Jim,
You may have seen one of those dual square boom logs
where the coax runs into back-end of one of the square
tubes and then all the way up the inside where it attaches
to the feed point at the front-end. The dual booms form
the parallel two-wire transmission line that feeds the
elements (coax center conductor connects to one tube
and shield to the other). It looks like its fed from the back,
because you see the coax going in from that end, but its
really fed from the front. I tried modeling a log cell yagi
once and made the mistake of thinking that I could feed
it from the back (it didn't work it I turned the feed around).
All those frequency independent 160/80 meter receive
antennas like the K9AY loop and the pennant are the
same way (they fire towards the feed).
73 de Mike, W4EF..........................................................
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Tom Rauch" <w8ji@contesting.com>; "towertalk reflector"
<towertalk@contesting.com>
Sent: Thursday, July 01, 2004 2:42 PM
Subject: Re: [TowerTalk] Funniest thing I've seen in weeks
> At 04:36 PM 7/1/2004 -0400, Tom Rauch wrote:
>
> > > the elements. For instance, a log periodic (which has all
> >elements driven)
> > > doesn't really care which end it's driven at (aside from
> >some matching issues).
> >
> >The log periodic uses crossfire phasing, and that means it
> >must always fire towards the feedpoint.
> >
> >There is no way around that, so the statement a "log doesn't
> >care which end is fed" is absolutely not correct in an
> >aperiodic design.
>
> Hmmm. I seem to recall seeing a log periodic fed backwards, but I'll have
> to take a look for the design and see if they were fooling with oddball
> transmission lines or loading.
>
>
>
>
> >As for the Yagi, optimum pattern and gain require elements
> >be excited in a binomial current distribution. While you
> >could feed the front and use two rearward parasitics in a
> >three element antenna (the middle in NO way would be tuned
> >accurately as a reflector, or the furthest to the rear would
> >have almost no current), by far the optimum feedpoint is the
> >center which will produce a nearly perfect 1:2:1 current
> >distribution and a null of at least 40dB with proper tuning
> >and spacing.
>
> The binomial case is just one aperture weighting that happens to be simple
> (both in analysis and in implementation). It's not necessarily optimum,
> depending on what you want the main and sidelobes to look like. Any sort
of
> tapered illumination will tend to broaden the main lobe and reduce the
> sidelobes. If one wanted a real narrow main lobe, and didn't care so much
> about side/back lobes, you could go with a reverse taper that has higher
> currents at the ends. Especially if you have lots of elements (as in a
> long boom design), one could in theory, construct any sort of
> illumination/current taper you wanted (such as a Chebysheff or Taylor),
> although, I think you might have a problem with realizibility or bandwidth
> (you don't have total freedom with mutual impedances... an interesting
> synthesis problem... good topic for a thesis project). And of course,
it's
> probably not worthwhile. Nobody is obsessing about 30 dB sidelobes for an
> amateur antenna.
>
> The depth of null is almost entirely going to be determined by design and
> construction tolerances. 40 dB nulls imply controlling the current to
> around 1%, which would be quite challenging, especially over frequency.
> I'll bet the wind induced motion in the elements would change the coupling
> enough to cause problems. As it happens, I have a paper I'm working on
> that addresses just this problem (i.e. how much does the mutual coupling,
> and hence the pattern, vary as a function of small displacements in the
> elements): For parallel dipoles spaced 0.2 wavelength apart(one driven,
> one shorted), a 1% change in spacing changes the phase of the radiated
> field by about half a degree. For small angular changes in dipoles half a
> wavelength apart, 1 degree results in a phase change of about half a
> degree. That half a degree phase change corresponds to about a 40-45 dB
> null. Normal element flexing in an antenna is probably on the order of
> several percent distance wise, and probably 4 or 5 degrees. I don't
recall
> the exact numbers for such large displacements (it's not important in my
> application) but it's probably roughly linear, so you're looking at phase
> shifts of 5-10 degrees.
>
> Of course, this is what makes the SteppIR so cool, it makes getting 30 -
40
> dB nulls actually possible over a wide bandwidth (just not all frequencies
> at the same time). (if only the wind didn't blow<grin>)
>
>
>
>
>
> >The idea of adding a reflector behind a reflector is a waste
> >of time, because there is essentially no field there to work
> >with. This is why Yagi's have developed into multiple
> >directors with single reflectors. Even trigonal reflectors
> >(staggered above and below the boom) have largely proven to
> >be a waste of material.
>
> Why would you say there's no field behind the reflector? True, in the far
> field where the fields from all the elements have cancelled, but certainly
> not in the near field, and particularly not in a superdirective array.
Try
> running a NEC model on a Yagi and looking at the H field (which is where
> the energy is stored).
>
> For instance, on a 3 element 20m beam, with 5.37, 5.31, and 4.70 meter
> elements, spaced at 2.72 and 3.89m, the H field has three distinct peaks
at
> the element positions, but the field extends about 1.5 meters off either
> end before it decays to less than 10% of the peak at the driven
> element. This is particularly noticeable if you do a cut at, say, 0.25 m
> above the plane of the elements. Indeed, the fact that it decays that
much
> implies that this particular antenna might not benefit much from an added
> element on either end, especially if it's spaced out comparable to the
> other two.
>
>
>
> >Of course if you DON'T properly tune an element for null
> >then adding a second reflector can help.
>
> I don't think that adding a reflector (or director, for that matter) to an
> existing optimized design will have all that big an effect (at least not a
> good one!). You'd have to optimize the whole shooting match, and you'd
> probably want the elements pretty closely spaced. You'd probably get into
> a situation where the coupled currents are so high that loss becomes an
> issue, and, in any case, it would probably be horribly narrow band.
>
>
>
> >I think the confusion in all of this comes from people
> >thinking reflector reflect and directors direct. That just
> >isn't what they do. What all of the elements do is cancel
> >radiation towards the undesired direction. The minute a
> >director starts "directing" as a primary gain mechanism it
> >barely affect pattern. Patterns are formed by squeezing
> >radiation by forming nulls, like squeezing a balloon.
> >Stretching the pattern forward is a very inefficient way to
> >obtain gain.
> >
> I would agree, although, superdirective arrays work by stretching to get
> gain, but have limits on how far you can take it.
>
>
>
> >Traveling wave antennas, like the Fishbone, Beverage,
> >Inverted V, and V Beam, use in-phase addition more than
> >nulling to achieve directivity. That's why they require huge
> >amounts of linear area for minimal gain. I can't imaging
> >why anyone would want to mess up a Yagi by making it work as
> >a traveling wave antenna, unless they pound for pound had
> >more element and boom material than brain material. Hi hi.
>
> If they needed very broad band characteristics. Of course, you would
> probably be better off with a log periodic or something else.
>
> _______________________________________________
>
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questions and ask for Sherman, W2FLA.
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_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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