[TowerTalk] Re: Force12

Thu, 11 Oct 2001 15:06:52 EDT

 In a message dated 10/10/01 6:04:40 PM Pacific Daylight Time, K3BU@aol.com 
writes:<< 
  In a message dated 10/10/01 8:21:01 PM Eastern Daylight Time, 
w8ji@akorn.net 
  writes: > 
  >  The only mechanism that increases bandwidth is loss in the line. 
  >  Nothing else.
  >  73, Tom W8JI

  Here is one "else":
  Quarter wave "bazooka" coax balun increases the bandwidth. Change in its 
  reactance with frequency works "against" reactance change of antenna with 
  frequency.  
  Yuri, K3BU >>

In a message dated 10/10/01 6:57:33 PM Pacific Daylight Time, 
W4EF@dellroy.com writes:
 <<  "The only thing that changes SWR on a feedline is the load at the end of 
the line".  This is pretty basic, and the facts have not been repealed either 
by modern electronics or by modeling.
  > 
 Yuri: Right on again.  For "Basic Review 101" once again a 1/4 wave open 
stub (open on both ends) and shorted stub (shorted on one end) have the 
"opposite reactance changes" with frequency.  The Collins 1/4 Wave Bazooka is 
equivalent to a shorted 1/4 wave balanced stub across the feedpoint and a 
dipole is equivalent to a 1/4 wave open balanced stub.  Above and below their 
resonant frequencies their reactance's vary in "opposite directions"--they 
can cancel if connected to each other.  So you have a "variable load" in a 
beneficial way with frequency. 

 In a Bazooka the 1/4 wave SHORTED stub also balances the unbalanced coax and 
the dipole is equivalent to the 1/4 wave OPEN stub.  They are connected in 
such a way they see each other at the load and play "reactance cancellation 
games" above and below the resonant frequency if they are resonant at the 
same frequency and preferably in the middle of the band.  A Bazooka will 
double the band width of a 75M dipole because the broadbanding is done "right 
at the feedpoint."   NO LOSS IN THE FEEDLINE IS NEEDED.  Identical stubs 
would cancel each other over a very wide range but they are physically 
different in this case as the open stub is a different orientation--flat.  It 
gives a very beneficial broad banding affect without Black Magic or other 
claimed delusions--since WWII and even before.  It is suggested, legal and 
technically instructive to read these Antenna Books from the last Century NOW 
however and even use these techniques today for the first time for some.  

  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 
back.  

 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 

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