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[TowerTalk] Re: Force12

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Subject: [TowerTalk] Re: Force12
From: (Joe Reisert)
Date: Sat, 13 Oct 2001 10:37:12 -0400

The beam you are referring to is the K8CC (not W8CC) Yagi.

One had to be very careful adding "clap trap" to a specific design without 
understanding the originators design intent. In this case, this particular 
Yagi is designed to be a high performance Yagi for 28--28,7 MHz, mostly for 
winning DX contests, and is NOT intended for operation above 28.7. Above 
28.7 MHz, the pattern blows up and the VSWR skyrockets! Trying to change 
the feed system etc. is not the way to improve bandwidth!

Let me be more specific. As a quick first cut on the K8CC 5 element 
10-meter Yagi on my modeling program, this particular antenna can be easily 
modified by just moving the driven element forward to 100" or so and 
slightly increasing the length of the driven element. The pattern etc. will 
not be significantly degraded but what will happen is that the driven 
element impedance will rapidly increase to 40 Ohms or higher (depending on 
how far forward you go). Then all one has to do is to center feed the 
driven element with a few ferrite beads on the coax and voila, a great 
wideband Yagi.

This is all explained in my article Yagi/Uda Antenna Design, A Different 
Approach" in Communicatons Quarterly, Winter 1998, pp. 49-60.

The bottom line is that if you want to increase the bandwidth of a Yagi 
antenna, the best way is to look at the design first and modify the model 
if needed before you start to add all these exotic matching systems. 
Simplest is usually best!


Joe, W1JR

>   I built the W8CC 5 element 10M beam and wanted to use a Folded Dipole DE
>for more bandwidth as 10M is a wide band.  I adjusted the DE tubing ratios so
>I had 200 ohms.  I like 200 ohm feedpoints as it lowers the RF losses in the
>DE feedpoint with a lower RF current.  It was 3/4" tubing with 5/8" tips and
>a .125"  wire suspended 2" below and fed in the center.  This creates a step
>up ratio of about 7:1 to create 200 ohms from 28 ohms.  The charts and graphs
>showing how to create different step up ratios based on diameter ratio and
>spacing are shown in those old books also as this concept was widely used.
>  A balanced 200 ohm feedline was created using a 1/2 wave of 50 ohm coax
>connected to the end of the main 50 ohm feedline and the other balanced
>feedpoint (center lead) and the shields are connected together.  This is also
>shown in many of the last Century Antenna Books.
>  Unfortunately I didn't get more bandwidth--I got less than typical for a
>gamma.  The reason is the 1/2 wave balancing coax was open on both ends and
>it has the same reactance change of the dipole DE.  Therefore the summations
>of all this hardware and off resonant frequency reactance was a higher
>reactance change than the DE alone and less bandwidth.  I noticed the same
>thing on a 6M DE.
>  I've used FD DE's before set for 100 ohms fed with 100 ohm balanced coax 
> and
>got exceptional bandwidth.  The FD was able to make it's contribution of
>reducing reactance change.  A FD is really 2 shorted 1/4 wave stubs--back to
>  There is another beneficial way to use 1/4 wave stubs in matching the
>antenna R to the feedline Zo and ALSO obtaining additional bandwidth.  I
>needed a 37 ohm 1/4 wave stub to match the split DE 28 ohms to 50 ohms.  I
>parallel connected 2-72 ohm coaxial stubs.  Not only did it give me the
>desired 1:1 match but I got tremendous bandwidth.  With the DE resonant at
>28.4 MHz I could operate at 29 MHz with a fairly low SWR not possible with
>the 200 ohm FD DE.  This bandwidth was possible NOT with RF loss in the
>feedline as has been suggested as the ONLY WAY to get bandwidth, but with a
>"Magic Opposite Reactance Generator" inside of and called a "!/4 Wave Stub"
>used also to match 2 Z's.  I coiled it up to see if I could reduce the RF
>Spill Over and measured it with the Palomar RF Current Meter and it didn't do
>much.  The beam worked great but the F/B wasn't as good.  Using 1/4 wave
>stubs to match lower R's has created wider bandwidth in every application
>I've used it in as has a Bazooka and I'm not about to stop doing it.
>I'd like to make a TT bet here of $100 for anyone to prove that wrong.
>$100 is all that is offered as there is doubt they could afford anymore when
>they lost.  They must also apologize for spreading band info.  I will publish
>this comparative bandwidth data in an article on THIS VERY TOPIC and could
>perhaps serve a useful purpose.
>  One of my next projects is to incorporate the 2 quarter wave matching stubs
>in one side of the 1/4 wave Bazooka.  This should increase the bandwidth even
>more than the Bazooka's action and preliminary tests show it works.  It's a
>unique way to get a Bazooka to match loads above and below the Zo of the main
>feedline.  Components that change their reactance in different ways has been
>used in many ways for years.  This seems to be the first exposure of this
>simple technique for many.  Frank Witts article in the ARRL Compendium Vol 4
>p30-37 referenced by W1JR is another example.
>  There was another article in 4/89 QST that used a coaxial 1/4 stub attached
>to the DE in a sneaky sort of a different way with a transverse connection at
>the feedpoint.  It not only allowed matching the DE R whatever it was to any
>coax Zo but--are you sitting down--increased the bandwidth by creating the
>OPPOSITE reactance--a technique actually widely used in HR by those in the
>know.  I will compare this design to the others when I get time.
>So as you can see these "Basic Fundamental Black Magic Broadbanding
>Techniques" have been around for some time.  If you are still not convinced
>it can't be done even with lossless feedline, it is suggested you actually
>try it.  Variations of these concepts are applicable to open wire line and
>it's for all practical purposes lossless--where it supposedly won't work.
>There is a constant flow of all the things that "can't be done" mostly by
>those who can't do them with example after example after example.  Lets start
>a new "TT Trend" of "what can be done" using "Basic Fundamentals" from the
>last Century and see if we can "advance the state of the art."  Why is it
>necessary to constantly review the most elementary basic fundamentals to
>justify simple concepts that work for everyone that actually try them?  This
>broadbanding was done without changing the diameter of the DE as claimed.
>Lets keep the facts straight.  Enlarging the DE diameter will broadband the
>system even more.   K7GCO
>List Sponsored by AN Wireless:  AN Wireless handles Rohn tower systems,
>Trylon Titan towers, coax, hardline and more. Also check out our self
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List Sponsored by AN Wireless:  AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!!

FAQ on WWW:     
Administrative requests:

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