Presentation on theme: "A Broadband Feed System for an 80 Meter Antenna System Jim George N3BB."— Presentation transcript:
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.htmlhttp://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)
NE Sloper selected as one of three sloping dipole antennas N3BB Data
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') 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 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.