Paul,
Interesting test. I am assuming you have some professional antenna
range which is what lots of us here would like to hear from on antennas.
Many of us have used the 150-160 DB224 and 450-470 folded dipoles int he
Ham bands. Same with the fiberglass. About all we can do is measure
the SWR, but always wonder about the gain and pattern differences.
Have you measured these antennas for Ham use. Often getting these used
is easy due to commercial guys getting rid of them while in mint condx.
73, ron, n9ee/r
Ron Wright, N9EE
727-376-6575
MICRO COMPUTER CONCEPTS
Owner 146.64 repeater Tampa Bay, FL
No tone, all are welcome.
On Sat, Jun 7, 2008 at 11:13 PM, Paul Plack wrote:
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 <mailto:[EMAIL PROTECTED]> <mailto:[EMAIL PROTECTED]>
To: Repeater-Builder@ yahoogroups. com
<mailto:[email protected]>
<mailto:[email protected]>
Sent: Saturday, June 07, 2008 7:14 PM
<mailto:[email protected]>
Subject: Re: [Repeater-Builder] Re: antenna suggestions for 440mhz
<mailto:[email protected]>
<mailto:[email protected]>
<mailto:[email protected]>
Paul, <mailto:[email protected]>
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.
<mailto:[email protected]>
<mailto:[email protected]>
73 Allan Crites WA9ZZU
Paul Plack < [EMAIL PROTECTED] net <mailto:[EMAIL PROTECTED]> > wrote:
<mailto:[EMAIL PROTECTED]>
"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
