RE: NEC-LIST:loaded dipoles

Another way to broadband a beam antenna, such as a yagi, is to couple in a
parasitic element somewhat shorter than the driven element. The parasitic
element is inductive at one end of the frequency range, and capacitive at
the other end of the range. Sometimes these are referred to as "exciter"
elements?

Spacing between the driven dipole and parasitic element at 45-70 MHz, for
example, is around 3". This is the technique used in the TV industry by
antenna manufacturers such as Antiference in the UK. NEC2d models of these
antennas show remarkable gain/SWR bandwidth, maintenance of the polar
diagram, with good gain (up to 10 dBi) and a back-to-front ratio of around
10 dB for a commercial Band 1 TV yagi.

A simple dipole with coupled parasitic element, also spaced 3", will cover
45-68 MHz with SWR around 2.0.

This technique has the benefit of not warming up loading resistors (when
transmitting).
Cheers, Bob.

Ian Roberts ZS6BTE

-----Original Message-----
From: Robert Haviland [mailto:bobh_at_iag.net]
Sent: 03 March 2002 11:37
To: nec-list_at_ee.ubc.ca
Subject: NEC-LIST:loaded dipoles

A way to "broadband" an antenna is to shunt the feed point with a resistor,
say 100 ohms for a dipole. The impedance presented doesnt stray too much
over
a wide range of frequencies. Of course the coupling efficiency to space does
change, but this is not important if the ambient noise is greater than
system
noise.
Such a "black box" was offered commercially a number of years ago.
bob w4mb

-- 
The NEC-List mailing list <nec-list_at_gweep.ca>
http://www.gweep.ca/mailman/listinfo.cgi/nec-list