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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