As I sent to YSD yesterday:
> I've read quickly through the .doc. Its only considering nearby or
> near field for people exposure. Using NEC for that is wrong, uses the
> wrong math. And NEC does not model a folded or coaxial dipole well.
> It does so bad that all the yagi optimization done by the successful
> gurus like K1FO use a straight dipole as the driven element. Then
> they often are built with a T match or a folded dipole and sometimes
> achieve the gain and pattern modeled. K1FO only accepts an
> optimization when it measures virtually the same on the antenna
> range. Which is to say coupling from a dipole or folded dipole to the
> director and reflector array is the same. There are differences in
> yagi design that affect that coupling more.
> There are two major schools of thought on a yagi feed, one that the
> feed Z of the simple dipole should be 50 ohms and Guru DL6WU designs
> accomplish that with very high quality yagis based on design, not
> iterative optimization. He likes a folded dipole feed because its
> mechanically sturdier and easier to feed balanced (usually with a
> half wave coax balun, a strict voltage balun) than a straight dipole.
> Gamma matches on a VHF yagi tend to skew the pattern and to hurt the
> gain, as exemplified by the 11 element crumcrafts that have been
> around 50 years, with gain claims 5 dB more than they deliver.
> DL6WU yagis have been optimized generally sacrificing the 50 ohm feed
> and often the optimization puts a first director very close to the
> driven element and knocks that feed Z down under 10 ohms because of
> the tight coupling. K1FO has admitted yagis optimized in that
> direction seem to model better than they perform, likely that the
> high circulating currents from the tight coupling cause greater
> losses that the model misses. A factor might be that the folded
> dipole couples less well to that closely spaced director than a
> simple dipole.
> While the .doc is regulation, I have little faith when the only
> external references are more than 50 years old and when they used NEC
> improperly close in and with closely spaced wires in the folded
> dipole. What they call a coaxial dipole may not be a bazooka and its
> not clear when they are assuming dipole vs folded dipole orientation.
> Being off the end tends to be in a null while being broadside gets
> maximum signal. So a vertical dipole would tend to have a null under
> it (nearest people exposure) while a horizontal dipole would have a
> maximum under it (broadside).
> Point is, the folded dipole and the straight dipole radiate the same.
> The folded dipole tends to be slightly shorter for resonance because
> of the ends and definitely has a higher terminal impedance. But the
> watts radiated are the same.
73, Jerry, K0CQ
On 12/9/2010 2:23 AM, Ken Brown wrote:
>> 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.
> DE N6KB _______________________________________________ TenTec
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