[TowerTalk] Open Wire Spacers - Summary II

[Bill Coleman AA4LR]

Well, my summary generated more comments, some in private e-mail. So, in 
the interest of completeness, here's a subsequent summary, integrating 
more feedback:

--

A week or two ago, when the merits of open wire line were discussed, I 
noted that the biggest impediment to using REAL open wire line is the 
requirement of fashioning spacers. Here's a summary of suggestions:

Material:

* Dowels boiled in Paraffin - (Not to mention the relative dangers of hot 
paraffin) This method has been mentioned in the ARRL handbook, as well as 
other articles on open wire. I believe this the the classic method for 
making spacers. K7CGO indicates he has had failures with this type, 
likely due to moisture and/or carbon tracking.  

* Tongue Depressors - Another classic. K7GCO says they crack easily. 

* Fiberglass Rod - This should be durable, so long as they are protected 
from UV. KC4UZ indicates mgs4u.com sells solid fiberglass rod. Cutting 
and drilling the rod may be a bit more difficult

* Delrin or Teflon Rod - GM3POI says that obtaining 10mm (3/8") PTFE 
(Teflon) rod isn't difficult to find. It's water resistance is 
recommended. K7CGO suggests looking in the Yellow Pages under plastics - 
his experience with Delrin spacers is quite good.

* 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. VK4JUD found a source of stuff PVC tubes that are risers for 
garden spray systems. UV proof and very inexpensive.

* Plexiglass - W8JI found it too brittle to work. Also failed when wet.

* UV Resistant Lexan - W8JI found it was best in his tests. W7GK says you 
can obtain 3/8" stock rod from electric sign makers. 

* Nylon - W8JI found it works OK.

* Ceramic - Great spacer material, but heavy. A traditional spacer 
material. According to W3BV, these are availble 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.

* K3HX noted a QST hint (QST AUG 86) to use 120-size plastic film spools. 
Free for the asking at professional film houses.

Construction:

* 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 coathangers 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 coathanger. 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.

Miscellaneous:

* K7CGO mentioned that sometimes tuners have trouble matching the complex 
impedance present across open wire. Sometimes a series Xc or Xc to ground 
is needed in link-coupled tuners, or change the link tap.

* K7CGO also mentioned you should take care and space open wire far from 
metal objects. He recommends 1 foot minimum.

Thank you everyone for your suggestions! Lots of creative folks on the 
list.





Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


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