[TowerTalk] Open Wire Line Spacing From Surrounding Objects-Matching.

[K7GCO@aol.com]

 In a message dated 12/10/00 4:43:47 AM Pacific Standard Time, 
majrabsr@rconnect.com writes:<< 
     Hello again guys,
 
 I need to run 450-ohm ladder line from my shack in the basement to outside.
 Basement wall is concrete blocks, with two blocks showing above ground
 level.
 
 Also, how far should the ladder line be spaced from the house?  Thanks to 
all. 
 
Ron, KA9ALC
 
Ron: A good rule of thumb is to connect some string to the line in a way you 
can pull short, medium or long sections away from the house or object metal 
or wood and observe any loading changes.  Try 1' for starters.  I've 
supported 135' of open wire above the ground shorted on both ends to grid dip 
it to 3.562 MHz and it has to be no less than 2' above the ground to totally 
isolate it for the measurement.  After the open wire line is installed, short 
both ends and grid dip again for the desired frequency.  

To get outside the house I've run open wire line through 2" of a wood floor 
and 6" through a wood wall in 3/8" sleeves used years ago for house wiring.  
It's a very short Velocity Factor (V.P.) change at that point.  A better 
procedure to get outside is to replace a glass window with a plastic window 
and feed through.  The beauty of open wire line is that it can have Zo 
changes due to spacing changes or V.P. changes due to surrounding objects 
without consequences.  The affect of all deviations from a so called clean 
surrounding just alters the Z slightly as seen by the tuner at the end of the 
feedline.  Don't worry about "impedance bumps."  A well designed tuner has 
the ability to match a wide range of Z's around the expected zone.  Therefore 
the "K7GCO First Law of RF Transfer" of power from the tuner to the feedline 
is "if the tuner can match the Z at the END of the feedline and presents 50 
ohms resistive (1:1 SWR) to the rig, 100% of power transfer occurs there 
minus tuner losses and this power level minus feedline losses, occurs at the 
beam feedpoint--and away it goes.  With open wire line, SWR losses are not a 
problem due to lack of dielectric along the way.  The voltage increases in 
standing waves reduces the feedline Current "I Squared R Losses" along the 
way and is another + for it's use.  So high SWR's and high RF voltage along 
the feedline are of no problem with open wire line as they are with coax.  

The same Z matching laws of power transfer apply to coax.  It just has far 
less lee way of SWR it can tolerate.  As the feedline load changes from 50 
ohms either way, a non-optimum Rload is reflected back to the tubes or 
transistor finals and less output occurs.  In a pi-network output, more power 
can be obtained from the tubes by readjustments but at a lower efficiency of 
operating--more heat out the stack.  This is tolerable as long as the finals 
can handle it and is certainly acceptable.  50 ohm output finals just put out 
less power, dissipation goes up, the fan comes on and you CAN'T retune it 
like you can the pi or link coupled finals.  This is why SWR bandwidth is of 
greater importance with 50 ohm output (CB) finals.  

Tuners are a necessary buffer of impedance matching with open wire line to 
assure the optimum load for the finals.  Typical feedline loads at the end of 
the feedline can be a very high Z around 3000 ohms although just by adding 
1/4 wave of open wire line, a low Z can often be obtained that the final pi 
or 50 ohm final can match directly--no tuner.  The Z matching range of tuners 
in the rigs of today are limited to Z values around 50 ohms only.  If a 1.5:1 
SWR reduces power output below what's needed to drive a final amplifier or 
antenna, a tuner (an impedance buffer) can match the 1.5:1 SWR to 50 ohms for 
full output again minus the tuner losses.  The "Link Coupled Tank Circuits" 
used before Pi Networks could match any Z up to 1000 ohms without another 
tuner and still present the optimum Rp to the final tubes.  

Now days each technical advancement to final amplifiers to improve operating 
performance ends up placing more restrictions and limitations on operating 
flexibility, creative RF expression, powerductitvity (power output) and 
antenna use.  I call it "RF Socialism."  I shutter the next great "break 
through."  Long live the "Link Coupled Tank Circuit." The output power could 
be varied from low to high by just varying the Swinging Link with no other 
adjustments as long as the series Xc is properly adjusted.  Eat your heart 
out Art Collins Pi Network users. You have no idea what it was like in the 
technically free enterprise days 40-70 years ago.  K7GCO
                       
                             "Now you know the rest of the Open Wire Story"

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