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Re: [TenTec] was OT: Indoor Antenna: re B&W type terminated dipoles

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] was OT: Indoor Antenna: re B&W type terminated dipoles
From: Ken Brown <ken.d.brown@hawaiiantel.net>
Reply-to: ken.d.brown@hawaiiantel.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Wed, 08 Dec 2010 22:23:09 -1000
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
> The efficiency of a dipole is nearly 100 per cent or 1.0.  Depending on 
> construction, it might be 99 per cent, (0,99, 0,98, etc.)  It is as 
> others stated a statement of power minus losses, giving what is radiated.
> A half wave dipole converts almost all the available power to radiation 
> is another way of looking at it.
> Coupling factor is something else, usually applied in my experience to 
> relations between tuned circuit inductors.
As a dipole driven element is coupled to parasitic elements, it's 
radiation resistance becomes lower. It is no longer a 75 ohm feedpoint 
impedance, but rather something much lower. This is why driven elements 
of yagis tend to have gamma matches, or other means of matching the low 
impedance of the driven element to 50 ohm coax. As the radiation 
resistance gets lower, the ratio of radiation resistance to ohmic loss 
resistance gets lower. The ohmic resistance of the conductor becomes a 
more significant part of the total impedance, so the efficiency goes 
down compared to dipole not coupled to parasitic elements.

One method of achieving a higher feed point impedance on the driven 
element of a parasitic array is to use a folded dipole instead of a 
plain dipole.

Remember TV antennas feed with 300 ohm ribbon transmission line? Many of 
those used folded dipole driven elements.

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