Unlike other commercial Off-Center-Fed-Dipole (OCFD) antennas, which are based on the classical 1/3 - 2/3 feedpoint split, the Aerial-51 Model 807-xx is based upon the theory of the Current-Sum Antenna, as suggested by Karl Hille, DL1VU (SK) in his paperback book entitled "Windom- und Stromsummen-Antennen" (ISBN 3-910159-14-1).
The problem with the classical OCFD design is, the current flow on 15 meters crosses the zero axis at this feedpoint, resulting in a very high impedance on that band. Although you can match it with an antenna tuner, losses in the balun will be high, which often leads to destroying the balun .
In order to obtain optimum performance on the 15m band, we have relocated the feedpoint to a different position, droping its feedpoint impedance into the same general range as the impedances of the other bands.
(See modeled and measured data at the bottom of this page).
UNIQUE: THE BALUN
Heavy Common-Mode-Current (CMC) has been an ongoing cause of trouble for many Windom and OCFD users. For that reason, many experts strongly advise against using this antenna.
Indeed, using the classical feedpoint split (33.3%) and simple baluns sold with other commercial OCFD antennas, these experts are right. You can and should expect trouble with these antennas.
Not so with the 807-xx antenna.
The 807-xx has a purpose-designed "Hybrid Balun" which impedes the flow of CMC on the feedline in all but the wost-case installations (i.e., one leg of the antenna passing over a garage with a metal roof).
The match is good enough. BUT the CMI isolation is OUTSTANDING. Take a look [at the graphs below]. While I like my own CBC [balun] I need a W1JR FT-240-31 w. 17t RG303 at the VNA to achieve similar results; your hybrid is tops of any commercial/DIY combos I have measured... Bar none.... Feel free to share the data.......
Mel, K6KBE *
*K6KBE was one of the co-founders of KLM Antennas. He was also a co- author of Bill Orr's (W6SAI) last antenna book.
See: California Hammin'
The first chart shows the CMC rejection across the hf spectrum.
"THIS" is what makes the Aerial-51 807-xx superior to all other
commercial OCFD antennas!
UNIQUE: THE RMU
The field-swappable Remote Matching Unit is a unique concept introduced with the 807-xx antenna. It enables setting 80m resonance for your favorite part of the band, yet easily changing the resonance any time you please.
Black Magic? Hocus Pocus? No, Simple Science!
The concept of raising an antenna's frequency of minimum SWR by inserting a capacitor at its point of maximum RF current has long been known (50+ years). This concept was first introduced to the OCFD antenna by Serge, ON4AA more than a dozen years ago in his paper on
For the purpose of Ham Spirit, Serge did not patent this design.
The RMU used in the 807-xx is our own design using high-power, high-voltage SMD capacitors and resistors, resulting in a flexible implementation of this concept. It enables setting 80m resonance wherever we like, and gives us the ability to easily change the frequency of resonance if ever required.
The Aerial-51 Model 807-xx family of antennas are excellent performing antennas, and generally free of Common-Mode-Current problems that are typical with competitors' OCFD antennas.
In this document you will find EZNEC modeled data and typical SWR curves, by band.