Back in the 70's, I did a project comparing the optical cross section a lot
of different growing things - mostly trees - to their radar cross section
at various frequencies, UHF and up. You can probably imagine why. Spent a
lot of time in a spooky, warehouse-sized anechoic chamber, I did. With the
passage of time, I now remember that as the good old days...
73,
geo - n4ua
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On Thu, Feb 1, 2018 at 3:18 PM, jimlux <jimlux@earthlink.net> wrote:
> On 2/1/18 10:25 AM, Jim Brown wrote:
> There's a piece in QST this month about trees absorbing RF
>
>> from VERTICAL antennas. They have much less effect on horizontal
>> antennas. I suggest you take a look at my applications note on Antenna
>> Planning.
>>
>>
>
> The modeling in QST (which was a nice article) only modeled the trunk of
> the tree, so it wouldn't find any H-pol effect.
>
> You'd really need to model the branches on the tree as well.
>
>
> A nice "further work for the student" would be to take the QST model and
> add horizontal branches, then run it with some horizontal dipoles. They've
> already done the hard work of figuring out the dielectric properties of the
> tree wood, and the equations for 4nec2 to convert that to RLC loads.
>
>
> There is a reference in the article to Tamir's 1977 paper, but that's more
> of "modeling the forest as a dielectric slab" for propagation, not the near
> field effects. Cavalcante's 1982 and 1983 papers extend on Tamir's work,
> but still ignore the near field effects (well, he describes it, gives the
> equation, and then blithely goes on with: "Since the interest here is the
> calculation of the fields in the far zone, the Hankel function which
> appears in (16) may be replaced by its asymptotic expression.In addition,
> one over the expression that appears in the denominator of (18) may be
> expressed in an infinite series form (binomial expansion)."
>
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