NEC-LIST: The HEMAC drive for a tree-trunk antenna

Hello,

The recent exchange conerning using a tree as a vertical antenna (see
below) is very interesting. It reminds me (probably related to George
Hagn's experience) of some work reported by a person (whose name I
unfortunately can't remember) at Ft. Monmouth, NJ, also in the 1960s.
The "tree antenna" drive was described as a "HEMAC," basically a coil
loosely wound around the tree trunk. This method of drive apparently
worked, although I don't recall any further details. I imagine that
George might have more information about it. One interesting aspect
of this kind of drive is that possibly it might be used, although what
the input impedance would be is another matter, for other vertical
structures, maybe a lamp post for example, as a temporary, expedient
for a field antenna.

> 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

----
Dr. Edmund K. Miller
3225 Calle Celestial
Santa Fe, NM 87501-9613
505-820-7371 (Voice & FAX)
ekmiller_at_prodigy.net