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Re: [TenTec] Open wire feed

To: tentec@contesting.com
Subject: Re: [TenTec] Open wire feed
From: "Rob Atkinson, K5UJ" <k5uj@hotmail.com>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Wed, 01 Feb 2006 18:48:18 +0000
List-post: <mailto:tentec@contesting.com>
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

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