The parasitic element that reflects back toward the driven element is called
the reflector. I think the name origin is obvious. If it were shorter than
the driven element it would not exhibit this characteristic. It is usually
placed 1/4 wavelength from the driven element so that the magnetic field set
up in the parasitic element is 180 degrees out of phase. This induces a
voltage in the driven element that is in phase with and increases the
forward radiation.
I witnessed a demonstration of antenna radiation patterns using a
transmitter/receiver in the 3-4GHz range. The antenna samples were made
with 22 gauge copper wire and were quite small. All of the 10 samples
together could be held in one hand. A simple Yagi with one reflector had
about 85 percent of the gain of another with one reflector and 4 directors.
There was also another sample that used two reflectors, and it did not show
a noticeable gain over the single. I think this is because there is little
RF to work with behind the first reflector.
The reflections caused by the ground and ionosphere are radically different
from antenna elements. The ground absorbs horizontally polarized waves.
That is why Yagi antennas are placed as high as possible, and vertical
antennas are so popular for the lower bands. The ionosphere is unstable and
usually causes the incident wave to be rotated to a different polarization.
Hope this helps.
Keith WD5DXL
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Al Williams
Sent: Wednesday, June 30, 2004 10:42 AM
To: towertalk@contesting.com; W0UN -- John Brosnahan
Subject: Re: [TowerTalk] Funniest thing I've seen in weeks
>----- Original Message -----
>From: "W0UN -- John Brosnahan" <shr@swtexas.net>
snip
>What struck me as funny is the title of the symbol:
>"Folded dipole aerial with 3 executives and 1 reflector sim. VDE"
>Guess something got lost in the translation and 3 directors became
>3 executives.
Why is the longer element called a reflector while the shorter element
called a director> What is the origin of the naming?
Basically, don't they both re-radiate because of the same reason?
What is the relationship, if any, between the Yagi reflector and ionosphere
and ground reflections?
k7puc
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
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