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 SystemFrom 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 CurveFor a Single NE Sloping Dipole

(N3BB Data)

SWR vs Frequency

0

0.5

1

1.5

2

2.5

3.4 3.6 3.8 4

Frequency (MHz)

SW

R

SWR

NE Sloper selected as one of three sloping dipole antennasN3BB Data

NE Sloper SWR

0

0.5

1

1.5

2

2.5

3

3.5

3.4 3.6 3.8 4

Frequency (MHz)

SW

R NE SloperSWR

NE by Itself vs NE alone in System

0

0.51

1.5

2

2.53

3.5

3.4 3.5 3.6 3.7 3.8 3.9 4

Frequency (MHz)

SWR

SWR (NE Sloperalone)

SWR NE Sloper(when all three are inplace)

SE Sloper SWR

0

0.5

1

1.5

2

2.5

3

3.5

3.4 3.6 3.8 4

Frequency (MHz)

SW

R SE SloperSWR

NW Sloper SWR

0

0.5

1

1.5

2

2.5

3

3.5

3.4 3.6 3.8 4

NW SloperSWR

NE/NW Combination SWR

0

0.5

1

1.5

2

2.5

3

3.4 3.6 3.8 4

Frequency (MHz)

SW

R

NE/NWCombinationSWR

All Three Slopers Selected Together

SWR All Three Slopers Selected

0

0.51

1.5

2

2.53

3.5

3.4 3.5 3.6 3.7 3.8 3.9 4

Frequency (MHz)

SW

R All Three SWR

SWR Comparison: All Major Combinations

SWR For All Major Combinations

0

0.51

1.5

2

2.53

3.5

3.4 3.5 3.6 3.7 3.8 3.9 4

Frequency (MHz)

SW

R

SE Sloper SWR

NE Sloper SWR

NW Sloper SWR

NE/NW CombinationSWR

All Three SWR

How It WorksFor 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') < 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 / Zinwhere 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

• 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