[Lowfer] Active Whips

[Douglas D. Williams]

Thank you, John, for that very good explanation.

73, Doug KB4OER

On Thu, Jan 24, 2013 at 4:18 PM, JD <listread at lwca.org> wrote:

> >>> I wonder why some "active antenna" manufacturers caution users that
> long
> >>> support poles are not necessary? Could it be that they are concerned
> >>> that too much antenna length ... could overload the amplifier and cause
> >>> IMD?
>
> That's exactly right, Doug.  My 40 foot "active whip" would be a terrible
> performer for that very reason if I were to try to use it for all-band
> receiving.  It's only because I roll off response below 9 kHz and above
> about 530 kHz in the front end that I am able to keep the signal levels
> within the amplifier's capabilities without taking extraordinary measures.
>
> >>> So, when we raise the height of our "active whip" ... it isn't
> >>> necessarily because the "whip" part of the antenna is higher, but
> >>> because  ...  we have effectively increased the length of the antenna
> by
> >>> using a longer support pole?
>
> That's exactly correct.  In the case of an antenna in the open, it's not
> hard to visualize.
>
> Even where there is shielding from proximity to trees, the amount of the
> whip (and support) entending above the trees is acting virtually the same
> as
> a monopole extending above the treeline by the same amount.  The location
> of
> the feedpoint doesn't matter much.  The tree "shields" the lower part of
> both antennas, but by the same mechanism in both cases...namely, its (the
> tree's) own local "near field" environment.
>
> Zack and Todd are quite right that the tree effect is real, but the
> explanation in the AMRAD whip article is not adequate and can be
> misleading.
> It furthers the misimpression that an "e-probe" is a totally electrostatic
> device, which it's not...you cannot just disassociate the electric and
> magnetic fields of a propagating EM wave.  You can, however, distort the
> impedance (the ratio of E and H) of some particular region of space that
> the
> wave is passing through by causing it to interact with an electrically
> conductive object...like a tree.
>
> The imaginary sheet the article discussed is not a very accurate
> representation of the region of space affected by semi-conducting objects,
> and the electric component of an EM wave does not simply drop to zero below
> that sheet.  There is interaction between the wave's own components and
> whatever near fields the wave has induced within all the various more or
> less conducting objects.  An imaginary inverted cone having a radius equal
> to the height of each object, centered on the object, would be a much
> better
> way of visualizing the impacted region.  Get some of your antenna up out of
> that cone, even just the top several feet, and you'll snare more signal.
> Move the base of the antenna out of the cone entirely, and you'll get even
> more.
>
> The simplest analogy may give us a quick, basic understanding of something
> we see, but it can also get in the way of deeper understanding, as I know
> all too well.  The better the model we can imagine for a given observed
> phenomenon, the more accurately we'll be able to use it, and the more
> likely
> we'll be to predict new and better applications for it.
>
> John
> ______________________________________________________________
> Lowfer mailing list
> Home: http://mailman.qth.net/mailman/listinfo/lowfer
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Lowfer at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>