[TowerTalk] (no subject)

[K7GCO@aol.com]

In a message dated 04.05.00 04:41:47 Pacific Daylight Time, aa4lr@radio.org 
writes:<< 
 * Dowels boiled in Paraffin - (Not to mention the relative dangers of hot 
 paraffin) this is a classic method. K7GCO indicates that these don't last 
 long, and I would tend to agree. 

******I didn't say that.  I said I heard a crackling noise in 4" boiled 
paraffin spacers with 600W on 20M--inside and outside the shack.  There was 
moisture apparently trapped inside the dowels forming a conductive 
patch--with noise.  I should have cut one apart.  Translation: It's a bad 
insulator.  There is nothing Classic about it (Classic Dumb maybe) and they 
do last if properly supported.  It was just a cheap and light bad insulator 
when ceramic insulators that I still have after 60 years were available.
 
 * Tongue Depressors - Another classic. K7GCO says they crack easily. 
 Dowels would probably be a better choice.
 
*******I didn't say Dowels (wood inferred) would be a better choice.  Delrin 
dowels are a better choice.  See above.for obvious reason.

 * Delrin or Teflon Rod - I don't know where you get these, but it sounds 
 good. 

*******Try the yellow pages under "Plastics." 

 * PVC - W8JI ran tests. Gray PVC hold water as it weathers - not 
 recommended. White PVC is good, but gets brittle and cracks. Can fail 
 when wet.
 
 * Plexiglass - W8JI found it too brittle to work. Also failed when wet.
 
 * UV Resistant Lexan - W8JI found it was best in his tests.
 
 * Nylon - W8JI found it works OK.
 
 * Ceramic - A classic, but they are very heavy and difficult to find.

********Ceramic open wire line spreader insulators are available from Daburn
 Electronics & Cable Corp. [224 Pegasus Ave. Northvale, NJ.  (201-
 768-5400) ] in 2", 4" and 6" sizes. They come in packages of 25.   Alan, 
W3BV  >>
 
 * K3HX noted a QST hint (QST AUG 86) to use 120-size plastic film spools. 
 Free for the asking at professional film houses.

 ******Those are ideal for the 400 ohm range of Zo of open wire line.
 Construction: Has anyone tested this plastic for UV etc?  Buy the Ceramic or 
use 3/8' Delrin
 
 * N6NZ came up with a neat idea: Take flat plastic pieces and cut them in 
 a stack:
 
 +---------------+
 |               |
 |   |       |   |
 +---+-------+---+
 
 Slap two of these together on the wire in opposite direction, and hold 
 together with cement, or use a UV resistant cable tie.
 
 * My variation on N6NZ's idea: Cut slots on opposite sides
 
 +---+-----------+
 |   |           |
 |           |   |
 +-----------+---+
 
 Cut slots slightly over center, then hold as above, or drill a hole in 
 the center and hold together with a screw and nut (makes the spacers 
 reusable)
 
 * K6LL provided an excellent suggestion: use plastic coat hangers for 
 material. But the interesting idea was his construction technique. Set up 
 a little drilling pattern, alternating little hole, big hole, little hole 
 (o O o      o O o) in the straight pieces of the coat hanger. Then, using 
 a diagonal cutter, cut in the center of each big hole, which will leave a 
 slot for the feedline wires. The
 tie-wires go in the little holes. This method could easily be used with 
 any type of plastic rod or narrow plastic sheet.
 
 Thank you everyone for your suggestions! Lots of creative folks on the 
 list.
 
 Bill Coleman, AA4LR, PP-ASEL    

******* A long life open wire line is only half the design goal.  The tuner 
used, the multiple Z's of the load and the length of the feedline is more 
critical than with coax use.  Since SWR's can be high, the reactive part of 
the Z at the end of the feedline can be hard to match with the wrong length 
of open wire feedline.  ARRL often shows the link circuits without a variable 
Xc to ground which illustrates they never used what they print.  I often use 
a no toroid 3 band balun to feed the link also.  The Johnson Match Box is one 
of the best tuners for balanced or unbalanced loads.  I have others. I use RF 
ammeters or shunted light bulbs in the output.  It doesn't need a series Xc 
in the link--most of the time.  It's a good idea to be able to change the 
link tap for the coax from 2,3,4-5 turns if it has trouble matching a load.  
If it still won't match a load, reverse the connections on the differential 
Xc.  I use a well insulated DPDT knife switch.  Use the configuration that 
gives the maximum output even though both may give 1:1 SWR.    K7GCO.

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