Discussion in 'General Technical Questions and Answers' started by KC9GUZ, Sep 15, 2006.
I had a local tell me he once made a dipole using lamp cord as the feedline. Has anyone tried this?
it may work to some degree, but I think the spacing might be too close for some things.
Yip. It's not the lowest loss stuff in the world, but I've used it for portable operation. It's about 75 ohms.
was in common use over 50 years ago
its a balanced feed line same as open wire
only closer spacing
with more line loss
and lower imp about 75 ohms
works ok on lower bands
line losses get you above 40 meters
remember you can use most
any kind of wire for a feedline
and you can bet its been tryed
ive used junk small gage
twisted pair telephone wire for
feed line on a 80 meter dipole
it worked fb
Twisted pair can be very low loss up to >600 MHz if it's twisted right.
CAT-6e, for example, is nothing more than twisted pairs and is rated to 600 MHz. It contains radiation very well and maintains constant impedance up to about that frequency, without shielding.
"Any old" twisted pair works pretty well on HF.
Yup, the answer has been pretty well given, but this question does come up occasionally. Another thing about lamp wire is that some of it is not the least bit resistant to Ultra-Violet. The insulation will crack and break off after just a very short time of exposure to the elements. But as has been said, for portable, short-term operation on the low bands, it is useable!
Ladder line is a far more suitable alternative, can be rolled up into a fairly small bundle, and works at much lower loss if you are planning to use it either on a higher band, or for a more permanent installation.
Good luck! 73, Jim
Seems I've read that the impedance runs something like 35 ohms. It sure is easy to handle.
73, JP, K8AG
Like every other transmission line, the impedance would vary as a relationship between wire spacing, and wire size. In other words, a piece of 16 gauge lightweight lamp cord would have a different impedance than a piece of 12 gauge heavyweight extension cord!
The impedance of the cord would not be all that much of a factor, though, because you would probably be feeding the antenna through a balun/tuner setup, and unless you went really wild with the thing ( 10 feet of dipole on 160 meters ! ) and you are able to match the setup to the rig, you will be able to use it. However , as Mac and others have stated, the losses DO increase with frequency, as the stuff is just not all that good at RF. After all, it was made to transfer 60hz AC , not RF!
Yep. Used lamp cord (known popularly as "zip cord" for several antennas and feedlines over the years. Once had a 75 meter dipole made of zip cord, and fed with zip cord, from a link-coupled transmitter. Worked wonderfully. I have no idea what the impedance was! Or the SWR. All I know is my weak (20 watts input, Central Electronics 20A barefoot, couple thorugh a homemade tuner) got out very nicely.
Go for it.
Yeah, I imagine the modern plastics used in zip cord are a LOT better than the old rubber of yore. Probably almost as good as "official" twin lead.
Someone even invented a special knot to use at the junction, so the thing didn't unzip any further under its weight. (Older handbooks had this). Hmm...I should invent a knot.
The ARRL Antenna Handbook, in its 17th edition (1994), devotes some space to an examination of zip-cord antennas & transmission lines.
The kind tested had a characteristic impedance of about 105 ohms.
So far as its use as transmission line is concerned, the conclusion is that "......a hundred feet or so of zip-cord transmission line on 80 metres might be acceptable, as might 50 feet on 40 metres. But for longer lengths and higher frequencies, the losses become appreciable".
Ahhh.. I found it. It's called the "UL Knot."
I looked at the wire table for home made ladderline in ant handbook.Made my own from 12 house wire spaced 1 in apart with uv resistant plastic bars,had some plexiglass plastic bars 1/2by1/2 by3 ft long cut each spaser abt 1.5 long,and used silicon pure the stuff comes in a tube ,its the pure clear stuff used to seal fishtanks or tubs.200 ohms,worked perfect made a 4to 1 balun where it connected to coax through the wall,1 small hole1/2in verses the 1.5 rectangle if I had decided to stay with the ladderline.
I got 100 ft of lamp cord really cheap, so I pulled the two halves apart and used homebrew spreaders and made ladder line out of it. It's been up for a couple of months, don't know how well it will hold up in the long run. Probably not well, but it was very cheap. I'll just replace it with something else when it goes bad.
I made my 20 meter full wave loop out of some multi-strand cable I had laying around here. The two strands I used were 16 ga. The other strands are smaller, but could be used if everything was supported, and the legs were not too long. What the heck, the stuff was a "throw-away" so it didn't cost me anything.
My point is that you can often find suitable wire for at least a short-term antenna project just about anywhere. Check dumpsters where they manufacture stuff, you might be lucky and find a big spool of something quite useful!
Good luck! Put together your own antennas and twin feed line, and get on the air the thrifty way ! 73, Jim
Zip cord dimensions are not that different from the 72 ohm line I used in the late 50's for the feedline for a 80M dipole that I had built. Coax was expensive and the Amphenol line was much cheaper. Worked just fine for 100W out of a DX-100. Imagine Glen remembers that stuff.
Just for grins I pulled out some real cheap zip cord and my MFJ analyzer some time back. The impedance was not bad, around 70 - 80 ohms. Seems like the velocity factor worked out to about 75%, but I may be remembering wrong.
The losses were terrible, though. From what I understand, it's not the wire or the size that's the problem, but the plastic insulation they use that causes the terrible losses.
I wouldn't use it for a feedline unless it was very short and certainly not above 40 meters. However, I didn't try any other brands, so maybe there are some with better dielectrics that would be useable.
Is there another way to measure impedance without a analyzer, maybe using a digital volt ohm meter?
Ah, I just looked up that UL knot and it sure looks familiar. I first saw it in one of those Sears-Roebuck "how to" books on electrical wiring and fixtures. Memories memories, funny how I never saw one in a commercially produced lamp, they just tied a half hitch in it.
GO, you WOULD use an ohm meter!