In a message dated 10/10/01 6:04:40 PM Pacific Daylight Time, K3BU@aol.com
In a message dated 10/10/01 8:21:01 PM Eastern Daylight Time,
> 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
Yuri, K3BU >>
In a message dated 10/10/01 6:57:33 PM Pacific Daylight Time,
<< "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 This is pretty basic, and the facts have not been repealed either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
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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