Can I use speaker wire as a transmission line?

Question

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?

Answer 1

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:

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

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

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?

Answer 2

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.

Answer 3

1. You can use any type of wire to construct twinlead type feedline. Back in the day before coax was popular, odd pieces of scrounged wire, lampcord, speaker wire, even extension cord was used with success.
2. The impedance of the feedline is unimportant. If directly connecting twinlead to a halfwave dipole (two quarter wave sections fed in the center usually but not always), there will be an impedance mismatch at the feedpoint, since a half wave dipole AT ITS RESONANT FREQUENCY will be approximately 72 ohms, depending on its height above ground. This will create reflected power in the feedline which u-turns at the transmitter and bounces up and down the feedline. Since it is a high impedance feedline, the voltage is high and the current is low, resulting in low I squared R losses. Using 300 ohm tv twinlead, or audiophile wire is a waste of money. Just get your hands on some cheap spools of wire and fabricate a twinlead feedline, making sure to install it properly by keeping it a few inches away from metal objects and twisting it every couple of feet to prevent it oscillating in the wind. Coax is inherently lossy (especially in long lengths) because of the higher currents in it. The nice thing about is that you can make your own for low cost. If you go out and buy 200 feet of "low loss" coax it will cost a packet. If you want a multi-band dipole, this is easily accomplished with twinlead feedline coming down to some type of matching arrangement at the transmitter to convert the high impedance line to the 50 ohms the transmitter would like to see. If you use coax expecting to use it on multiple bands you will have a great deal of difficulty, and the losses on some frequencies will be very large. If you refer to any edition of the ARRL Amateur's Handbook you will find very concise explanations of antenna and feedline basics. Fianlly - stop obsessing over SWR! I hope this helps. Added Oct 17 - Here's another idea if you have some tv twinlead lying around. Make a folded dipole for the lowest frequency you will operate on, and feed it with the 300 ohm twinlead. You will have a very close impedance match at the feedpoint, hence low reflected power in the feedline, reducing losses even more. Simply connect the feedline to a transmatch as usual. The 300 feedline will be able to handle 300 watts or more due to the low current. The folded dipole will also give a little more gain than a straight dipole. Easy to make with cheap materials! Covers multiple bands. Keep the coax between transmitter and tuner as short as possible. Disconnect and short out when electrical storms are approaching!