Transcript A Broadband 80 Meter Antenna System

A Broadband Feed System for
an 80 Meter Antenna System
Jim George N3BB
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.
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
W4RNL-W6NL Broadband Feed System
From the W4RNL Technical Paper
Double Dip SWR Curve
(Dipole in Free Space)
From the W4RNL Technical Paper
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
Distinctive Double Dip SWR Curve
For a Single NE Sloping Dipole
(N3BB Data)
SWR vs Frequency
2.5
SWR
2
1.5
SWR
1
0.5
0
3.4
3.6
3.8
Frequency (MHz)
4
NE Sloper selected as one of three sloping dipole antennas
N3BB Data
NE Sloper SWR
SWR
3.5
3
2.5
2
NE Sloper
SWR
1.5
1
0.5
0
3.4
3.6
3.8
Frequency (MHz)
4
SWR
NE by Itself vs NE alone in System
3.5
3
2.5
2
1.5
1
0.5
0
SWR (NE Sloper
alone)
SWR NE Sloper
(when all three are in
place)
3.4
3.5
3.6
3.7
3.8
Frequency (MHz)
3.9
4
SE Sloper SWR
SWR
3.5
3
2.5
2
SE Sloper
SWR
1.5
1
0.5
0
3.4
3.6
3.8
Frequency (MHz)
4
NW Sloper SWR
3.5
3
2.5
2
NW Sloper
SWR
1.5
1
0.5
0
3.4
3.6
3.8
4
NE/NW Combination SWR
3
2.5
SWR
2
NE/NW
Combination
SWR
1.5
1
0.5
0
3.4
3.6
3.8
Frequency (MHz)
4
All Three Slopers Selected Together
SWR
SWR All Three Slopers Selected
3.5
3
2.5
2
1.5
1
0.5
0
All Three SWR
3.4
3.5
3.6
3.7
3.8
Frequency (MHz)
3.9
4
SWR Comparison: All Major Combinations
SWR
SWR For All Major Combinations
3.5
3
2.5
2
1.5
1
0.5
0
SE Sloper SWR
NE Sloper SWR
NW Sloper SWR
NE/NW Combination
SWR
3.4
3.5
3.6
3.7
3.8
Frequency (MHz)
3.9
4
All Three SWR
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 50Ohm SWR 2.36 1.83 2.47 50-Ohm Coax (200.6') < 1 wl exactly 1 wl > 1 wl Impedance at coax end 110.1 j26.7 91.6 - j0.5 121.3 + j15.1 75-Ohm Match (44.3') < 1/4 wl exactly 1/4 wl > 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 = Zo^2 / 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.
Three Slopers on Hilltop
Positioning of Coax and Stackmatch Inside the 45G Tower
80 Meter Stackmatch Control Box in N3BB Shack
(Note Use of Labels Made with a Label Machine)
N3BB SO2R Operating Position Details
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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
Detailed View, Band Decoder and Antenna Switch Box
N3BB SO2R Operating Position