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Author Topic: Zip Cord Feedline?  (Read 1896 times)
KC2VDM
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« on: March 25, 2012, 04:43:18 PM »

I've been reading up on some antenna designs and heard about lamp cord (zip cord, whatever you want to call it) being used as feedline. I read an article from an old QST about it, and I think I'll try it sometime soon. I'm just wondering if anyone else has done it. How well it worked, and if it's worth trying.

-Alex
KC2VDM
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W1JKA
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« Reply #1 on: March 25, 2012, 05:19:03 PM »

  I have made and used several of theses zip cord dipoles for portable qrp operation only.Quick and cheap to make.I connect one feedline wire to radio antenna point and other wire to radio case ground,no bnc or other connectors needed or baluns,they work just fine.you will see all kinds of these on the various qrp antenna websites.Swr,efficency,rf and other issues? I don't know,I don't care because for qrp power they work.Try one and see for yourself.  73  Jim
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W5DXP
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« Reply #2 on: March 25, 2012, 05:21:21 PM »

It "works" along with high copper and insulation losses. If that is acceptable to you, then do it. Personally, being interested in high efficiency, I wouldn't use it and don't know any other ham who does.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W1JKA
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« Reply #3 on: March 25, 2012, 05:36:17 PM »

W5DXP: Cecil,I respect your opinion on this and your other post and learn a lot from them.For me these zip cord dipoles depending on conditions work just as well as my par end fedz and  homebrew efhw antennas,yes they are all compromise antennas of a sort.73  Jim
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K0ZN
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« Reply #4 on: March 25, 2012, 07:51:45 PM »

Hi.

I am currently using about 40+ feed of BROWN lamp cord ( from Home Depot ) to feed my 30 M dipole. I have used it several times before on dipoles with very good/satisfactory
results, but only short (less than 50 ft.) lengths. Based on results and operation, which has been perfectly good and totally comparable to coax feed, I can't see
any problems or signs of loss. BUT... that is with BROWN lamp cord.

For some reason, the white colored stuff DOES seem to be a problem and cause losses, so do some of the clear "vinyl" insulated zip cord/speaker wire, etc.
I have played/experimented with several types of these wires. There definitely IS a difference in how they perform as a low Z transmission line.
I suspect there is a lot of zinc oxide in the white cord....not sure why the clear stuff is so bad, but it was; all signals were several S-units lower when I used
this line. I have not had that problem with the brown line, such as I have on this antenna. I can work what I hear and frequently have DX QSO's with very weak
signals both ways. That would NOT be the case if line loss was high or excessive. Again, all that said, I would not want to run 150 or 200 feet of it !! ...but
at 40 ft. lengths, it sure seems to work well for me.

Note that the impedance of "Zip cord"/lamp cord is about 90 to 100 ohms.  My 30 M dipole is at a height that gives about 90 ohms of feedpoint impedance and
the 90 ohm lamp cord impedance works beautifully. I use a 2:1 balun between the end of the 90 ohm lamp cord and the 50 ohm coax. Depending on the height and feedpoint impedance of your antenna, zip cord with 90 ohms of impedance may yield a moderately high MINIMUM SWR if your antenna has a lower impedance. Remember that
height above ground is primarily what determines the feedpoint impedance of a dipole and it runs from about 40 to 90 ohms depending upon height.

One way to use the zip cord if your antenna has a lower (~40 to 55 ohms feedpoint Z, due to height) is to feed it with an ELECTRICAL Half Wave of the zip cord.
There is no way of knowing the exact velocity factor of zip cord, but I would *guess* it is probably around .80 or so....just a guess. A half wave of line
repeats the impedance of the antenna feedpoint, so even if the antenna has, say 45 ohms, and the zip cord is 90 ohms, a half wave of it would show
a Z of about 45 ohms and could be connected to a 1:1 balun with good results.

Obviously, the line/cord you used MAY BE DIFFERENT.  No guarantees in this game !

BOTTOMLINE:  From what I have experienced, the BROWN zip cord seems OK for shorter lengths of line.  

(TRY it and see how it plays. The stuff is pretty cheap.)

73,  K0ZN
« Last Edit: March 25, 2012, 08:00:36 PM by K0ZN » Logged
WB6BYU
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« Reply #5 on: March 25, 2012, 08:22:06 PM »

I used zip cord to feed a full wave 40m loop when I was in Alaska and broke a
pileup on Okino Tora-shima with 2 watts (about 2000 miles/watt).  The higher
impedance worked fairly well with a loop.

The characteristics (for at least one type of Zip cord) are included in VK1OD's
transmission line loss calculator here:  http://www.vk1od.net/calc/tl/tllc.php
You can check out the losses with various impedances at different frequencies.

Loss is about 2dB / 100' on 80m and close to 3dB on 40m.  It is 3.5dB on 30m,
so keeping the length to 50' or less is a good idea.  Losses increase quickly
from there, to about 6dB / 100' on 15m (with a perfect 105 ohm load, half
a dB higher in a 50 ohm system.)


I've also used some thin vinyl-coated speaker wire as a feedline.  I don't know
what the impedance was, but I could never get the dipole to load properly on
40m when I was using something close to a quarter wave of feedline. 
(If the dipole was 50 ohms and the feedline 100 ohms, the rig would see
200 ohms.)  It was rather frustrating to sit in the middle of the wilderness
and listen to KC4AAA at the South Pole booming in but not being able to
get full output from my HW-8 due to the poor impedance match.  (Getting
the antenna higher wasn't an option - there was only one tree on the
hilltop.)


I now use RG-174 coax for my backpack feedlines - the losses are lower than
for Zip cord (about 2.1dB / 100' on 40m) and it has a 50 ohm impedance.  And
it is smaller and lighter than zip cord.


I'd say it is worth trying if you want to experiment, or if that is what you have
on hand.  You're likely to get better results on 80m and 40m than on 15m and
10m due to the high losses, but if you are running 100W into 100 feet of the
stuff you're still radiating more power than I do with my 5 watt station, and
I make plenty of contacts.  Just plan for the 100 ohm impedance and you
should be OK.
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KC2VDM
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Posts: 145




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« Reply #6 on: March 25, 2012, 08:27:44 PM »

Thanks for all the info guys! I'll have to give this a try sometime soon. I'll remember to look for brown cord.
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W5DXP
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« Reply #7 on: March 26, 2012, 05:36:28 AM »

For me these zip cord dipoles depending on conditions work just as well as my par end fedz and  homebrew efhw antennas, ...

Like I said, being an efficiency freak, I would not use any of those antennas.Smiley For efficiency, it is really hard to beat an open-wire-fed dipole.
« Last Edit: March 26, 2012, 05:40:24 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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