Allan,
I question the relevance, but here goes. I just modeled an ordinary half-wave
dipole in free space in EZNEC. 20 MHz low at 450 MHz is about 4.5%.
At 4.5% above design frequency, the difference in the pattern of the single
dipole is negligible, and the gain rises 0.04 dB.
At 4.5% below design frequency, the difference in the pattern of the single
dipole is negligible, and the gain drops 0.04 dB.
For entertainment's sake, I modeled it at 100% above design frequency.
Impedance is 1754 ohms, for an SWR of 44.9:1, but assuming you could match it
without loss, you'd enjoy 1.79 dB gain at the horizon, slightly elongating the
major lobes in a polar plot.
Is it your position that combining a bunch of dipoles in a colinear phased
array does not change their behavior compared to a single dipole? If that's
true, we're all been wasting lots of money.
By the way, my recent modeling experience has been almost exclusively with
half-wave dipoles, fed in-phase, spaced a half-wave apart, for applications
involving single-site low-band repeaters using separate antennas to achieve
isolation through vertical separation. In this application, the null in the
vertical axis is much more important than the beamwidth at the horizon.
I acknowledge that the available bandwidth before the pattern decays may be
different in the commercial antennas being discussed. If someone can tell me
the spacing and phasing of the elements in the popular folded-dipole arrays,
I'll try modeling them at some point, and see how they behave differently from
my application.
I've also played a little with antennas spaced at 3/8- and 5/8-wave, with
phasing leading or lagging by 45 degrees, and some very interesting "fill"
patterns can be created.
73,
Paul, AE4KR
----- Original Message -----
From: allan crites
To: Repeater-Builder@yahoogroups.com
Sent: Saturday, June 07, 2008 7:14 PM
Subject: Re: [Repeater-Builder] Re: antenna suggestions for 440mhz
Paul,
Perhaps you can now explain how the radiation pattern changes on a single
center fed, 1/2 wave length simple dipole when the frequency is changed both
above and below the dipole resonant frequency, and how that relates to the
statements you have made below.
73 Allan Crites WA9ZZU
Paul Plack <[EMAIL PROTECTED]> wrote:
"No, parallel-fed antennas do NOT suffer uptilt/downtilt as frequency is
varied unless the harness was special-ordered for factory downtilt. If the
antenna wasn't ordered with downtilt, all of the elements are fed in phase, and
they will always be in phase regardless of frequency."
Jeff, the pattern depends on both phasing and spacing. As frequency drops,
the interelement phasing, expressed in degrees, remains the same, but the
spacing, expressed in degrees or wavelengths, drops. If you model a colinear
array of parallel-fed dipoles in an antenna software program, and don't scale
the dimensions as you scale the frequency, you'll see the main lobe shift up or
down, and "butterfly" lobes appear, as you get a few per cent off-frequency.
In an extreme case, a pair of vertical colinear dipoles fed in phase with
half-wave spacing has the familiar big lobe toward the horizon. As frequency
rises, the pattern degrades until, at a frequency of 2X, it becomes an end-fire
array, with most energy directed straight up and down. This happens with no
change in phasing or spacing.
73,
Paul, AE4KR
