The method of putting beads on coax on the low Z side of a balanced network
that matches open wire feed to low Z unbalanced is widely used now, i.e.
that's probably the method employed in all of the commercially manufactured
balanced tuners.
For transmatch components, you can get new air variables and inductors from
Cardwell, and nice roller inductors and capacitors used from Surplus Sales.
Of course, they aren't cheap. that's why we have flea markets.
Someone wrote something to the effect that open wire feed works with dipoles
because the feedpoint Z is less than the characteristic Z of the feedline.
That's not relevant. The open wire feedline doesn't care what the f/p Z is.
the rf current generates fields that no longer collapse when the two
sides of the feed diverge to opposite directions and cease being parallel
with each other.
While I'm on the subject of antennas, someone wrote a while back that they
only use resonant antennas and used the Gap Titan as an example. The Gap
Titan is probably never resonant on any ham frequency. It is a kind of
bizarre combination of off center vert. dipole (the feed point i.e. the
"gap" is only about 10 feet from the top) fed with stubs, a capacitor, and
using linear loading (what Gap calls "tuning rods") and a bottom hat on 40
m. to present an acceptable vswr at the end of the yellow pigtail coax they
supply with it. I know because I have one. What I suspect is going on is
that some folks are confusing a low vswr with resonance. the two are not
related unless by coincidence, you have a feedpoint Z of pure resistance at
some frequency that is the same as the characteristic Z of your feedline,
and also happens to be the design impedence of your vswr measuring equipment
and PA tank circuit output.
the open wire feedline doesn't care about mismatch and vswr because at any
point on the line the currents and voltages are equal and opposite; the
electromagnetic fields moving around each side perpendicular to the
direction of current flow revolve about the line in opposite directions,
oppose each other, collapse and that results in no loss to radiated energy.
(all this works the same way on rx too.) Bizarre R and X on the line which
vary constantly with frequency and location on the line doesn't matter IF
the I and E on the line are always equal and opposite (i.e. balanced). You
can kind of think of it as extending the feedpoint of the balanced antenna
such as a center fed dipole down to the shack. It's important to use a
balanced antenna. Avoid anything asymmetric such as off center fed dipoles,
or end fed zepps. Loops are usually okay. So, the only problem for the
ham, is transforming the usually high Z on the line down to 50 ohms.
With genuine open wire line, the dielectric is air which is very difficult
to heat, so heat related losses due to high standing voltage resistance is
minimized. So is the risk of high standing v. flashover, two things you may
have to worry about with coax which can get hot, melt, flashover or
experience center migration.
Coax works reasonably well, when the ends are terminated in a Z that is
reasonably close to the characteristic Z of the line, usually around 50
ohms. That's because even though it is unbalanced, R and X are fairly
constant along the line, ditto for voltage and current, and radiation is
minimized due to current and field being internalized to the surface of the
center conductor, the dielectric and the inner surface of the cylinder that
surrounds the dielectric.
Let's say you divide your coax outside and connect the center to one side of
parallel wire feed and the shield to the other. it's not unusual for the Z
at the end of the twin lead to be for example, 1500 ohms. several things
happen on your coax: The extreme between v. max and v. min is wide (high
vswr). You have high coax loss, heating, and depending on ur power level,
maybe even a flash or migration. you will have a lot of reflected power
and power lost in the coax. you are better off with a wide range matching
network between the parallel line and low Z coax.
rob / k5uj
_________________________________________________________________
On the road to retirement? Check out MSN Life Events for advice on how to
get there! http://lifeevents.msn.com/category.aspx?cid=Retirement
|