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Speaker wire is really cheap and available everywhere, and it doesn't seem that different to 300 ohm twin lead. It seems to me it would make an excellent balanced transmission line.

What would the approximate impedance and loss be? Can I use a balun / matching transformer at the radio end and then tune the antenna to the speaker wire impedance?

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    $\begingroup$ Speaker wire is not similar to twinlead in that the distance between the wires is very different. Does your speaker wire have a rated impedance? $\endgroup$
    – user10489
    May 23 at 23:39
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    $\begingroup$ There are impedance calculators out there if you wanted to test your assertion. You'll have to estimate the dielectric constant. Speaker wire makes a fine transmission line if you split it and make ladder line from it. Though, I can't help but think that If speaker wire as-is was a Good Enough transmission line, cheap hams would already be using it. $\endgroup$ May 24 at 1:04
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    $\begingroup$ cat5e is I think 100 ohms and very stable. A 2:1 balun would make that good for coax. Unfortunately, the current carrying capacity of the wires is pretty small, so you might be limited to QRP. $\endgroup$
    – user10489
    May 24 at 11:23
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    $\begingroup$ So-called audiophiles should not be held up as paragons of experimental and practical physics. $\endgroup$ May 24 at 12:05
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    $\begingroup$ Checking a range of prices, speaker wire is not really cheap compared to other solutions designed for antennas such as 300 ohm or 450 ohm ladder line. In fact, DX Engineering has ladder line available for costs that overlap and compare nicely to the 100 foot rolls of speaker wire (assuming 100 foot lengths). Shop around, I actually sold 250 foot of 450 ohm ladder line for $50 a year ago -- I was not trying to get highest price though, just trying to get rid of it rather than throw it away. $\endgroup$
    – K7PEH
    May 30 at 16:10

2 Answers 2

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I can think of a reason expect this not to work very well. The impedance of twin-lead transmission line is dependent on the ratio between

  • the diameter of the conductors, and
  • the distance between their centers.

In twin-lead or any parallel-conductor transmission line, the insulation is designed to keep that distance stable. On the other hand, in speaker wire, the insulation is usually quite soft, and a small amount (<1 mm) of incidental squishing of the cable will quite significantly change the distance between the conductors, and thus the impedance.

But, that's a theoretical answer, on why we don't in general use cheap speaker wire or lamp-cord for transmission lines. What about in practice?

Well, I've got a piece of speaker wire handy (30 ft or so, 16 AWG). I unplugged it from my speaker and plugged it into a resistance decade box and my antenna analyzer, and made some measurements. Impedance, 0-30 MHz:

Impedance chart

So, an adequate piece of transmission line around 110 Ω.

Does it have obvious impedance discontinuities measured in my analyzer's "TDR" mode?

TDR chart

Not very much! (The spike on the left end is the connection to the analyzer, and the wiggle on the right is the connection to the termination.)

I'd guess that the practical reasons not to use speaker wire are:

  • It's not a standard impedance, so you'd need matching on both ends
  • Any particular product might have a different impedance
  • It's not coax, so you have to treat it like any twin-lead and keep it away from other things, but people generally prefer coax unless they're building a high-performance HF station, at which point why use the cheap stuff?
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  • $\begingroup$ But if you were desperate or poor, you could use speaker wire ... with matching at both ends ... and get good results. $\endgroup$
    – Andrew
    May 24 at 7:45
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    $\begingroup$ Matching at both ends might cost more than using the correct wire with a rated impedance. $\endgroup$
    – user10489
    May 24 at 11:11
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    $\begingroup$ To be fair, you might only need impedance matching at the radio if the antenna can be tuned to the impedance of the line, such as a custom dipole. $\endgroup$
    – foreverska
    May 24 at 16:19
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It depends substantially on the particular speaker wire — available gauges range from 14-2 to 22-2, insulation materials and spacing vary, and the impedance is probably somewhere between 80 and 150 ohms, depending. The loss will be worse than 300-ohm ladder line because of the lower impedance (meaning higher current for a given power), because speaker wire has a higher "fill factor" (percentage of the space between conductors occupied by insulator rather than air), and because the insulator is usually PVC (which is much lossier than PE or PTFE), and will probably be on a par with very cheap coax.

But since there's no standardization for this stuff in terms of its RF properties, you can't really tell what you'll get from a particular roll without buying it and measuring it. It's probably fine for a "whatever dipole" but not for any design that requires careful impedance matching.

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    $\begingroup$ Great answer. I've been curious about the use of PTFE. I like PTFE for it's heat tolerance and abrasion resistance but didn't realize it also helps with loss. I've made a few baluns with it and they work really well. $\endgroup$
    – wbg
    May 24 at 15:45
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    $\begingroup$ You are right on about the loss because of the poor dielectric properties of the insulation. I didn't search, but I've read about many tests confirming this. $\endgroup$
    – Mike Waters
    May 27 at 19:44
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    $\begingroup$ How about audiophile-grade speaker wire? Those seem to have negative impedance and even add stuff that isn't on the original recording. $\endgroup$
    – osiris
    May 28 at 1:33

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