[TowerTalk] (no subject)

[K7GCO@aol.com]

The Summary of open wire spacers by W8JI is right on.  Johnson had great 
ceramic white spacers in the early days of open wire line that lasted forever 
and I still see them at the flea markets.  I like Delrin the best of current 
insulators.  One of the worst ever for longevity recommended is the bit of 
using wooden dowels boiled in paraffin.  They just don't last and did a lot 
to reduce the use of open wire line.  Wooden tung depressors are cheap 
initially but crack easy and have far more wind resistance.  I had a Lazy H 
on 20M using wooden dowels boiled in paraffin for spacers.  Running 600W I 
could hear a cracking noise in 4" spacers.  Use 3/8" delrin or teflon.  6" 
spacing gives around 600 ohms and a high resistance spacer.  Drill holes in 
the end for the preferably insulated or at least enameled #12 or larger wire 
and secure spacer with wire pigtail, epoxy or RTV silicone rubber.  I make 
some with a SS screw clamp.  Or put little bends in the wire on each side to 
keep them from moving.  Velocity factor is around .98 for spacers every 3'.  
It's lower with more spacers.  They can be wider on straight runs but uneven 
spacing just doesn't look neat.  Where it bends I use one every foot like in 
the shack.  Where coax wire spacing needs to be constant to maintain 50 ohms, 
open wire spacing can actually vary all over the place.  The end result of 
all this is to alter the Z at the end of the feedline some and it's matched 
by the tuner for full transfer of power.  High SWR's are no problem with open 
wire line with the right lengths and tuners.  The low loss doesn't change and 
it ignores rain.  It can't be buried and has to be supported on insulators 
and kept at least 1 foot from metal objects.  I've replaced glass windows 
with plexi glass and installed feed through insulators.  Now I just drill a 
small hole for the wire.  I have also just drilled a 1/4" hole in the 
basement window jam and floor at the corner and run the insulated wires 
through it into the shack.  This just alters the VF slightly which can be 
compensated for.  GD the line shorted on both ends when installed and adjust 
the length for the desired 1/2 wave multiple frequency as suggested below.

I made up an open wire line with 1/4" copper tubing and bent it into a sine 
wave shape.  I told every one "I ran high power and this resulted due to a 
high SWR".  With a straight face they walk away mumbling to themselves.
  
There is a "Magic Length" that reduces odd impedance loads.  It's a 1/2 wave 
or multiple on all bands 80-10M or very close.  It's calculated by 492 X VF 
(.98) / 3.562 = 135.4'.  It's a 1/2 wave at 3.562 and multiples at 7.125, 
10.688, 14.25, 17.812, 21.375, 24.937, & 28.5 MHz.  If you short both ends 
and support it at least 5' off the ground, it will grid dip right on, on all 
bands.  The Magic Length for .66 VF coax is 91' 2". 

It's great to see the interest my Post on open wire line seems to have 
created.  Coax has many problems, high costs for low loss and longevity 
problems. The right tuner is still the key with open wire line for the 
maximum utility, longevity, least cost and I've used them all.  Some of 
ARRL's suggested tuner circuits don't work well.  Their constant 
recommendation of "any length", then using a 4:1 balun and wooden dowels 
boiled in paraffin is in the Top 5 All Time Dummities ever printed.  Open 
wire length is critical as it controls the Z at the end of it the tuner has 
to match.  Using lengths that are multiples of 1/2 or even odd 1/4 WL assures 
a resistive and low reactive load for the tuner.  Either Hi or Lo-R loads are 
easy to match but high reactive values cause tuner problems.  Ground rules 
are basically simple but space and lack of diagrams possible here limits 
proper coverage to the wonderful land of open wire line and tuners that has 
lacked coverage, really perform and are interesting to use.  The ARRL "Series 
Tuner Circuit" for Lo-R values seldom works well and also needs a variable Xc 
in the link circuit to ground or other lead of a 70-600 ohm balanced line as 
does the parallel circuit for Hi-R loads.  Link circuits that connects to the 
shield of coax and chassis creates a lot of RF floating around in open 
tuners--bad circuity.  Enclosed RF tight tuners keep it inside like the 
Johnson MatchBox.  In open tuners I have fed the link circuit with a 3 band 
no toroid no loss 1:1 balun.  It's one of the best tuners with just 2 knobs 
to adjust that doesn't need a variable Xc in the link circuit for most loads. 
 Adding a switching circuit that allows selection of the number of turns in 
the link like 2, 2.5, 3, 4 & 5 turns adds Z matching range.  

