# Audio Frequency Transmission line?

Discussion in 'General Electronics Chat' started by electronice123, Apr 29, 2014.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
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Can a transmission line be used in the audio range if it's wound on a high permeability core?

The reason why I ask this is because I recently read that the effective length of a transmission line is the actual length of the line time the sqrt of the core permeability. (Ref "A tutorial on transmission line transformers" -equation 5)

So if a core with a μe of 2500 were used this would reduce the required length of the line by a factor of 50.

Then you could take a 1000 ft cable and it's effective length would be 50,000 feet (9.49 miles). With a velocity factor of .7 this equates to a full wave frequency of 13.7kHz if my math is correct. So a frequency 1/4 of 13.7kHz (3.4kHz) could be used to form a quarter wave line?

2. ### crutschow Expert

Mar 14, 2008
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What is the purpose of this audio frequency transmission line? Normally transmission lines are not used for audio unless you are talking about something like the old analog long distance telephone lines.

3. ### wayneh Expert

Sep 9, 2010
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I assumed it was for the muzak in a space elevator.

4. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
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I always hear that xmsn lines are only used for RF.
After reading more I understand better.

Usually people just say the line length is the reason.

However, the above paper I referenced seems to show that the length of the line is not the reason.

After more reading I found out that the propagation velocity changes a lot over the audio range, as well as the characteristic impedance.

Still wonder if there are any cases out there where a xmsn line is used in the audio range for impedance transformation?

5. ### studiot AAC Fanatic!

Nov 9, 2007
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Yes but you would have to find a core material that has this permeability at audio frequencies. Does yours do this?

The normal way to create a transmission line in audio is to use mechanical means rather than working on the electrical signal, unless you want to do it digitally.

6. ### alfacliff Well-Known Member

Dec 13, 2013
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yes, transmission lines are used at audio frequencies, just check out all those at 60 and 50 hz. if you check into transmission line theory, it works the same, just that they call swr power factor, and power factor correctors are the same (with larger l and c components) circuits used in rf transmission lines. the power companies have to work with the wavelength at 60 or 50 hz in their systems too.

7. ### crutschow Expert

Mar 14, 2008
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There may be transmission line considerations in long power lines but a power factor different than unity is not usually caused by the same thing as a SWR different from unity. Power factor is determined by reactive loads. SWR is a transmission line effect from a difference in the load impedance as compared to the transmission line characteristic impedance. An SWR different from unity can be caused by an incorrect resistive load value whereas a power factor different from unity requires a reactive load.

8. ### studiot AAC Fanatic!

Nov 9, 2007
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It is probably worth mentioning that we normally operate audio feed lines far far away from the maximum power transfer theorem region in transmission line theory for damping and other reasons.

9. ### alfacliff Well-Known Member

Dec 13, 2013
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and what affeects the load impedance? just like in rf, and as you mentioned with power factor, inductive and capacitive loads. to have impedance, you have to have complex reactance, r,l,c. a load can act with a leading (inductive ) or lagging (capacitive) power factor, to match the load, you have to introduce inductance or capacitance. an autpmatic power factor corrector adds inductance or capacitance to match the load, just like an automatic antenna tuner. antenna too short, add inductance. antenna too long, series capacitor.

10. ### crutschow Expert

Mar 14, 2008
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But that's the big difference. In a power line you are not matching the load to the source, you are simply minimizing the reactive currents to maximum power line efficiency so that most (100% ideally) of the generator power reaches the load. The total load impedance is still much less than the source impedance.

In a transmission line you are matching the load to the source to get maximum power transfer. The load impedance must match the source impedance with a maximum of only 1/2 the total power being transmitted to the load.

11. ### alfacliff Well-Known Member

Dec 13, 2013
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the last transmission line I measured had 100 watts going in and 80 watts out, with losses due to loss in the line. that is more than 50% getting to the load.

12. ### crutschow Expert

Mar 14, 2008
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You are looking at the power into the transmission line but aren't including the loss in the driver impedance which is at least as high as the load power. Are you not familiar with the maximum power transfer theorem?

13. ### alfacliff Well-Known Member

Dec 13, 2013
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the driver was a harris 100 watt transmitter, putting out 100 watts into the coaxial line. the load was a decibel products 4 pole antenna with almost no reflected power (swr) there was about 20 watts loss in the coax. the transmitter was running with about 75 % efficiency, and no room for 50% loss anywhere.
the refered Jacobi law seems to be mosly the same as disapation in series resstances. maximum power in ac circuits occurs when the impedance is matched between source and load. in my example, the transmitter had 50 ohm output, the coax had 5o ohm impedance, and the antenna has 50 ohm impedance. therefore, the loss in the circuit is the loss in the coax and connectors. that is simple transmission line theory.