[TowerTalk] Open Wire Feed

[K7GCO@aol.com]

In a message dated 10/4/00 10:52:48 AM Pacific Daylight Time, aa4lr@radio.org 
writes:
 On 10/1/00 6:19 PM, K7HPH at k7hph@xmission.com wrote:
 
 >I have an 80 Meter Dipole fed with 450 ohm ladder line. At my
 >new house, I must run my coax rotor cable, etc. through a 20 foot 3 inch
 >conduit to get to the shack. I am thinking that I need to terminate the
 >ladder line to a balun designed for such applications and run the coax
 >output through the conduit to the antenna tuner to run multi-bands on the
 >dipole.
 
 This is known as a remote balun. I have a similar installation at my 
 house. It works great. A few notes:
 
 1) Use only solid dielectric coax for the tuner to remote balun run. 
 Since you may have a complex impedance on this run of coax, it is 
 important to keep the voltage breakdown of the coax high. Foam insulated 
 coax has a much lower voltage breakdown. I use RG-213 for this run.
 
 2) Use the shortest length practical for the coax run. Again, with the 
 complex impedance, you're going to incur greater loss in the coax. Keep 
 it short to minimize losses.
 
 3) Use a big balun. Typical toroidial balun cores can get lossy when 
 dealing with complex impedances. Don't fret about the turns ratio -- with 
 a complex impedance, a 4:1 or 1:1 works equally well. Use a core that is 
 much bigger than you would for a resistive load. I run 100 watts, and use 
 a 2 kW balun. use a 5 kW design for a full gallon. Check your 
 installation on multiple bands with power. You shouldn't see any 
 appreciable heating on any band.
 
 While the remote balun doesn't have the same low-loss characteristics of 
 an all-open-wire installation, it is much easier to route the 
 transmission lines into the structure, and retains many of the advantages.
 
 Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org

    Bill: Excellent suggestions.  However what is the source of validity that 
solid dielectric has a higher voltage break down than foam coax?  It might be 
right but foam is at least half air in the dielectric and the center 
conductor is only about .02" larger.  Has anyone actually conducted the 
proper test on this? 

Running ladder line in say 3" PVC tubing with centering insulators or spacers 
through walls works real well and has the least loss of all.  I eliminate 
baluns where ever I can.  There are some porcelain 3/8"x6" insulators I've 
run #10 insulated wire through in walls many times.  6" of wood around open 
wire line is no big deal.  It's just another long insulator.  Just keep it 
away from metal.  K7GCO  
             

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