The MatchBox can be used with balanced or unbalanced loads.  The KW model has 
a compromise bandswitch shorting connection on 10&15M.  The 275W model has no 
compromise bandswitch connections and will take 600W of SSB on 10M and more 
on the lower bands.  There was a series of them with the 50 ohm link tap on 5 
turns and the 300 ohm tap at 2 turns.  Check this. If 1:1 SWR is obtained by 
adjustment of the 2 controls, maximum power transfers--most of the time.  
There are some loads that this is not true and arcing can still occur in the 
Xc or even the band switch.  Here is the trick that solves the problem.  For 
these odd loads, change the tap on the link or reverse the connections of the 
differential Xc.  I actually added a well insulated large knife switches to 
do that quickly. They are available at the hardware stores or use clip leads. 
 I also add RF ammeters in each output lead.  I have small knife switches 
shunted across them to extend their range.  Or use small light bulbs with 
clips to connect across certain lengths of the open wire line.  Tuners work 
great into balanced 100 ohm coax but there is a concept you need to know I 
discovered that can occur space doesn't allow here with a simple solution.  I 
have built MatchBox equivalents with plug in coils, from 2 rotary inductors 
with a differential gear between them to get the right rotation and made my 
own variable differential Xc from 2 split stator variables of 100 uufd each 
side. 

There is a balanced L tuner circuit I've developed that I really like for 
many reasons.  It also has a reactance canceling feature that can give 1:1 
SWR over the whole band and has to be seen to believe.  Some see it and still 
don't believe it.  I also have a neat circuit with the 2 variable Xc's and 
inductors all on 1 shaft for a "1 Knob Quick Tuner".  I use the same 
mechanical quick tune 1 knob idea on an Unbalanced L for Lo & Hi Z loads.  
I've shown these at various conventions and club talks since the late 40's.  
When I get time I'll write them up for the mags.  With open wire line and 
balanced coax feedlines, the world of no baluns and lossy toroids is nice.  
With all the coax problems that continually appear on TT, it seems that the 
ham public is finally receptive to a better way to transfer RF.  In 60 years 
I never found a better way or wasted a lot of money on coax except for short 
runs and the shack.  Many have to be dragged and kicking screaming into 
better techniques--into the last Century.   K7GCO   

In a message dated 20.04.00 14:39:04 Pacific Daylight Time, 
w8ji@contesting.com writes:<< 
 
 > The hard question about open wire line is - what to use for insulators?
 
 I went through that two years ago, when I was installing a 1500 foot 
 long open wire line. 
 
 I bought UV resistant Lexan and mostly used it, but I also installed 
 sample pieces of other materials of various types.
 
 I have a bench punch, and was hoping to find a material I could pop 
 holes in instead of drilling. No such luck. So I drilled and cut.
 
 My line is number eight bare copper with 2.75 inch centers, 450 
 ohms. Here's what the score is over a year later in the Georgia sun:
 1.) The gray PVC surface powdered, and held water. It carbon 
 tracked and failed by shorting.
 2.) The white PVC is still OK, but breaks (cracks) easy.
 3.) The regular Plexiglass? (I found a 4X8 sheet 3/8 inch thick  
 along the road) cracks when stressed.
 4.) The regular plastic fell apart.
 5.) The nylon is still OK
 6.) The UV resistant Lexan (1/8 inch thick) is still good, and never 
 has broken. The #16 tie wires break before the insulator does. 
 That's why I like it best.
 
 By the way, I've melted both plexiglass and PVC in an open wire 
 line (#16 wire) feeding two half-waves in-phase on damp days. Don't 
 know if it was more of that surface powdering trapping dirt and 
 moisture or what because the insulators just totally melted. 
 
 I guess what someone uses would depend on what they are 
 actually installing and how long they want it to last and how high 
 the voltage is on the line.   73, Tom W8JI  w8ji@contesting.com
  >>

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