RadioBanter - View Single Post

January 20th 04, 04:31 PM

(Mark Keith) wrote in message . com...
" Art Unwin KB9MZ" wrote in message news:bVUOb.100367$I06.445073@attbi_s01...
Mark
Let me only respond to the technical things that you are mistaken on
Radiators do have parts that are inefficient which you
apparently do not accept.

No, I don't accept it. To me, you are misapplying terms. All radiators
are efficient unless they are so thin, or of a material as to have a
lot of excess resistance.
All radiators are equally capable of being efficient radiators AS LONG
as you can actually transfer power to them. Efficiency is a poor term
to use for a radiator quality. A half size dipole is just as capable
of being an efficient radiator as the full size dipole. Really no
less, or more so. The fun part is actually transfering the power from
the radio/feedline to the radiator in an efficient manner. The only
thing you are altering when you shorten an antenna element is the
pattern, and gain in a certain direction. And the change is not that
drastic. The pattern is still a fig 8, and the gain has dropped to
about 1.8 dbi, instead of appx 2.1 dbi. You do not alter efficiency
per say. The efficiency is the percentage of power lost in the
transfer of power to the radiator. Or you can gauge the efficiency of
the whole system as a whole. You do not gauge efficiency of radiating
elements, except as already stated.

BTW, if I'm wrong on any of this, anyone feel free to jump in and
correct...
I don't want to create any excess old wives either...

Unfortunately Mark this is not going to happen.
Having lost so many talented people from this group the
tendency now for those that are left are to avoid the
hard questions, provide quotes from books that leave you
hanging afterwards or intentionaly or other wise confuse
and divert from the specific issue.
I would like to make one point clear. I was refering to
efficiency per unit length which somehow people will not accept.
In the case of the amateurs losing the ends of the dipole and not
noticing the difference is purely because the difference is not
perceptable to the ear as your figures pointed out which is why
capacity hats are so usefull.
On the ARRL question and gaps between dipoles. In all my copies
the gain curves all stop at zero gap between dipole ends which is
absolutely absurd as the ends of a dipole has nothing to do with
the situation of gain. Gain is determined as a vector addition
in combination with phase.
I also agree that less loss would be a better term but if one
moves away from convention all hell breaks loose
Best regards
Art

ie power factor or cos phi.
you have read them or

Radiation is created by current. If current was uniform over a radiator
length then the length of the radiator is reduced from 1/2 wave to
wavelength over pi.
This is because voltage becomes more dominant than current at the ends of a
radiator.
If you divide the current curve into uniform radiator length
it should become clear to you that the area under the current curve per unit
length diminishes as the curve moves to zero. This is fundermental but if
you still have problems with this concept by all means continue a technical
dialogue.

Dunno... I'm not really getting the point of all this...

Loss less feed systems.
This term is used quite a lot in academia. One can relate it to such things
as household circuits where the radiation is so small it is not considered a
factor in calculations.
. A 'loss less' feed system in say an antenna would comprise of something
short with respect to wave length and would be voltage dominated so that
radiation is minimised by the low value of current.

I'd feel better if you dropped the "lossless" term, and changed it to
"low loss". Or maybe lower loss...

Regarding efficiency of magnetic loops.
It is clear in this case that we are dealing with a radiatior that is not
only one tenth of a wavelength but also has an impedance dominated by
resistive losses which means that the efficiency will be extremely low and
possibly only a tenth of what you surmised.

Not sure...I don't bother with such antennas, but I was under the
impression the efficiency could be fairly decent with those if the
proper techniques were used in feeding them. I could have been
mistaken on the appx 70% number...

There are ways to ensure that low impedance
problems can be overcome, we see similar problems overcome
in very high gain yagi's which tend to have low impedances as efficiency
increases. This problem can be readily overcome in many cases by adding a
second reflector where its proximity to the driven element
reverses the decline in impedance.by adding a coupling effect.
If I have forgotton something technical that you brought up please let me
know.

I'm not sure if I really agree on this, but I'll leave this for now...

Ah yes, the yagi syndrome.
Yagi gain is based on boom length assuming other requirements are met. In
the amateaur world boom length is not really a problem for half of the bands
but it is a problem in that boom length and gain have a limit in scope
as well usuitable for many bands. So I would expect that future enginners
will move away from just yagi's and explore methods where direct coupling of
radiators will occur to remove problems of fractional wavelength portions
spacings as one sees with the yagi aproach. and explore other areas, where
turning radius becomes prominent rather than boom length..

They have been, still do, and surely will continue...
MK