I sent this to Dino directly late last night--but I was reluctant to
send it to Towertalk for a number of reasons. It was totally
unedited--not even reread (still hasn't been) and I was tired last
night from all of the Xmas rush. And I didn't want to discourage
others from answering his questions with even better thought out
replies. And Dan and Guy have certainly jumped in from their
own perspectives. But I think what I wrote may add something to the
discussion. So I will toss it out into the forum with apologies for
not making it more readable or well written.
Happy Holidays to all on the Towertalk reflector--
John W0UN
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I'll try to give you a couple of quick answers to the questions you raise.
The calculations for the details are often difficult to do accurately enough
because they depend on subtle differences that are often beyond the
control of the designer. Things that depend on exactly what the
builder does and the way he does it are often minor variables.
There is an old saying that explains things better. "I built it just
like QST, EXCEPT........ When you make minor exceptions there
are often things that change enough to make the calculations incorrect
or at least "off" somewhat.
Things like the size and shape and general method of the mounting plate
that attaches the elements to a boom can make a significant difference.
The spacing of the element from the boom or whether the element goes
through the boom or not and if it is insulated from the boom or not.
And the diameter of the boom. All of the things have effects and you
can either duplicate the published design EXACTLY or understand
the effects that the changes that you make will have. But this is often
difficult to model in advance. So when modifying a design it is often
necessary to "tweak" the lengths a bit to counteract the effects of
any changes made.
My crew used to make fun of me because I had a saying...."I don't care
if it is right or wrong--I want them all the same". The point being that
subtle changes can affect things--and if you are building a production
run you need to make them ALL identical. Maybe there is a better way
to do it--but if you change the way they were done it MAY affect the
exact tuning.
OK, enough philosophy, now for some details.
>Antenna #1. A simple J-pole. The design is straight forward. A simple
>1/2 wave antenna with a 1/4 wave matching stub tapped at the 50 ohm
>location. What I have not been able to find is information that fully
>explains the DISTANCE between the main radiator and the matching
>stub. The following web site is one of the few that even addresses this
>distance with calculations but still doesn't say WHY (dimension D)...
>
>http://www.packetradio.com/jpol.htm
On the J-pole antenna the point where the short section is attached to the
long section they are
shorted together. So if you attach the feedline at this point it would
see an impedance of 0 ohms.
The short section and the long section form a quarter wave line that is
shorted at the bottom end
and shows a very high impedance at the top (open) end of the 1/4
wave. Somewhere along
these two parallel conductors (that act both as an antenna and as a
transmission line) is a
point that is 50 ohms. (Since we know they are 0 ohms at the bottom and a
very high inpedance
at the top. But the EXACT point is dependent on some of the construction
details of your
particular antenna. If you build it exactly like the original design and
if it is mounted in
an identical environment then you will get the same results. But if the
J-pole is side-mounted
(for instance) on a tower and the spacing is different that the
original--then the antenna will
have a minor difference where the 50 ohm point is because, although the
bottom (shorted)
end is still 0 ohms the TOP of the 1/4 wave will be a different impedance
because of variations
of spacing from the grounded tower and other subtle differences. So if you
build it EXACTLY
like the original and mount it some what differently then the SWR may be a
bit different. Which
is OK--but many hams want to tweak their antennas until the are perfect--so
they then
adjust the tap point a bit to make it 1-1 SWR.
But the key issue is the POINT of attachment is just like setting the wiper
arm on a potentiometer.
You need to set it at the 50 ohm point and it is 0 ohms at the bottom end
and some very high
value at the top end of the 1/4 wave section. Just that you may not know
the EXACT value at
the top--therefore you won't know EXACTLY where the 50 ohm point is. But
if it is a good
design and you have copied it reasonably well then this point whould be
pretty close to
what was published.
>Antenna #2. A colinear VHF antenna. This is a simple and effective
>antenna utilizing stacked 1/2 wave elements with a 1/4 wave matching
>stub. The feed point is 200 ohms that requires a 4:1 balun. The text
>says that the "balun" needs to be .33 wave length. Why?! Wouldn't a
>commercial built 4:1 balun work to match a balanced 200 ohm load to an
>unbalanced 50 ohm feedline? Maybe the "loop" is really a phasing line,
>but the text still lacks the supportive text to explain it. The website
>is here...
There are a number of ways to make impedance transformations. This is
important if you
have a transmission line of (say) 50 ohms and an antenna that is something
different. A
lot of antennas were designed to have balanced feedpoints (coax is
UNBALANCED) and
an easy way to make this transition was with a 4-1 coax balun. Once
toroid cores
were invented and different kinds of transformers and balanced to
unbalanced translations
were understood they became an alternate way to make this impedance
transformation
(and balanced to unbalanced connection --BALUN).
