Re: Digest for hamnerds@googlegroups.com - 2 Messages in 1 Topic

[john nowacki]

Thanks Tom, I like the idea.

73, John  W3NA

On Thu, Jul 19, 2012 at 9:16 AM, <hamn...@googlegroups.com> wrote:

  Today's Topic Summary

Group: http://groups.google.com/group/hamnerds/topics

• Simple, cheap, fast way to make open wire feed line [2 Updates]

 Simple, cheap, fast way to make open wire feed line

Thomas Liebert <tbli...@gmail.com> Jul 18 12:31PM -0400  

I've discovered a simple, cheap, and fast way to make open wire feed line.
 
You need the following:
 
1) Bare copper wire (I got 100' of 14 gauge from KN1JLK (thanks, Kevin!))
2) 1/4" polyethylene tubing. Lowe's has 25' for $2.96:
http://www.lowes.com/pd_15275-104-SPEB25_0__?productId=3130913&Ntt=polyethylene+tubing
(note their website says it's 20', but it's actually 25')
3) A bag of 8" cable ties. Lowe's has a bag of 100 for $4.95:
http://www.lowes.com/ProductDisplay?partNumber=76328-1781-46-104UVB
I found bags of 100 at Big Lots for $2.50.
 
Cut the polyethylene tubing into short spacers (I made mine 2" long).
It's really easy to cut; I put it on a cutting board and used a big
kitchen butcher knife.
You can speed it up by first cutting a bunch of 12" pieces, then
cutting them in parallel to get the 2" spacers (kind of like cutting
the ends off a dozen
roses before putting them into water.)
 
Once you have a bunch of spacers, repeat the following: Pass a cable
tie through a spacer, around one wire, back through the spacer,
around the other wire, through the "ratchet", then tighten as hard as
you can with pliers. Clip the excess "tail" with a pair of dikes.
 
Spread the spacers out along the wire...seems like every 8" or so
works well. If you cinched down the cable tie tightly it should stay
put, but if you're worried you can put a dab of glue on one or both
sides of the spacer to keep it in place.
 
That's it!
 
If anybody knows a cheaper or faster way to make open-wire feedline,
I'd sure love to hear about it. (OBTW, I found the basic idea in a
YouTube video, but
that fellow used plastic pen bodies for spacers. My innovation is to
use 1/4" polyethylene tubing instead.)
--
73 de Tom NV1T
 
(603) 812-6777 Cell
506 Sherburne Road
Portsmouth, NH 03801
Lat/lon: 43.0652 -70.8059 Maidenhead grid: FN43ob


 

"AI2Q" <ai...@roadrunner.com> Jul 18 08:44PM -0400  

Yup. I've seen this done, Tom. Poking the cable ties makes it fast and cheap
and, well ------ effective!
 
I prefer my traditional approach, but maybe that's just 'cuz I'm partial to
aesthetics. HI HI.
 
Vy 73, AI2Q, Alex
 
----- Original Message -----
From: "Thomas Liebert" <tbli...@gmail.com>
To: "AI2Q Alex Mendelsohn" <ai...@roadrunner.com>
Cc: <hamn...@googlegroups.com>
Sent: Wednesday, July 18, 2012 12:31 PM
Subject: Simple, cheap, fast way to make open wire feed line
 
 


 

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[Thomas Liebert]

I made a short piece as a proof-of-principle, and it doesn't look too
bad (see attached).

Questions:

1) My spacers are 2" wide. Is that too wide, too narrow, or about
right? Does it make much difference?
2) How far apart should the spacers be? I'm thinking 8" - 10" should
work fine. What spacing did you use on yours, and why?
3) Does it matter if the wires are bare or insulated? I have about
350' of 14 gauge insulated copper, and I don't want to have to strip
it.
4) Does it matter what gauge the wire is; i.e. is there any reason not
to use the 14 gauge?

-t

[AI2Q]

 

Tom, with respect to making your own open-wire homebrew transmission line, I think it's a great idea.

 

14-ga. wire seems a tad heavy to me, as the cumulative weight of the line may cause the antenna center to sag if not supported. In my case, I have a center insulator support on my doublet, so that's not a problem, but it could be, as 14 gauge wire is fairly heavy and might be overkill for the application. It's up to you to decide. If you support the feedline periodically along its length with insulators (as NE1S does), that is a solution.

 

Insulated wire should be just fine, and it affords a great degree of weather proofing.

 

Spacing between placement of the spreaders is entirely up to you. The thing is to avoid twisting that could cause shorting, and to keep the weight of the insulators appropriate (same reasoning as to the use of heavier vs. lighter wire for the feedline).

 

A spacing of 2-in. would be fine, ensuring that you could run a kW into the line and not have to worry about arc-over at high-VSWR points. OTs used to use 6-in. spacing, but I personally feel that such wider spacing buys you little, and may affect how the line works at higher frequencies, such as the 10M band. The ratio of conductor diameter to spacing establishes the characteristic impedance of the line---but who cares?

 

Unlike the coax cable devotees, who worry about impedance matching at the antenna, you're going to disregard the complex impedance presented by the antenna and feedline, and compensate for it at the "tuner," which really does not "tune" your antenna at all. What the tuner does is transform the impedance of the line at the shack into 50-ohms unbalanced, so the 50-ohm port on your transceiver looks into 50-ohms resistive (with current and voltage in phase).  If the antenna feeder exhibits a capacitive reactance (as well as a resistive portion), the conjugate is presented by the "tuner."

 

Some people use baluns to force the tuner to "see" a balanced load, but this is a stopgap approach and far from optimum. The balun is really only happy over a very small range of impedance mismatch. For example, if it's a 4:1 balun, and one side is 50-ohms (the rig) the other side should be at 200-ohms balanced. But, the open-wire feeders may present a 1000 ohms reactive to the balun, and it won't be happy.

 

Also, a balun core can arc at high voltage points, and it can magnetically saturate if overloaded (over powered).

 

A better approach is to put the balun at the input of the tuner, and let the output side of the transformer see the tuner. Then the tuner can transform the balanced unknown impedance to a balanced 200-ohm impedance.

 

Better than that, though, IMHO, is the old fashioned technique used by hams in the 1930s; use a completely balanced tuner with no baluns whatsoever. That's what I use, and it got me on the DXCC Honor Roll (no rotary beam required--HI HI). I have a homebrew balanced tuner that works that way, and also use an old EF Johnson Matchbox, which is also entirely balanced and uses no balun cores.

 

Hope that helps.  Take a look at the QST article that was circulated recently that showed the three approaches (balun at output, balun at input of tuner, and no-balun completely balanced network).

 

Lastly, think porcelain and glass, especially where the feeders pass through walls, windows, etc.

 

Vy 73, AI2Q, Alex
Member: ARRL, FOC, RSGB, CWops, QRP-L, Antique Wireless Association, New England Radio Discussion Society, DXCC Honor Roll, 16ØM DXCC, 8ØM DXCC
http://home.roadrunner.com/~alexmm

 

 

[Thomas Liebert]

Thanks for your thoughtful and thorough response! I've got some
reading to do...

-t