire horizontal and feed it at 
its center.  But if you have a healthy 150-ft. tree in your yard and 
absolutely no way of reaching even its lowest branches, then you 
might try exciting it as a top-loaded vertical for 160 meters.  :-)

73 -Chuck, W1HIS