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Author Topic: Coax at LF  (Read 787 times)
G3RZP
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Posts: 1159




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« on: March 27, 2019, 02:06:08 PM »

To avoid too much thread drift on another topic...

We normally assume that the loss in coax - matched or not - decreases with frequency. There are a number of coax cables around today with a thinly copper plated steel inner, which means that because of skin effect, the losses increase as frequency goes down as the inner becomes more resistive. This makes it more important to define frequency in MHz rather than mHz....
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W9IQ
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« Reply #1 on: March 27, 2019, 03:08:39 PM »

Here is another fun coax fact.

Losses increase for a 1/2 wavelength of coax as frequency decreases. Of course, the same can be said for any fractional wavelength.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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« Reply #2 on: March 27, 2019, 03:31:10 PM »

For a given type of dielectric material, generally the larger the diameter of the center conductor, the lower the losses of the coax.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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« Reply #3 on: March 27, 2019, 05:06:51 PM »

The magnitude of ZO is generally several orders of magnitude greater at low frequencies. ZO will stabilize at its quoted value in the 10 kHz range.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
AE5GT
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Posts: 389




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« Reply #4 on: March 27, 2019, 06:14:39 PM »

To avoid too much thread drift on another topic...

We normally assume that the loss in coax - matched or not - decreases with frequency. There are a number of coax cables around today with a thinly copper plated steel inner, which means that because of skin effect, the losses increase as frequency goes down as the inner becomes more resistive. This makes it more important to define frequency in MHz rather than mHz....


Most coax uses an aluminum center with a copper plate. A coax with a steel inner would not bend well , however there are coaxes with a steel messenger wire that are used for stringing between telephone poles like 9117.

Losses at low frequencies are more likely due the higher penetration depths causing the coax to "leak" RF , thats why standard RG6 is a poor choice for HF frequencies. RG 6 is much mor suited for L-Band microwave because the RF penetration depth at those frequencies is low enough that most of the fields are contained within the foil shield.   


Standard RG6 leaks like a sieve at HF.
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G3RZP
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« Reply #5 on: March 28, 2019, 06:34:47 AM »

There is cable around over here meant for UHF use with a copper plated steel core for the inner which has caused some grief when used at HF. Similarly, some brands of RG174 have copper plated steel wires for the inner.

Somehow life seems to be getting more and more complicated!
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W1VT
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« Reply #6 on: March 28, 2019, 07:06:50 AM »

RG-8X is a good compromise at LF because the conductors are 100% copper in the examples I've seen. 

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G8HQP
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« Reply #7 on: March 28, 2019, 09:38:40 AM »

Quote from: W9IQ
For a given type of dielectric material, generally the larger the diameter of the center conductor, the lower the losses of the coax.
I am not certain this is true.

The larger the diameter of the cable, the lower the loss, other things being equal.

If you simply make the inner conductor bigger then you get a lower characteristic impedance. This means more current, to get the same power through the cable, so more loss for a given resistance. These two effects work in opposite directions, so you get a broad minimum of loss at about 50 ohms impedance (IIRC - or is it 75R? I know one is minimum loss and the other is maximum power handling).
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W1VT
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« Reply #8 on: March 28, 2019, 09:44:32 AM »

At typical LF/HF/VHF frequencies, you can ignore the dielectric loss of typical cables, and just look at conductor losses.

Though I  do recall that the rubber dielectric used in house AC cords is lossy at HF and should be considered if it is used a coax dielectric.  Shocked
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KE7YD
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« Reply #9 on: March 28, 2019, 09:53:05 AM »

Please, oh please, show me any major manufacturer of coax that has higher loss at lower frequencies. That may be true for the Chinese crap that is coming into the ham market but not for quality coax. Here is a direct copy of the loss chart for mil spec cable from Times.  RG-142 is double shielded teflon dielectric with a steel core that is copper then silver plated center conductor.

Frequency (MHz) 10 30 50 100 400 1,000 1,500 2,000 2,500 3,000 10,000 k1 k2
M17/RG-316       2.5 4.3 5.6 8 16 26 32 38 42 47 - 0.787 0.00120
M17/RG-223       1.2 2.1 2.8 4 8 13 17 20 22 25 51 0.384 0.00126
M17/RG-142       1.2 2.1 2.7 4 8 13 16 19 21 24 49 0.368 0.00120
M17/RG-214       0.7 1.2 1.6 2.2 4.7 7.9 10 11.9 13.7 15.3 37 0.210 0.00126
M17/RG-393       0.6 1.1 1.4 2.0 4 7 9 11 13 14 31 0.202 0.00120
Attenuation at Any Frequency = [ k1 x SQRT (Fmhz)] + [ k2 x Fmhz ]; dB per 100 feet

Sorry the chart didn't translate well onto the post.


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G8HQP
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« Reply #10 on: March 28, 2019, 10:49:24 AM »

What happens well below 10MHz?

If copper core then the attenuation continues to fall down to the kHz region and then stabilise.

If steel core than at some frequency it could begin to rise for a while until it eventually levels off.
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N1BNC
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« Reply #11 on: March 28, 2019, 12:03:02 PM »

What part of the spectrum are you concerning yourself with?
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KE7YD
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« Reply #12 on: March 28, 2019, 12:11:49 PM »

If your really that curious, Times conveniently included the attenuation formula at the end.

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G3RZP
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Posts: 1159




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« Reply #13 on: March 28, 2019, 02:36:11 PM »

Back in the 1980s, in 'RF Design' Magazine, there was a full mathematical analysis - which wasn't challenged in correspondence. Possibly a Google search will dig it out .....

Obviously, it didn't deal with steel cored coax inners...
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W9IQ
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« Reply #14 on: March 28, 2019, 03:22:55 PM »

Quote from: W9IQ
For a given type of dielectric material, generally the larger the diameter of the center conductor, the lower the losses of the coax.
I am not certain this is true.

The larger the diameter of the cable, the lower the loss, other things being equal.

If you simply make the inner conductor bigger then you get a lower characteristic impedance. This means more current, to get the same power through the cable, so more loss for a given resistance. These two effects work in opposite directions, so you get a broad minimum of loss at about 50 ohms impedance (IIRC - or is it 75R? I know one is minimum loss and the other is maximum power handling).

My statement was for coax of the same impedance.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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