‰  NOW 35 WPM  ‰  TEXT IS FROM SEPTEMBER 2019 QST PAGE 32 ƒ

A SLIGHTLY OFF CENTER FED DIPOLE BRIAN MACHESNEY, K1LI/J75Y MANY STATIONS
CAN BENEFIT FROM A SINGLE ANTENNA THAT PRESENTS A REASONABLY LOW SWR ACROSS
A NUMBER OF HAM BANDS.  FOR EXAMPLE, STATIONS PARTICIPATING IN HF AUTOMATIC
LINK ESTABLISHMENT HF ALE USE A STANDARDIZED ROTOCOL TO AUTONOMOUSLY QUERY
EACH OTHER FOR SIGNAL REPORTS ON AGREED FREQUENCIES FROM 160 THROUGH 10
METERS TO INCREASE THE CHANCES FOR SUCCESSFUL COMMUNICATION.  THE
TERMINATED FOLDED DIPOLE TFD IS A POPULAR ANTENNA FOR HF ALE, BUT A COSTLY
HIGH POWER RESISTOR IS REQUIRED AND IT UNDERPERFORMS A CENTER FED DIPOLE.
THE OFF CENTER FED DIPOLE OCFD SEEMED LIKE A GOOD STARTING POINT FOR
EFFORTS TO DEVELOP A CHEAPER, MORE EFFICIENT LTERNATIVE TO THE TFD.  OCFDS
AIM TO SOLVE A PROBLEM THAT PREVENTS THE USE OF A CENTER FED DIPOLE ACROSS
SEVERAL OCTAVES OF FREQUENCY HIGH IMPEDANCE RESONANCES AT EVEN MULTIPLES OF
THE DESIGN FREQUENCY MAKE IT DIFFICULT TO MATCH TO A 50 FEED LINE.  AS AN
EXAMPLE, AN 80 METER CENTER FED HALF WAVE DIPOLE HAS HIGH IMPEDANCE
RESONANCES ON 40, 20, 15, AND 10 METERS.  OCFDS TYPICALLY FEED THE ANTENNA
AT A POINT ONE THIRD FROM THE END OF THE ANTENNA.  THE OCFD SWR REFERENCE
IMPEDANCE IS 200, WHICH CAN BE EASILY MATCHED TO 50 COAX BY A PROPERLY
DESIGNED 4 1 CURRENT BALUN.  FIGURE 1 SHOWS THE SWR OF A CONVENTIONAL 80
METER OCFD HALF WAVE ANTENNA.  THIS ANTENNA COVERS SIX HAM BANDS, BUT GIVES
‰  END OF 35 WPM TEXT  ‰  QST DE W1AW  ƒ