A Broadband Feed System for an 80 Meter Antenna System

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A Broadband Feed System for an 80 Meter Antenna
System

• Jim George N3BB

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This presentation describes a system of three
half-wave sloping dipoles which allow CW and SSB
operation with no additional tuning. RTTY
operation is not guaranteed. ? The system is
based on a concept by W6NL and a technical paper
by W4RNL described on the Internet at
http//www.cebik.com/trans/wb.html The system
was designed and constructed at N3BB in January
and February, 2006. While this presentation
describes the three-sloper system, it is valid
for a single half wave dipole antenna.
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Broad Objectives

• Allow operation on both 75 and 80 meters w/out an
  antenna tuner or changing antenna length
• Use in both domestic and DX contests
• DX Contests obtain gain to Europe,
  Caribbean/South America, or JA/Asia with
  selection of a single sloper
• Domestic Contests Combine the NE and NW slopers
  to obtain a footprint to both coasts
• Be able to use any combination of the three
  slopers

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W4RNL-W6NL Broadband Feed SystemFrom the W4RNL
Technical Paper
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Double Dip SWR Curve(Dipole in Free Space)From
the W4RNL Technical Paper
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Practical Tips on Cutting Coax to Proper length

• The Velocity Factor (VF) published is not exact.
  It typically is within 5-10
• Cut the coax 5 to 10 longer than the stated
  length for the VF and the design frequency
• Using a noise bridge, grid dip meter, or MFJ
  259B, set the frequency such that the coax length
  is a half wave length
• Apply a PL259 male at one end, and short the
  other end of the coax length to be pruned
• Tune the input frequency for a minimum resistance
  reading (zero)
• The frequency should be lower than the desired
  frequency as the length is too long
• Cut some off the coax, and re-short the open end
• Repeat until the frequency is equal to the design
  frequency

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Distinctive Double Dip SWR CurveFor a Single NE
Sloping Dipole(N3BB Data)
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NE Sloper selected as one of three sloping dipole
antennasN3BB Data
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All Three Slopers Selected Together
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SWR Comparison All Major Combinations
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How It Works For those familiar with Smith
charts, the wide-band matching system shows
itself in the tracking of SWR curves. However, we
can also develop an appreciation for what is
happening without the chart by thinking more
directly about what goes on along a length of
transmission line--or several lengths in
succession. The easiest way to get a good grasp
on how the wide-band matching system works is to
look at the following table of figures. Remember
that the antenna is self resonant at 3.75 MHz. At
3.65 MHz, the antenna is short and the feedpoint
impedance is capacitively reactive. At 3.85 MHz,
the antenna is long and the feedpoint impedance
is inductively reactive. The 50-Ohm coax run
(using a cable with a VF of 0.765) is 1 wl long
at the same 3.75 MHz frequency. A full wl returns
exactly the same impedance as at the antenna
terminals. However, at 3.65 MHz, the coax run is
short of the 1 wl mark. The continuous impedance
transformation has not completed a full cycle.
The value it returns at its length (0.97 wl) is
higher resistively than the antenna feedpoint
impedance at this frequency and less capacitively
reactive. At 3.85 MHz, the coax run is longer
than 1 wl (1.03 wl) and has begun a new cycle of
transformation. It reaches a resistive value
higher than the feedpoint value, but is less
inductively reactive. These are the values going
into the so-called 75-Ohm matching section.
Frequency 3.65 MHz 3.75 MHz 3.85 MHz Dipole Feed
Impedance 85.2 - j45.9 91.6 - j0.5 98.1 j44.5
50-Ohm SWR 2.36 1.83 2.47 50-Ohm Coax (200.6') lt
1 wl exactly 1 wl gt 1 wl Impedance at coax end
110.1 - j26.7 91.6 - j0.5 121.3 j15.1 75-Ohm
Match (44.3') lt 1/4 wl exactly 1/4 wl gt 1/4 wl
Impedance at match end 47.7 j 9.8 61.4 _ j0.3
45.4 - j 3.7 50-Ohm SWR 1.23 1.23 1.13 0.25 wl
matching sections transform resistive impedances
according to the following simple formula Zo
SQRT (Zin Zout) or Zout Zo2 / Zin where Zo
is the characteristic impedance of the matching
section transmission line, Zin is the impedance
on the antenna side of the section and Zout is
the impedance on the station side of the section.
The length of the matching section is precise for
3.75 MHz and for the 0.66 VF 75-Ohm cable used.
The reactance at 3.75 MHz is too small to make a
difference, so we can plug 75 Ohms and 91.6 Ohms
into the second version of the equation and get a
Zout of 61.4, just what our model shows. At
3.65, the matching section is shorter than 1/4
wl. Moreover, we have significant reactance going
into the section. However, even if we ignore
these deviations, we get a simplified impedance
for Zout of 51.1 Ohms, only 3.4 Ohms off the
model. Likewise, the section is longer than 1/4
wl at 3.85 MHz, and Zin has reactance. Still, the
simplified equation predicts a Zout of 46.4 Ohms,
only 1 Ohm off the modeled mark. These
convenient transformations of impedance do not go
on indefinitely on either side of resonance. The
band edges of 80-75 are a little over 6.5
removed from the center frequency, and already
the combination of impedance transformations in
both the 50-Ohm and the 75-Ohm sections yield
impedances at the matching section end that
produce high SWRs.
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Three Slopers on Hilltop
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Positioning of Coax and Stackmatch Inside the 45G
Tower
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80 Meter Stackmatch Control Box in N3BB
Shack(Note Use of Labels Made with a Label
Machine)
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N3BB SO2R Operating Position Details

• Top-Ten Systems Automatic Antenna Switching
  System
• Dunestar Six-Band Band-Pass Filters
  Automatically Selected
• Top-Ten Systems DX-Doubler SO2R Controller
• Top-Bottom manual Rotator Controls
• 40 Meter Yagis Selection (Two)-Stackmatch Control
  Box
• 80 meter Slopers Selection (Three)-Stackmatch
  Control Box
• LCD Monitor (to Reduce RF Birdies)
• Dedicated DOS PC (66 MHz 486DX)
• FT1K-MP Radios with Full INRAD Filters
  (250/400CW, 1.8/2.4 SSB)
• Alpha 87A and Alpha 76PA Amplifiers
• TR-Log 6.78 Contest Software
• 40 Meter Coax Stub Filter to Reduce RFI to 20
  Meters

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Detailed View, Band Decoder and Antenna Switch Box
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N3BB SO2R Operating Position