>On 3/19/2013 11:10 AM, Joe Subich, W4TV wrote:
>>
>> They give the designer additional degrees of freedom with which to
>> control input impedance and side lobe structure.
>
>I was with you until you said "side lobe structure." I don't claim to
know much a
>bout the design of Yagis, but I'm under the impression that pattern is
largely
>determined by the tuning and spacing of parasitic elements. Am I wrong?
>
>73, Jim K9YC
The radiation pattern of a Yagi is determined by ALL of the elements,
including the driven element itself.
As a volunteer editor for the German-English VHF/UHF magazine DUBUS, I
worked on several of the original articles by Justin Johnson, G0KSC,
that first introduced the LFA (Loop Fed Array) and I have watched with
interest as the concept has developed. Recent years have seen a big leap
forward in Yagi design, and the LFA has played a major part in this.
G0KSC's original design work was aimed at extreme weak-signal
applications such as moonbounce and terrestrial VHF/UHF DX, where the
ultimate signal/noise ratio is often spoiled by the pickup of noise from
minor lobes of existing long Yagi designs. Suppression of minor lobes to
levels of -20 to -30dB requires very precise cancellation between the
radiation contributions from all of the Yagi elements summed together;
and at this level of detail, broad generalizations don't work any more.
Progress can only be made by very careful computer modeling and
optimization - and by questioning all the old assumptions about what
makes "a good Yagi".
To cut the story short, G0KSC found that a horizontal open-loop feed
allowed a Yagi to designed with an excellent combination of
characteristics: high gain (for the boom length employed), minor lobe
suppression of at least 20-30dB, a 50-ohm feedpoint impedance, and broad
bandwidth in two quite different respects - not only a good VSWR
bandwidth but also a good "pattern bandwidth", meaning that the
radiation pattern remains relatively stable across a wide frequency
range.
As W4TV says, the loop feed gives additional degrees of freedom in the
design. It seems to open the door into a particularly favorable area of
parameter space, where several good performance characteristics come
together and computer optimization can offer some very useful practical
designs.
Of course these computer-optimized designs then needed to be proved in
practice - and to be fair, I think they now have been. The InnovAntennas
company has been built on the good reputation of the LFA designs among
moonbouncers and experienced VHF/UHF DXers, who have tried the antennas
and measured reduced levels of noise pickup from the rear and side lobes
compared to their previous arrays.
Now here's where the story becomes even more interesting. While
continuing to develop the LFA concept, G0KSC also revisited other more
conventional Yagi designs that had been written off 50 years ago as
'evolutionary dead ends' - in particular, some designs with a very low
feedpoint impedance around 12.5 ohms. Guess what? With modern
optimization techniques, these conventional straight-element Yagis can
deliver very similar sidelobe suppression to the LFA.
Spurred on by that discovery, other designers such as YU7EF, DK7ZB and
G4CQM joined the charge and began to create their own designs using
conventional straight elements, with similar good results.
The latest development area seems to be in bent driven elements, such
the C-shaped 'OP-DES' concept of G0KSC and the V-shaped elements
favoured by some other designers. Once again these specially shaped
driven elements are giving the designer some extra degrees of freedom to
optimize the overall performance.
We now know that many different types of Yagi design can be optimized to
give excellent suppression of minor lobes across a good bandwidth... at
least within a computer optimization environment. The real
differentiators are probably much more practical: ease of translation
from the computer design into real hardware without loss of performance,
ease of feeding from 50 ohms and of course a dependable mechanical
design.
Every contributor deserves credit for these new developments in Yagi
design, and in particular everyone else gives credit to YU7EF. But it's
also fair to say that G0KSC has led the charge with his innovative
designs and prodigious work rate, and has raised the bar for everyone
else to match.
73 from Ian GM3SEK
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