[Lf] Different types of antennae]

[Andre' Kesteloot]

Pierre Thomson wrote:

> Hi Jim,
>
> While your logic is good in the case of a quarter wave vertical (with
> 1/100 wavelength metal trees around it), the picture is quite different
> when the radiator is an "infinitesimal monopole" such as the average
> Lowfer installation.  Here the antenna and ground system act as a
> big capacitor, and the bit of radiation that does get out results from
> the current on the vertical conductor between the plates.  Any poor
> dielectric (and moist trees are pretty bad) close to the capacitor
> will degrade the Q to some extent.  This is why trees closer than
> about 100 feet are bad.  And resistive trees are actually WORSE
> than metal in this respect.
>
> Another factor is that the E-field around such an antenna is primarily
> vertically polarized, and any vertical conductors nearby will absorb
> a portion of the energy.  This also applies to reception; you can notice
> the effect with your car radio tuned to a distant AM station.  It fades
> noticeably when you drive through dense forest, as compared to open
> fields.
>
> Based on a few years of LF work and a few decades of HF, I would
> contend that trees have more effect on Lowfer transmitting antennas
> than, say, on horizontal dipoles.  The latter can be strung through a
> tree with great results; a Lowfer antenna suspended in a tree is
> doomed from the start.
>
> Just my observations...
>
> 73 Pierre Thomson KA2QPG / RI
>
> > -----Original Message-----
> > From: WB9UWA Jim Shaffer [SMTP:wb9uwa at gte.net]
> > Sent: Sunday, May 21, 2000 7:59 PM
> > To:   Pierre Thomson
> > Subject:      Re: [Lowfer] Different types of antennae
> >
> > Has anyone modeled trees on antenna software on these frequencies?
> > It would seem to me that if you model the trees in a worst case
> > conditions (no resistance) then you have metal poles near by that
> > are a very very very very very small fraction of a quarter wavelength
> > high working against ground. These near by metal poles as such
> > should have very little effect on an antenna that would be over a 1000
> > foot tall if it were a quarter wave length. When these metal trees are
> > modeled, the effect should be to modify the pattern of the LF
> > antenna. Of course a real tree might absorb the signal rather
> > then reflect it.  Hams have been stringing wire antennas in and around
> > trees for years with good results. Certainly the effect on the signal is
> > going to be greater on HF then on LF.
> >
> > It would be my guess that this study shows more the effect of signal
> > blockage by land then by trees although I have not seen the study first
> > hand.
>         ...
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