[Lowfer] Active Whips

[JD]

>>> 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