G4CQM Coaxial Dipole Exposed!


Background and working...

Original Drawing...
Original Drawing

The G4CQM Coaxial Dipole was first conceived and tested during the month of January in 1994. It is in fact a conventional Split Dipole with just one difference. There is a centre wire conductor (insulated) running through the internal bore which terminates at either end. It was intended as a drop in replacement used in 50Ω Direct Feed yagi antennas designed and manufactured by PowAbeam Antennas. It does not in itself provide matching, this occurs as a consequence of the combined interreaction of all the elements (parasitics and driven)!

It was also adopted in 1999 by Communication Aerials Ltd of Westbury Witshire as a replacement driven element for their new range of yagi antennas. A requirement of many professional deployments is the presentation of a DC loop for continuity, monitoring and testing purposes. The centre wire conductor provides for this need!

Coaxial dipoles are not new, over the decades there have been numerous implementations, and a range of performance claims. Their modus operandi has often confounded the so called experts!

Basic Circuit...
Basic Circuit

At VHF frequencies RF currents flow on the outer surface of conductors (skin effect). External surfaces of the G4CQM Coaxial Dipole carry the flow and distribute RF current in exactly the same way as a Split Dipole, there is no difference. However, the wire inside the bore terminating at either end creates two shorted end λ/4 coaxial stubs opposing one another, as a result no RF current flows on this wire! From a DC current point of view this wire provides a loop for continuity, monitoring and testing purposes. In commercial application feeder cables usually terminate on a bulkhead patch panel which is grounded. So, the G4CQM Coaxial Dipole enables static charges to harmlessly discharge through the internal wire and then to earth. Meanwhile nearby lightning strikes that induce currents on the dipole structure are prevented from reaching destructive transient voltages because they are shorted and discharge to safely to earth!

When the G4CQM Coaxial Dipole is used in conjunction with a suitable slip on or clip over ferrite placed close up to the feedpoint or alternatively the 'λ/4 Half-Loop Trick' then no Balun is required.

Also see:

Coaxial Stubs...
Coaxial Stubs

 



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