ethernet twisted pair ad lightweight feedline

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eHam.net Forum : QRP : ethernet twisted pair ad lightweight feedline

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ethernet twisted pair ad lightweight feedline	Reply
by IZ4KBS on April 5, 2009	Mail this to a friend!
I'm interested in QRP operations and lightweight aerials, that are also quick and easy to erect in the field. After googling quite a bit I have not really found anything regarding the possibility of using an ethernet twisted pair as a lightweight feedline for a QRP dipole (think of the NorCal Doublet antenna and you get the idea). A single twisted pair (UTP) from a four-pair Cat5 ethernet cable is supposed to have a 100 ohm impedance, with a velocity factor of 0.7, and certainly a low-loss jacket due to the fact that it is designed to work ad 100 MHz or more. Would it be worth a try, or the fact that the two conductors are very closely spaced rules it out from the start ? (higher losses, etc). I'm very interesting in gathering skilled opinions on this.

RE: ethernet twisted pair ad lightweight feedline	Reply
by WB2WIK on April 5, 2009	Mail this to a friend!
It's not bad for short runs, but because the wire gauge is so small it's pretty lossy. I'd use it for 10-20' or something, but for 50 feet or more, I'd pick something else with less attenuation. "Lamp cord" (or "zip cord") that is #18AWG isn't bad. It's in the 70 Ohm range or so, and has much less loss than a twisted pair from CAT-5. WB2WIK/6

RE: ethernet twisted pair ad lightweight feedline	Reply
by IZ4KBS on April 6, 2009	Mail this to a friend!
TU for your reply. In fact I wanted to use Cat5 because it is designed for higher frequencies, so I thought it may be less lossy at HF than the usually dirt-cheap lamp cord jacket, at least at typical QRP power levels, but I may be wrong. I'm glad to learn that others have played with the idea though :-) One of these days I may actually take some measurements to try and assess the facts. I think that simply measuring the RF voltage at the beginning and at the end of a properly resistor-terminate length of Cat5 UTP should provide an answer.

RE: ethernet twisted pair ad lightweight feedline	Reply
by WB2WIK on April 6, 2009	Mail this to a friend!
I've already measured it, many times. At 28 MHz, a pair of #24 AWG conductors in CAT-5 cable measures ~6 dB/30m loss. At 7 MHz, it's more like 3 dB/30m. #24AWG with insulation is rated .577A, which at 100 Ohms would be about 33 Watts max continuous power. For CW-SSB it should handle a it more, due to the low duty cycle. CAT-5 is rated 0.19 Ohms/m loop resistance, and it really is about that bad due to the small conductor size. Lamp cord is typically #18AWG and has a lot more surface area, can handle a few hundred Watts easily and has about half the loss at HF. Although nothing guaranteed is about it, most lamp cord is around 70 Ohms Zo or so. WB2WIK/6

RE: ethernet twisted pair ad lightweight feedline	Reply
by IZ4KBS on April 6, 2009	Mail this to a friend!
Well, thank-you for the figures. Given those numbers, I guess the small gauge of the computer ribbon cable that is used in the NorCal Doublet antenna, which is the one I'm trying to mimic, will exhibit similar losses. What you think ?

RE: ethernet twisted pair ad lightweight feedline	Reply
by WB6BYU on April 6, 2009	Mail this to a friend!
RG-174 coax is only 4.2dB/30m at 28 MHz if you are feeding a dipole or other tuned antenna, which is lower than either of the parallel conductor lines. And you can plug it right into the rig, saving the weight of a tuner.

RE: ethernet twisted pair ad lightweight feedline	Reply
by WA7NCL on April 8, 2009	Mail this to a friend!
Zip cord and cat 5 use poor dielectrics. They have high loss. Even if you get zip cord with a big conductor, the dielectric is not designed for RF. Air is very good, thats why they use foam in coax.

RE: ethernet twisted pair ad lightweight feedline	Reply
by IZ4KBS on April 11, 2009	Mail this to a friend!
== Lamp cord is typically #18AWG and has a lot more surface area, can handle a few hundred Watts easily and has about half the loss at HF. Although nothing guaranteed is about it, most lamp cord is around 70 Ohms Zo or so. == How to I measure losses in a piece of twin-lead wire ? I have got a reasonably reliable aerial analyzer to work with. Thanks.

RE: ethernet twisted pair ad lightweight feedline	Reply
by WB6BYU on April 12, 2009	Mail this to a friend!
One simple way to measure feedline loss is to short circuit the far end of the feedline and measure the SWR at the other end. Actually if your analyzer can tell you the return loss relative to the feedline impedance (100 ohms in this case) then half the return loss is the feedline loss. The higher the loss, the closer the feedline input will be to the characteristic impedance, even when the far end of the line is shorted or open. The ARRL Antenna Book used to have graphs that showed the total line loss as a function of the SWR measured at the input. You can use such a graph by measuring the impedance of the shorted line and calculating the SWR relative to 100 ohms. If you have a power meter of some sort you can measure the power at each end of the line and calculate the loss from that. This will require a bit of care to take the measurements accurately at 100 ohms with a 50 ohm meter. One approach would be to put a dummy load on the meter and measure the output right after the tuner. Then move the meter to the far end of the line and readjust the tuner for best match and compare the readings. There will be some difference in tuner loss when matching different impedances, but that should be small, especially if you use a 1 : 1 balun rather than 4 : 1.

RE: ethernet twisted pair ad lightweight feedline	Reply
by IZ4KBS on April 12, 2009	Mail this to a friend!
== The higher the loss, the closer the feedline input will be to the characteristic impedance, even when the far end of the line is shorted or open. == My analyzer is a VK5JST and it only shows resistance, reactance and SWR. Applying the method that you suggested to a 40ft line made of speaker wire I measured an impedance of about 120 ohm, almost flat across the whole HF spectrum. Shorting the far end of the wires I get approx. 136 ohm at 25MHz, so I must conclude that such wire is quite lossy at RF.

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