G4CQM 50Ω Coaxial Dipole - YAGI DESIGNS BY G4CQM
Preface - Dipole or Tripole?
Several academics and literary technical authors have explained in lengthy and illustrated written works why baluns are required when feeding a dipole or driven element with coaxial cable, one popular theme being the Dipole or Tripole argument.
Why is it then that those of us who don't use a recognised balun still have solid QSO's, even working exceptional DX, as good or better than those with conventional aerials?
The simple answer is all about resonance and matching! In short, RF energy will take the path of least opposition. A truly resonant and well matched dipole or driven element will draw most of the power from its source and radiate it. However, if there are resonances on the feedline some of this power can stray and radiate from the outer surface of the braid/screening due to the skin effect. How much will be determined by the quality of resonance and matching of the dipole and those resonances of the feeder braid/screening.
Fortunately with the yagi beam and modern computer software it is possible to design for 50-Ohms impedance presented at the driven element terminals avoiding the need for matching devices. True resonance can be assured by tuning out inductive or capacitive reactance. These two distinctive and essential requirements are achieved by moving the driven element back and forth on the boom within the software environment to find the 50-Ohm location and also altering the dipole length to minimise +/- j reactive components, it's that simple!
Non-resonance of the driven element is the killer in any yagi beam. If sizeable values of reactance are present particularly at the HF end of a yagis frequency response other problems like wet weather instability will occur!
In the factory we see 'lively aerials' prior to proper tuning where handling and moving the feeder causes the resonance and VSWR to jump all over the place.
So, a well designed and correctly tuned yagi beam can operate without the need for a balun, as an example in the USA the famous Mosley Electronics discovered this fact more than 50 years ago!
The G4CQM 50Ω coaxial dipole can be described as a non-folded dipole made from two open conductive cylinders, each λ/4 long and potentially benefits from the stub compensation of a folded dipole by passing an insulated wire through the axis and connecting it to the ends of both limbs.
There can be no multiplication of the terminal resistance in this case because the wire forms the inner conductor of a co-axial line and cannot radiate.
Like the half-wavelength folded dipole, which is regarded by some as a self-balancing antenna, it is also possible for this dipole to maintain a balanced radiation pattern even when fed directly from an unbalanced transmission line.
G4CQM COAXIAL DIPOLE ANIMATION WATCH!
Considered as a pair of short-circuit λ/4 co-axial stubs connected in series with one another, appears in parallel with the dipole’s terminals. This creates a point of equilibrium at the centre of the inner wire which is also the centre of the dipole and in turn forces the two cylinders (that form the dipole) into being equal, i.e. having the current anti-node (maximum) at the dipoles centre and equal magnitude voltage anti-nodes at its ends. This is as if it were a λ/2 dipole (sometimes referred to as a split dipole) fed from a balanced source and in practice this system has proven to require no additional balun! Fortunately the inner wire has been found to be remarkably uncritical in tests and doesn't even have to run centrally inside the tubes. However, to achieve this outcome the dipole has to be electrically isolated from the boom, the PowAbeam DE molding was purpose designed to fullfil this requirement.
Evidence (Exhibit A)
Information provided by David Ross GI4SNA - Here is a real world example of the G4CQM COAXIAL DIPOLE working in the original 6M7N50 yagi compared to its computer model based on direct feed with a simple split dipole driven element...
When the G4CQM COAXIAL DIPOLE is deployed in a yagi as its driven element, intrinsic impedance at resonance is determined by the design of the yagi which is set at nominal 50-Ohms and therefore suitable for direct feed.
DC tested from the shack (inner to outer) a loop (short circuit) will be seen and will enable static charges to harmlessly discharge. It is good practice therefore to terminate coaxial feeder cable at the shack end on to a bulk head patch panel which is suitably grounded through a heavy conductor.
Evidence (Exhibit B)
Images provided by Dave Manasseh (Production Manager - Antennas) Shakespeare UK - In this real world test the G4CQM COAXIAL DIPOLE is completely on its own and not located in a yagi structure. See how the centre wire improves bandwidth and flatens the frequency response particulary at the HF end as in the 6M7N50 yagi example.
Evidence (Exhibit C)
Images provided by Derek Hilleard G4CQM - YAGI BANDWIDTH IS DETERMINED BY THE WHOLE AND NOT JUST THE DRIVEN ELEMENT - IN YAGI A PLOT BELOW SEE HOW SPECIAL TUNING CAN DRAMATICALLY WIDEN THE FREQUENCY RESPONSE AS SEEN IN YAGI B
Evidence (Exhibit D)
Images provided by Richard Mason G6HKS - Forward and reverse power seen at G6HKS with his prototype CQM16 432MHz yagi using the G4CQM 50Ω coaxial dipole...
Read why the JN53LK Contest Group abandon low impedance designs in favor of G4CQM's 50-Ohm 'Coaxial Dipole'...After 28 ohm antenna bad experience we decided to try new antenna concept based on 50 ohm direct feed dipole...g4cqm design...
THE BOTTOM LINE WITH G4CQM's COAXIAL DIPOLE: NO MATCHING, NO TUNING, NO PIPE BENDING, NO (DUBIOUS & LOSSY) BALUNS, NO MESSY COILS OF COAX, NO LOW IMPEDANCE, NO HIDDEN LOSSES AND NO COMPROMISE!
Thousands of G4CQM 50Ω coaxial dipole's in use for both Amateur Radio and Communication Aerials since 1995...