It turns out that 4-1 transformers are the easiest to make (after the most
simple 1-1
job). On a core if you have TWO windings in series and think about the
transformer
as an autotransformer--ie, a winding with a tap, you can see that if the
input side
is hooked half way up the winding and the output is hooked up the entire
length--then
you have a 2-1 VOLTAGE increase. But, since the power stays the same, you
also
have a 1-2 current DECREASE. Since impedance is E/I and you have doubled the
E and halved the I the resulting R is 4 times as high. This is a bit more
complex because
there are different ways of hooking up the wires and using more wires that make
the baluns either "current" baluns or "voltage" baluns but this is another
topic. In any
case the 2-1 step up of the voltage makes for a 4-1 step up of the
impedance--hence
50 ohms in and 200 ohms out.
But you can also do this with coax. This gets a little more tricky to
explain with words
rather than pictures--but I am not going to waste time drawing pictures now
that it
is time to go to bed. The key thing about any input whether it is an
antenna or any
black box is that the two connections be driven out of phase. If the
voltage across the
two connections is exactly IN phase then there is no voltage DIFFERENCE across
the input terminals and no power is delivered to the load. Coax provides
an unbalanced
signal from the center conductor to the shield and these are out of phase.
But if you have a BALANCED system you should have a balanced feed to this
system
for a lot of reasons that are beyond my quick note. The transformer balun
on a toroid is
ONE way to do it. Another way is with a COAX balun.
The coax balun has one output at the end of the main coax--but this is
unbalanced. If
you then connect another length of coax at this point, the voltage on the
main feedline
will drive the one antenna feedpoint and it will also drive the added
section of the
coax. 1/2 wave down this added coax is a signal that is now out of phase
with the
signal at the other end (because it is a 1/2 wave of coax. So connecting
this second
end of the added coax to the other terminal of the antenna will result in
the antenna's
two feedpoints being driven out of phase--a requirement for maximum power
transfer.
So how does the 4-1 impedance action happen? It is a bit harder to think
about--
but let me work it backwards as a way of illustrating it. If the first
antenna connection
were considered an unbalanced connection (like a gamma-match--or like the
tap on
the J-pole you first asked about) and this point was a 100 ohm point, and
if the
second connection (at the end added 1/2 wave of coax) was also a 100 ohm
point--then
you can think about the second 100 ohm point being connected to the first
100 ohm
point by a 1/2 wave of coax. So they are now in parallel and 100 ohms in
parallel with
100 ohms is 50 ohms--the correct value to match the feedline. And the two 100
ohm points are actually in SERIES. Take a driven element for a Yagi. If the
balanced "Tee" match is adjusted properly--ie, each shorting bar of the Tee
is set
to the 100 ohm point as referenced to the center of the driven element (aka
ground)
then the two 100 ohm feedpoints are in series and that 1/2 wave of coax
converts
the 50 ohm coaxial line into a 200 ohm balanced matching section.
The example listed the coax as 1/3 of a wavelength. But this is the
physical length
of the cable--not the electrical length. We need a 180 degree phase
shift--which
is always 1/2 wavelength ELECTRICALLY but the cable has a velocity factor
that is a function of the dielectric material. For solid polyethelene coax
this is
0.66 and 1/2 wave time 0.66 is 1/3. If you use foam coax or Teflon coax
then this
physical length will be different--but still needs to be 1/2 wave electrically.
A coax balun is easy to make--but it is not a broadband device. A toroidal
transformer can be more problematic--you need to understand the power
handling capability of the core and issues like current or voltage
drive--but it
is broadband. So you can buy a broadband toroidal balun and it will probably
work if the designer does his homework. But you can make a narrow band
balun from coax (it only works at the frequency that it is 1/2 wave long
(or odd multiple to be correct--but no one uses them in that mode).
So if you think about the balanced input of an antenna being driven by
X voltage (referenced to ground) and this same voltage is also applied
to the 1/2 wave of coax and this same voltage (1/2 wave away) is also
applied to the other feedpoint (again referenced to ground) then the
differential voltage from feed point to feed point is TWO-X. So once again
we have doubled the voltage--and therefore the current must be half--so
the impedance is transformer by this ratio SQUARED. So the
impedance (for a 50 ohm system through a 1/2 wave coax balun) is 200 ohms.
I have tried to simplify this to keep it short--so I can go to bed. But this
should give you some idea of what is going on for your questions.
BTW Technically the extra 1/2 wave of coax in a 50-200 ohm balun SHOULD be
100 ohms--since it is hooked to a 100 ohm point on the antenna. But a
property of mis-matched coaxes that are 1/2 wave multiples is that the
mis-matched impedance is repeated every half-wave. So the 100 ohm
1/2 wave of coax has a 2-1 SWR on the line--but the cable is short and
the added loss of the mismatch is relatively insignificant. And when the
two 100 ohm points are paralleled where the coax from the shack hooks to
1/2 of the antenna and to the 1/2 wave piece of the antenna this becomes
50 ohms and is 1-1 back to the shack.
Don't be discouraged--there is a lot of this type of stuff that only becomes
apparent after you have played with things for some time. There was
no single book that tells all of this--you just have to read and assimilate
the info until it all fits together.
I hope this helps a little bit. I am too tired to go back and proof read it--
so just hope I didn't make any serious typos tonight.
Hey, if it was easy everyone would be an antenna expert or a rocket
scientist--or whatever! ;-)
73 es gl John W0UN
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